Main bearings clearances question.
 

So I recently brought an aluminum 930 sc3.0 to a highly reputable Porsche machine shop for a line hone. The case was something ai purchased on here last winter and it had a botched up line bore and when I installed the crankshaft with the correct main bearings I could barely rotate it by hand. However, now that it’s been corrected two main bearings have a clearance measurement at .004”. The machinist (whom I fully trust, but my OCD is making me crazy,) states that it won’t be an issue because I’m running a gr3 pump. I’ve built some high reving small block Chevys with mains measuring in the mid .003” range so I know that I’m only a few tenths higher.
Does anyone have any insights?
Thanks

Sorry, there’s a few spelling errors. I don’t have my glasses so I can barely read what I’m texting. Therefore, it’s not a Gr3 pump but a GT3 pump

Because of the multi bearing design 8 bearing on a six cylinder engine, the main bearings are rarely an issue. That said, .004" is large enough where it might cause excess oil pressure loss. I would be disappointed with this machining error.

I am aware that if I wanted to thicken the bearings I can have Calico coatings coat them, which gives me the option of decreasing the clearance by either a a full thousandth or a half.

The issue would be clamping force. Bearings are designed to have a specific amount of compression when clamped together. If you are thinking about coating the bearings to improve clamping force, you should apply the coating to the back of the bearing.
This may offer an additional concern: will dry film coating on the back of a bearing create a propensity to shuffle.


Side note: an old school trick to reduce damage do to bearing shuffle (fretting) was to put lanolin on the back side of a bearing. A silly notion but widely used by engine builder of the past.

I would not be happy with .004" oil clearance either. IMO, the sweet spot for main and rod bearings is .0020-.0030, preferably .0025". . That's enough to assure plenty of oil flow and cooling, but not enough to cause low pressure at low RPM.

Hi
As Henry says above the issue is loss of bearing crush and the risk that the shells will shuffle in the case.
You could plate the back of the shells with zinc or something harder to increase the bearing crush and reduce the clearance between the bearing and the crankshaft.

The link below discusses bearing crush, clearance and housing bore displacement.
https://enginepro.com/bearing-crush-and-vertical-clearance/
Cheers

In the olde days, it was acceptable to add shims under the bearing shells to adjust clearances. Obviously, these were very thin, on the order of .001"-.002". But engines were not as highly stressed as later designs.

To identify the actual source of the problem, I would recommend torquing the case up without the 1-7 bearings, 8 nose bearing to align case, zero a bore gauge in a mic set to 65.00mm, go and probe every bore, recording size/ovality/taper, If you are not over 65.019, your tunnel is within Porsche spec. Then you need to look at crank journal size and main bearing thickness. If it comes down to the crank being on bottom size or a bearing thickness issue, look for some Yellow gt3/blue mains or send bearings to calico and ask for high build.
If it ends up being the tunnel is over 65.019, Call your machinist.

We use to line hone to the +.19 max spec and never had any oil pressure problems, but you do loose bearings crush. I would find that the main bearings rattled and worried, we quit doing that.

Is there an option to have a non-sacrificial coating applied to the backside of bearings, on the order of .001" to make up the difference? Something like DLC instead of the usual dry film lubricant coating that more typically would be applied to the face of bearings.

Gee.. who was the highly reputable Porsche machine shop?

Read this post and figured I would suggest some things to think about for others in the future.

0.004" clearance and often more is common today even with very lightweight oil. I have seen OE'sd call out up to 0.006" and recommend 5W 50 oil. I understand that a lot of other factors come into play. In the end you have to go with what makes you confident and not what others say. At least you will sleep at night.

This is not what is the "right and only way" to do this, this is how we approach this recon work. First off, my opinion on Line honing may not be the same as others. There will be times when this is all that needs to be done to get a result.

Line honing machines came about when Steel blocks were the norm. Before engines had Aluminum blocks. I cannot remember the first engine that had an aluminum block, my first I think that I was around was the Cosworth DFV, in the 70's.

Finish surfacing by Line boring in steel blocks was never the best so the final few thousands was removed by honing. The honing bar will always follow the hole its in. It will not straighten the bore just remove where the hone stones touch. We get a far better finish today on steel blocks and aluminum blocks with just line boring. Tooling has come along way. Still, steel blocks in my opinion should be finished sized by honing.

The engine in the above post, the machine shop may have a different opinion on honing. We all have our ways. As long as the result is the same. Maybe the +0.004" is a result of the hone removing material just where it touched as the first line bore was not straight.

There are many ways to check this. Every shop will have their own way. It can be done on the Line boring machine with a indicator fixed to the bar. As long as the bar is straight and the hanger bushings are accurate. Or it can be done with a ground sleeve before to establish the case halves are aligned and measured quickly with feeler gages. This gives a fast alignment to tell if it is indeed not straight and need boring.

The housing can be round and on size but misaligned some. Mag case especially are often like this. We also use this alignment sleeve to align the two halves and fit a dowel in the front lower of the case so the case alignment is correct when boring the case.

We have these sleeves in nominal 62.00mm and 65.00mm sizes along with short ones and countershaft alignment sleeves. The countershaft sleeves become very important when the line housing is off enough where the sleeves cannot be fitted. The countershaft bore is then used as an alignment.

As I said, this is how we do this work maybe not how others do it. This is how I was trained to do this work. I was privileged to work for some of the very best and in F1 you did everything yourself. There were no shops that you sent the work out to have done. You were trained as an engine person which involved all machining and assembly work. Today, there are not many who have had the same opportunity to learn both sets of skills. I remember my mentor in F1 telling me, the best engine builder is a person with both sets of skills.

The biggest percentage of assembling an engine is knowing what is right and what is not. Today, much of the critical parts of these engines is done by outside vendors and the assembler sees none of it.

Remember, "there are two types involved in this business. Those that see it wrong and can fix it and those that do not see it at all".

Hope this is of use to anyone about to have this work done. An understanding of what is involved can help when needing this recon work done.
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