ACL rod bearings?

DRLane wrote

Epoustouflant wrote

Bought ACL mixed set + ARP bolts. The value proposition isn't there for BEs for 2x the price IMO.

Damn man, you're the last person on this forum I would've anticipated to cheap out a few hundred $.

I'm just not seeing it to be honest.

I fell into that bearing rabbit hole (the bolts Drama, the ACL measurements, the VAC critiques, the numerous BOs etc) and came away with a relatively poor impression of how BE represented itself against other long standing manufacturers of bearings.

It seems like that attitude only increased during their lengthy backorders as competitors visibly made up ground and ate into market shares.

My shop who will do the work builds race engines and will measure each bearing prior to install, which seems to be the main value driver of BE over others now that other manufacturers offer clearance adjusted bearings.

The few data points we have of clearance (or coating) adjusted bearings coming out of engines have all looked good.

My 2 cents, I'll definitely not claim any title to this bearing debate. I'll sleep well with ACL / ARP, ultimately it's what we're buying; sleep aids 😂

Hi folks - a new post to refresh this thread.

I've read all 12 pages on this and hope I understand things well enough.
Looking to do RB change on my 2008 E93 M3. Has 78k miles. I am the third owner. I suspect RB are original. Been doing Blackstone analysis with yearly oil changes (4-5 k miles generally) and lead and copper are low and below the universal averages.

I'm not skilled enough to do the replacement so I have an indi mechanic who has worked at BWM on tap to do the change. He knows a race shop and they recommend ACL - I'm still trying to confirm with him if they prefer the H or HX.

I buy as many parts as I can from FCP Euro for the lifetime replacement. They carry the HX shells and the OEM rod bolts. I'd like to go this way if possible to reduce part costs down the road. On Amazon I see HX and H packs that sell for $125-$130 each - if this covers one RB change then the cost is similar to FCP but with a mixed clearance.

So curious if H or HX or a mixed set is the prevailing thinking now? And how the early adopters of ACL are fairing?

Thanks!

Jim
Denver Colorado

jimmtman wrote

Hi folks - a new post to refresh this thread.

I've read all 12 pages on this and hope I understand things well enough.
Looking to do RB change on my 2008 E93 M3. Has 78k miles. I am the third owner. I suspect RB are original. Been doing Blackstone analysis with yearly oil changes (4-5 k miles generally) and lead and copper are low and below the universal averages.

I'm not skilled enough to do the replacement so I have an indi mechanic who has worked at BWM on tap to do the change. He knows a race shop and they recommend ACL - I'm still trying to confirm with him if they prefer the H or HX.

I buy as many parts as I can from FCP Euro for the lifetime replacement. They carry the HX shells and the OEM rod bolts. I'd like to go this way if possible to reduce part costs down the road. On Amazon I see HX and H packs that sell for $125-$130 each - if this covers one RB change then the cost is similar to FCP but with a mixed clearance.

So curious if H or HX or a mixed set is the prevailing thinking now? And how the early adopters of ACL are fairing?

Thanks!

Jim
Denver Colorado

Edit, I'll start off by recommending you reconsider the OEM bolts. There is a lot of room for error and they do not let to have a do-over.

If you are entirely sold on ACL as your bearing of choice, Dean from Mporium out of Texas would be my suggestion. He was hand measuring custom sets of ACL bearings using the H+HX mix to achieve a believed to be proper tolerance, if you don't like H or HX on their own. He may be back to using BE since the inventory was back around. You should also be aware that virtually EVERYONE else who may tell you they hand measure each set of mixed shells is basically blowing smoke up your a.. If you're not sold on ACL, then I would buy a set off those BE folks and maybe SERIOUSLY reconsider the OEM bolts.

TX wrote

If you are entirely sold on ACL as your bearing of choice, Dean from Mporium out of Texas would be my suggestion. He was hand measuring custom sets of ACL bearings using the H+HX mix to achieve a believed to be proper tolerance, if you don't like H or HX on their own. He may be back to using BE since the inventory was back around. You should also be aware that virtually EVERYONE else who may tell you they hand measure each set of mixed shells is basically blowing smoke up your a.. If you're not sold on ACL, then I would buy a set off those BE folks and maybe SERIOUSLY reconsider the OEM bolts.

