Main bearing tolerances
Turbo Mini Forums

Hi all
Just after a bit of advice if possible. I’m still pulling the bits together for my k build and am looking at bearings. I’ve got my hard crank from SC and on measuring all the journals I get the mains as 1 thou over 2” and the big ends as 9thou under 1.75”. So my guess is that it’s had 10thou taken off the big ends but that the hardening process has meant all the journals have grown by a thou. Is that a problem will standard bearings for the mains and +10thou for the big ends be ok or will it all be a little tight?

Any reccomendation for which bearings to use while we’re at it?
Cheers
Andy

Edited by Andy500 on 11th Oct, 2020.

Depending on which of the specialists you speak to depends on the recommended clearance. There is a massive variance between what the Haynes states and some of the specialists.
Ive had to assemble bottom ends in the past and send the crank back to be polished to bottom end tolerance to get the desired clearance.
Id suggest assembling and checking the actual clearance with Plastigauge or internal micrometer and going from there. You may find a little variance between using different bearing manuufaturers.
I use this as a rule of thumb https://www.mahle-aftermarket.com/media/loc...cl77-1-205r.pdf
Also note worthy is some of the crank tolerances are .0001's so would need a micrometer to suit.
Ive used ACL race and I'll be honest the same brand but ACL Duraglide where just as good at a fraction of the cost.

Shane

Edited by shane on 11th Oct, 2020.

Thanks Shane that’s a great article as well. I’ve just measured all the journals and they come in as 2.0010 for all the mains and 1.7410” for the big ends so I’m pretty confident there within tolerance now after having a mooch on the internet as well for an A+ crank with +10 on the big ends.
Cheers

Coming back to this in a less inebriated state.
I may be wrong but I thought the crank mains tolerances were 2.0012" to 2.0017" which would mean that your two tenths undersize on the lower limit, and the big end tolerance is 1.7497" to 1.7504".
This is why I commented above about having a micrometer to suit?
A dry build for measuring is good practice to check your crank clearances and your piston to deck height, further machining may be required.
Its worthwhile checking the bearing housings in the block and con rods for ovality also.

Shane

Edited by shane on 12th Oct, 2020.

Thanks Shane, I think the actual standard mains sizes is one of is one of the problems as The HBOL doesn’t state (or at least my version up to 1993 doesn’t)the A+ sizes as different to the earlier 1275 which I think they are?

According to mini sport the standard CAM6232 main journal sizes are 2.0005 to 2.0010” meaning mine is now at the upper end of the tolerance???

Either which way I will do a dry build as suggested and use plastigauge to check clearances.

The variance of clearance within HBOL for the mini and metro A and A+ is laughable.
There is a section in the darker blue (up to P reg') BOL which mentions about the colour code/paint daub on the crank and their respective journal sizes (in metric), this lead me to wonder if the main journal housings in the block have a different size as I noticed on a friends block this week there is some similar colouration on the mains caps to his crank, I've never heard of cranks being matched to the blocks before tho? But after asking many specialists no-one either wanted to or could give any definitive sizes of the housing.
Shane

I think the bottom line is check the clearance in the block and go fromt there,

basically your aiming for 1 thou per inch of journal diameter, so 1.75 thou on the bigends, and 2 thou on the mains.

Thanks Joe.. sorry for the novice question but when you say measure clearance in the block, are you saying with it dry built and then either with a micrometer or with the crank in and with plastigauge? Either option better than the other?

You can install the bearing into the housing and torque to spec and measure with an internal micrometer then subtract the crank journal diameter to give clearance.
The issue with using a micrometer is "feel" when using them, some people are more ham fisted than others when using one which could lead to incorrect measurement.
I have had some varying results with different brands of "Plastigauge" so would suggest using the genuine.

Shane

A shane says,
as the bearing material is softish it can be tricky to measure without marking it, so I just use plasigauge ( plus I dont have an internal mic that size anyway..)

I didnt see the Mahle link shane put up, that covers what i said about clearance, although it say you can go tighter than i mentioned, Std Mains clearance specs are 0.5 to 2 thou, so im basically saying aim for bottom limit. I'd bet a pint that if you pasigauge it it will come out close to that anyway.

I think the main problem you might encounter is main bearings nipping up due to miss alignment, Ive had this and its usually due to worn mains dowels, check they are a good fit, if they arnt bin them and get a new set, the last couple of engines I built the std ones were out of stock or really pricy so I ended up using Swiftune billet ones. they are SNUG!

https://swiftune.com/parts-shop/engine-bloc...p-dowels-2.html

apart from that double check everythings clean, I always line to break the sharp edge on the block & rods & caps with a needle file so it dosent dig the back of the bearing ( litrally a couple of swipes will do) and flash over the back of the bearings with a bit of fine wet and dry to check none of the stamping is proud. Oh also when you plasigauge dont do it on dru bearings as the plasigauge will stick to them (i use a bit of wd40)

Super thanks both really is much appreciated. Joe I was planning on getting new dowels when I order the bearings so thanks for the link as well.

One last question.... when you use plastigauge and torque up the cap nuts do you still use lube on the threads I expect so but wanted to double check as you guys have done this far more than I

It's good practise to lube up the threads.
I use silicon spray on the bearing face that contacts the Plastigauge to prevent transfer and it allows clearer reading of the deformation.
Word of warning when replacing the dowels is to check the housing alignment to ensure its not canted to one side / out of round, also when fitting and removing the caps don't force or strike them as this can easily loosen or deform the dowels.
A bore gauge / DTI can be purchased cheaply to check also to compare the housing diameters.
Misalignment can cause it to nip up as Joe has mentioned.
I'll second Joes comments Re cleanliness and deburring.
Shane

Edited by shane on 15th Oct, 2020.

👍👍👍 Thanks shane

I like the silicon spray, might steal that!

It doesn’t need to be anything swanky, this is all I use;

https://www.screwfix.com/p/flomasta-silicon...ray-200ml/6699x