Main bearing clearance...

[Chris01573]

Hi All, installed the crank today in my engine. Using plastigauge I am seeing a bearing clearance on the main bearings of .00325 I am thinking this is too much clearance with .001 per 1" being the norm I was looking for around .002 ideally. Any engine experts able to advise ?

[flying farmer]

Does sound a bit on the large side to me.

Is it a used crank or one that has been reground?

To be honest I have never actually measured the clearance on any of the engines I have rebuilt, I have only measured the journal diameter and if that was to spec I have just bolted it together and not had a problem.
However, I have never had a used crank that didn't need regrinding in any of the engines I have bought.

The two engines that I built myself with properly hardened cranks have been ok, even the one that had been ground 60thou under on the mains. I stripped that one after HGF at 70,000 miles as I was fitting it to another car, both the crank and bearings were perfect.
Neil

[Chris01573]

Hello Neil. its a crank that has been reground to -0.020 with King shells to suit.

[Chris01573]

So putting a micrometer on the Journals they are all 2.105 so based on the ROM that means they have been ground down by 0.0215 that seems to correlate with the results from the plastigauge. So basically 1.5thou too much has been ground off them I think ?

[flying farmer]

What is the journal diameter in the ROM?

I have googled it and it says standard is 2.125, yet my Haynes manual says 2.126 so there is a 1 thou discrepancy between those two.

FWIW I have just measured the mains on the crank that came from the stripped down engine that used to be in my TR. That measures 2.115 and they have 10 thou undersize shells that I am probably going to reuse as they are a set of original early type lead faced bearings. I bought those from an autojumble some years ago.

I have no idea what clearance they were running but it never suffered from low oil pressure at idle and they have about 20,000 miles total use with about 1000 miles of that covered on track days.

I bought that crank as a spare from a seller at the Stag national day some 20 odd years ago as it was only 10 thou undersize and the one in my first Stag engine was 60 thou under on the mains.

I do wonder as there are two different figures floating about, that there may be a good possibility of incorrect size regrinds.

If it was an exchange crank I would send it back and try and get a correct sized regrind if the standard figure is 2.126

Neil

[jbuckl]

If you built the engine and the oil pressure was 35 psi cold and 25 psi hot at idle it would be ok.

If that wasn’t what you wanted to see, it’s best to attend to the clearances now, before the build.

Dont just build it and then change the PRV spring to up the pressure…. Thats the non preferred bodge…. Which will most likley cause premature wear on the cam drive system.

[Chris01573]

Hi chaps thanks for the advice, crank is with machine shop being reground to -.030 better safe than sorry. Moral of the story check tolerances more thoroughly !!!

[Chris01573]

So what is the collective view on clearances for the mains and then for the big ends ? I am thinking 0.0015 on the mains and 0.0020 on the big ends based on what little info I can find

[jbuckl]

FWIW clearances must be referred to as diametrical (1x clearance) or radial (2 x clearance) does that make sense?

diametrical is journal, shell, oil thickness.

Radial is 1/2 journal, shell & 2x oil thickness.


afaiaa 1 thou (0.001”) is good for all the way around / whole diameter oil film thickness.

however, I have found that some grinders target their grinds to be 0.0015 oil film .

often their rebores are similarly over done.

Upon quizzing them I got : - yep, we go a bit further, because they are old engines….

this doesn’t make sense to me.

[Chris01573]

Thanks Julian so aim for diametrical .001 equivalent to radial .002