Yes, for a few years now we've been offering WPC treated ACL bearings which we size into matched extra clearance sets. We give our customers the choice even when BE are available as the ACL service is also a little less expensive. Either offering will outlast the main bearings, so it's basically a one-and-done solution.

jimmtman wrote

Hi folks - a new post to refresh this thread.

I've read all 12 pages on this and hope I understand things well enough.
Looking to do RB change on my 2008 E93 M3. Has 78k miles. I am the third owner. I suspect RB are original. Been doing Blackstone analysis with yearly oil changes (4-5 k miles generally) and lead and copper are low and below the universal averages.

I'm not skilled enough to do the replacement so I have an indi mechanic who has worked at BWM on tap to do the change. He knows a race shop and they recommend ACL - I'm still trying to confirm with him if they prefer the H or HX.

I buy as many parts as I can from FCP Euro for the lifetime replacement. They carry the HX shells and the OEM rod bolts. I'd like to go this way if possible to reduce part costs down the road. On Amazon I see HX and H packs that sell for $125-$130 each - if this covers one RB change then the cost is similar to FCP but with a mixed clearance.

So curious if H or HX or a mixed set is the prevailing thinking now? And how the early adopters of ACL are fairing?

Thanks!

Jim
Denver Colorado

Hi Jim!

As I probably mentioned earlier in this thread I went with extended clearance (HX) and ARP 625 bolts in July 2018, done abour 15.5k miles on them. FWIW, so far all good! (continue with 10W60, or course)

I see quite a few go with H/HX mixed sets today and to honest I'd probably do the same should I redo them. They still increase clearance a bit vs OEM, and to be fair majority of the OEM shells still run out there even with wear. So again, a tad more open I believe goes a long way while still maintain the essential even oil film pressure (which is jeopardized with increased clearance).

HOWEVER, mixing these shells must be done correctly. From what I understand some shops simply cut a H and a HX pack in half and sell off. You can read in threads from guys like Deansbimmer that the shells need to be measured in order to get the proper pair mixed and achieve targeted clearance. If you can't confirm this to be done I'd say just go with a H or HX set.

Regarding OEM bolts they are assumingly doing as good of a job as for instance ARP. The reason poeple are using them - and shops prefer them - is that OEM takes much more time due to its torque procedure. If thats fine with selected shop, go OEM (new bolts cause stretch) and save a few buck.

Good luck!

Helmsman wrote

.

Regarding OEM bolts they are assumingly doing as good of a job as for instance ARP. The reason poeple are using them - and shops prefer them - is that OEM takes much more time due to its torque procedure. If thats fine with selected shop, go OEM (new bolts cause stretch) and save a few buck.

Good luck!

I would add that, not only the ARP Bolts torque procedure is simpler, but also they are reusable - while OEMs aren't -, and mainly they do a smaller bore distortion to the shells, which contributes to avoid the contact with the crank and consequent wear.

But I agree with you, OEM bolt is acceptable.

jvictormp wrote

I would add that, not only the ARP Bolts torque procedure is simpler, but also they are reusable - while OEMs aren't -, and mainly they do a smaller bore distortion to the shells, which contributes to avoid the contact with the crank and consequent wear.

But I agree with you, OEM bolt is acceptable.

Choose your ARP bolts wisely. Not all are without bore distortion.

http://wiki.rcollins.org/core/index.php?title=Engine_Specs_-_S65/S85_Rod_Bolts

Green-Eggs wrote

Choose your ARP bolts wisely. Not all are without bore distortion.

http://wiki.rcollins.org/core/index.php?title=Engine_Specs_-_S65/S85_Rod_Bolts

Would you help me think clearly?

It seems like, with 45 ft/lb of torque, the ARP 2000 is the only one that causes a positive distortion.

And in the group of those that make negative distortion, (ARP-2000 with 45 and 55ft/lb, ARP-625 with 50, 55, 60 ft/lb, and OEM Bolts), the OEM Bolts seem to make the smallest negative distortion.

Am I seeing it right?

What do we want:
Positive bore distortion?
Zero bore distortion?
Some distortion, but the less the better?
Much distortion, the more the better, but only until some limit?

I can't answer your question specifically, but as I recall the measurements at rcollins site show that the ARP 625 bolts (with 55lb/ft IIRC?) are the ones that give excessive distortion.
Hopefully Green-eggs will give you specifics.

Hi Folks - thanks for all of the replies! Very helpful.

My mechanic favors the OEM rod bolts and he is familiar with them. They are tougher to install than ARP but cheaper. Also FCP Euro carries them so they will be lifetime replacement. If I were skilled enough to do this repair myself I probably would opt for the ARP but I don't think I ever will be. So happy to let my mechanic pick which bolt he would like.

On bearings my mechanic reached out to a shop on the East Coast he knows as experts on this. They suggest the standard H bearings. I spoke to ACL tech Robert in Nashville TN today and he was very helpful. He also suggested the H standard size. He said if I raced this car and it saw redline alot or it were boosted then HX would make more sense. But I rarely go above 7k rpm so he thought standard would be better. He said all H bearings are sized for 0.001" clearance of rod journal which for S65 at 52 mm would be 0.052 mm clearance. The wiki lists BMW at 0.0381 mm clearance which is 0.00073" per rod journal inch. So even the standard H is a bigger clearance than the BWM ones. Robert thought the HX would be a little big for the driving I do. He says they do sell alot of HX's but those users must push their cars more. So my current take is to go for the H sized bearings. He also thought that if the oil film is maintained this bearing should get very little wear. Maintaining oil film is his opinion tougher if the clearance is larger and may require upgraded oil pump (but I don't think anyone here running HX has done that). So using 10W60 with H he thinks the bearings should last a long time. ACL gives no mileage to replace at. ACL will give the metals breakdown to Blackstone to allow checking for wear but again Robert thinks this bearing should last a long time....like as in may not have to replace again.

Please let me know your thoughts on all that I heard on this and if anything doesn't sound right.

Thanks!

The OEM bolts are perfectly fine, but take about 3 times as long time install. If you are paying for labor, ARP or Carillo work out to the same cost. The torque sequence is also a pain in the rear. Not hard, just painstaking to do every bolt in 3 stages 3 times. I did it back in 2014, but the stock bolts were cheap then. Presumably your mechanic knows all this and is not thinking the torque sequence is just like it is for an S52 or an N54 or other common BMW motors.

jimmtman
Yes the HX's (nominal) clearance is a little too large, although I haven't heard of any problems with that to date here.
As for the ACL tech @ Nashville, he seems to be contradicting his own company's specs for the nominal clearances for the H's, which are 0.0355mm for the S65.
I originally worked it out/saw it from the following posts (this is from my notes).

ACLs own specs (ref posted email https://www.m3post.com/forums/showpost.php?p=23295448&postcount=36 & from Helmsman https://www.m3post.com/forums/showthread.php?p=25741508#post25741508)

ACL std H
Nominal Rod Bearing Clearance 0.0355 mm 0.00140 inch
Bearing Clearance Variance 0.013 - 0.058 mm 0.00051 - 0.00228 inch
(Redish Motorsport find they get ~0.05mm on plastigauges)

ACL std HX (extra clearance)
Nominal Rod Bearing Clearance 0.0615 mm 0.00242 inch
Bearing Clearance Variance 0.039 - 0.084 mm 0.00153 - 0.00331 inch

To see the official figures directly (and work out the nominal clearances) checkout the following pdf from ACL, copy the bearing part number then word search (ctrl+f) for the specs in the Tech data section at the end.
https://aclraceseries.com/images/77610_ACL_Bearing_Race_Series_Guide_US_Version.pdf

Anyway, from what I found the nominal ACL H clearance is less than the OEM nominal clearance that the BE bearing wiki found.
That said, as per my notes above, Redish motors found with plastigauge* that they normally come out at about 0.05mm, which is pretty much what BE found measuring a set of H's. It's interesting that the ACL tech you spoke to also gave that figure, perhaps ACL bin them to aim for 0.05mm clearance?? If so they aren't following their own specs (which is good! lol), but it leaves uncertainty. It would be interesting if you could ask him what that's about!?

That is why I went for mixed and measured ACL H/HX shells from deansbimmer, no uncertainty then from the shells!
If I wasn't able to get mixed and measured shells then I would have gone for the H shells, measuring them with plastigauge* and hoping that I'd get 0.05mm. The trouble is, if it's much tighter, then what? Order another set??

*Re measuring with plastigauge, it isn't as accurate as dial gauges, but 1. It's possible to use in situ, and 2. It's good enough for BMW and Suzuki, (the latter who use it to choose the correct thickness shells), so it's good enough for us. It is not meaningless as some people claim . Check out these videos of plastigauge vs dial gauge ([edit ffs, the forum is removing the start times I put in once you click on it!]result table @5m:25s, for the full run down I'd suggest watching the 1st video from the start)
and (2 of his results @10m:11s)

Thanks for your reply! My mechanic used to work at BMW and has done a few RB replacements so he is familiar. I asked him if he would reduce the labor time and cost for using ARP bolts and he didn't. So without the labor cost savings ARP cost $300 and stock bolts are $150. So I went stock bolts as cheaper and my mechanic prefers them. Perhaps down the line I will regret this - time will tell.

Curious if everyone plans to change their ACL bearings on a certain # of miles or with UOA a certain metal content? If these ACL last 50k miles (probably conservative) and I drive about 5k miles a year that 10 years. I don't even know if I will own the car this long.

Thanks for your reply! I need to double check the numbers and make sure I have this right. I don't want H if the clearance is less than stock. I don't have time to delve into it now and I'll need to go back and reread your post on it.

Where is deansbimmer located? I'm in Denver. I'm just thinking ahead if I do want to go the mixed H and HX route perhaps he can supply them and my local mechanic installs them.

Thanks!

He's one of the forum vendors here, Mporium, I think he's in Texas?? ...... yes he is, see here https://mporiumbmw.com/contact/

Btw, re replacement aftermarket bearings, as far as I'm concerned they fix the clearance issue (and so the problem), and so never need changing again (bar being damaged some other way).

Assimilator1 wrote

I can't answer your question specifically, but as I recall the measurements at rcollins site show that the ARP 625 bolts (with 55lb/ft IIRC?) are the ones that give excessive distortion.
Hopefully Green-eggs will give you specifics.

The point I'd like to make are that multiple vendors sell APR-2000 bolts. Not all of them provide the same torque specifications. One vendor says 45 ft-lbs which causes bore distortion, and the other has custom made bolts at 50 ft-lbs with little or no bore distortion. I'm not trying to sell any products or hurt anybody's business, so that's why I didn't mention any names here; but I'm sure people can figure it out. :-) Hopefully that clears this up.

Assimilator1 wrote

jimmtman
Yes the HX's (nominal) clearance is a little too large, although I haven't heard of any problems with that to date here.
As for the ACL tech @ Nashville, he seems to be contradicting his own company's specs for the nominal clearances for the H's, which are 0.0355mm for the S65.
I originally worked it out/saw it from the following posts (this is from my notes).

ACLs own specs (ref posted email https://www.m3post.com/forums/showpost.php?p=23295448&postcount=36 & from Helmsman https://www.m3post.com/forums/showthread.php?p=25741508#post25741508)

I'm not sure I followed those links before, but today I did. I noticed some information about BE that needs correcting. The claim was made that BE implies that their single set measurements are what everybody who purchases those bearings can expect. My shop and BE has said the exact opposite, and said it many, MANY times. Most often, the claim is made that "BE measured the updated 702/703 bearings and found that BMW increased the bearing clearance." I see that claim all the time, and I've tried to correct it every time i see it. Every time I see it, I remind people that BE only measured one single set of bearings that you can't draw any meaningful statistical conclusions from that single set of measurements.

Another claim is made that BE used a more restrictive definition of "shaft diameter," but used the actual BMW specs for the rod bore size. This is true, but might be slightly misleading (maybe because it was misunderstood).
Since BMW didn't publish any shaft diameter specs, using measured specs is all we have available. ACL used a different set of specs which is more liberal.
However, when you run the math on both sets of shaft diameter specs from BE and ACL, they both come up with the same EXACT nominal shaft diameter. So it ends up being a distinction without a difference. In fact, if you understand how manufacturers calculate bearing clearance variance, the ACL spec works against them and makes their variance look worse than it probably is.

*Re measuring with plastigauge, it isn't as accurate as dial gauges, but 1. It's possible to use in situ, and 2. It's good enough for BMW and Suzuki, (the latter who use it to choose the correct thickness shells), so it's good enough for us. It is not meaningless as some people claim . Check out these videos of plastigauge vs dial gauge ([edit ffs, the forum is removing the start times I put in once you click on it!]result table @5m:25s, for the full run down I'd suggest watching the 1st video from the start)

and (2 of his results @10m:11s)

BE did their own comparison as well. This has been previously documented and posted, but the video has never been published. A customer called BE and said they measured 0.0012 with Plastigage and was very concerned. Bert was concerned too, asked them to send the bearings back and he replaced them. When the bearings were returned, Bert ran the same experiment -- and video'd the entire procedure. He confirmed the customer findings of approximately 0.0012 clearance with Plastigage. However, when he used a bore gauge and micrometer (also shown in the video), IIRC, I think he said he measured 0.0026 inch clearance. I've asked Bert to post that video many times...I'm tired of asking. Maybe some day. Anyways, that's what happened and that's our view of Plastigage as well.

jimmtman wrote

Thanks for your reply! I need to double check the numbers and make sure I have this right. I don't want H if the clearance is less than stock. I don't have time to delve into it now and I'll need to go back and reread your post on it.

Where is deansbimmer located? I'm in Denver. I'm just thinking ahead if I do want to go the mixed H and HX route perhaps he can supply them and my local mechanic installs them.

Thanks!

I'll break this down as simple as I can.

If you don't believe there's a bearing clearance problem, then don't change your bearings. If you do believe there's a bearing clearance problem, then change your bearings. If you change your bearings because you believe there's a clearance problem, then you should not use bearings with the same clearance as stock.

I don't recommend to trust the info from the guy at ACL because he doesn't seem to know his own specs (links posted above). His own specs say that the H-Series shells are very closely aligned (basically they are identical) to OEM clearances. He also apparently doesn't know that the S65 oil pump is a variable rate pump and will automatically increase output when it detects a drop in pressure. So I recommend that you don't take is advice as gospel, and continue to do your own research.

Dean's Bimmer (MPorium) is located in the Dallas, FT. Worth area. They are about 20 miles north of Dallas IIRC.

Thanks for your reply. I do think there is a clearance issue here and want to replace with something larger. The wiki shows 0.00073" of rod bearing clearance per inch of rod journal versus the commonly used 0.001". And BMW specs 10W-60 oil and higher viscosity oil per other sources would raise the clearance required versus lower it. I think the clearance has to be part of the reason for the numerous engine failures and at relatively low miles (< 50k).

So I need to work through the numbers myself.

jimmtman wrote

Thanks for your reply. I do think there is a clearance issue here and want to replace with something larger. The wiki shows 0.00073" of rod bearing clearance per inch of rod journal versus the commonly used 0.001". And BMW specs 10W-60 oil and higher viscosity oil per other sources would raise the clearance required versus lower it. I think the clearance has to be part of the reason for the numerous engine failures and at relatively low miles (< 50k).

So I need to work through the numbers myself.

The nominal clearance is one thing. The real risk is the variance which can allow for 0.00029"/" with 088/089 bearings. That is the kicker.

BE removes one variable by hand measuring every set. That is worth it in my opinion. deansbimmer does the same with ACL shells.

Statistics is a harsh mistress. I would recommend BE or properly measured ACL shells with BE ARP bolts. OE bolts are fine, but they have a complex torque procedure which can introduce risk during installation.

Cheers,

I would like to understand how you did the calculations to come up with the nominal clearances for H and HX. Can you give an example?

In my simple view of things the bearing thickness should be key for the clearance. ACL H from their website has a thickness of 1.998 mm. From the wiki the 702/703 have thicknesses of 1.9977 mm top and 1.9939 bottom. For ACL H the total thickness for 2 bearnings is 2*1.998 = 3.996 mm and for 702/703 it is 1.9977+1.9939 = 3.9916. So ACL H is thicker in total than 702/703 so I would think this means lower clearance. 088/089 is shown as 2.0028 mm top and 1.9977 mm btm so 4.0005 mm total. If this is right than 702/703 have more clearance than 088/089. H is slightly more clearance than the original 088/089 bearings which I think I have in my 2008 E93.

The HX thickness is given as 1.985 mm which is similar to BE's 1.9876 mm - smaller by 0.0026 mm. If did a mix of mixed set of H and HX the average thickness would be (1.998/2+1.985/2)=1.9915 mm - larger by 0.0039 mm than BE. From this HX looks closer to BE clearance than a mixed set.

Hoping to understand how these clearances are calculated as I am confused as to how it is done. It sounds like ACL calculates the clearance some other way and often doesn't show that calcuation in their datasheets.

Thanks!

Jim

jimmtman wrote

I would like to understand how you did the calculations to come up with the nominal clearances for H and HX. Can you give an example?

In my simple view of things the bearing thickness should be key for the clearance. ACL H from their website has a thickness of 1.998 mm. From the wiki the 702/703 have thicknesses of 1.9977 mm top and 1.9939 bottom. For ACL H the total thickness for 2 bearnings is 2*1.998 = 3.996 mm and for 702/703 it is 1.9977+1.9939 = 3.9916. So ACL H is thicker in total than 702/703 so I would think this means lower clearance. 088/089 is shown as 2.0028 mm top and 1.9977 mm btm so 4.0005 mm total. If this is right than 702/703 have more clearance than 088/089. H is slightly more clearance than the original 088/089 bearings which I think I have in my 2008 E93.

The HX thickness is given as 1.985 mm which is similar to BE's 1.9876 mm - smaller by 0.0026 mm. If did a mix of mixed set of H and HX the average thickness would be (1.998/2+1.985/2)=1.9915 mm - larger by 0.0039 mm than BE. From this HX looks closer to BE clearance than a mixed set.

Hoping to understand how these clearances are calculated as I am confused as to how it is done. It sounds like ACL calculates the clearance some other way and often doesn't show that calcuation in their datasheets.

Thanks!

Jim

I'm pretty sure the math formulas for theoretical bearing clearance have been posted in the bearing wiki by "regular guy" (bert@bebearings). The thread is long, so good luck finding it though. I can always ask bert to add the theoretical formulas and clearances to his own wiki page, but not sure how fast he would do it, or how motivated he might be to help BE help a competing vendor make a sale. This is the problem with this issue, nobody but BE has done any of the work.
https://www.m3post.com/forums/showthread.php?t=892838

There's also more to the calculations than the bearing thickness itself. You also must accommodate the rod journal variance and the rod big-end bore variance as well. And in the end, all you'll get are theoretical clearances.

In the real world, the length of the strip material also factors into it. Change the strip material by a few 1/10000ths of an inch, and you change the clearance by up to 0.001 inch. This happens because the length of the strip material factors into the bearing crush. The longer the material, the more crush. Too much crush, and you distort the bore of the connecting rod big-end while the bearing is installed. For this reason, Bert's wiki posts actual measurements.

jimmtman wrote

I would like to understand how you did the calculations to come up with the nominal clearances for H and HX. Can you give an example?

Here's a few other points that I missed the first time.

In my simple view of things the bearing thickness should be key for the clearance. ACL H from their website has a thickness of 1.998 mm. From the wiki the 702/703 have thicknesses of 1.9977 mm top and 1.9939 bottom. For ACL H the total thickness for 2 bearnings is 2*1.998 = 3.996 mm and for 702/703 it is 1.9977+1.9939 = 3.9916. So ACL H is thicker in total than 702/703 so I would think this means lower clearance. 088/089 is shown as 2.0028 mm top and 1.9977 mm btm so 4.0005 mm total. If this is right than 702/703 have more clearance than 088/089. H is slightly more clearance than the original 088/089 bearings which I think I have in my 2008 E93.

The HX thickness is given as 1.985 mm which is similar to BE's 1.9876 mm - smaller by 0.0026 mm. If did a mix of mixed set of H and HX the average thickness would be (1.998/2+1.985/2)=1.9915 mm - larger by 0.0039 mm than BE. From this HX looks closer to BE clearance than a mixed set.

It's time for that weekly reminder that the BE page only measured one set of shells. You shouldn't try to draw statistically meaningful conclusions from measuring a single set (e.g. 088/089's are thicker than 702/703's, and therefore have less clearance). Another factoid: when Bert measured those shells, he was using a ball-anvil micrometer. Today, he uses a much more accurate data-logging drop-indicator. Wouldn't it be great if Bert remeasured the shells with the drop indicator? Answer: yes, but don't hold your breath for it.

Hoping to understand how these clearances are calculated as I am confused as to how it is done. It sounds like ACL calculates the clearance some other way and often doesn't show that calcuation in their datasheets.

Thanks!

Jim

If ACL really does measure theoretical clearances differently, then they would be wrong. It's kind of like the guy on m4post who insisted that two different car accelerometer devices (VBox and Draggy) calculated slope differently.