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What should a brand new forged crank measure? I'm finding the journals nearly a thou up, and only have a thou clearance on the thrusts.
So, will I get the rods and mains caps honed, or the crank polished?

You have not mentioned what crank or the sizes you are getting. You would expect mains to measure 2.001 rods can be 1.6259 give or take. You can not polish a crank and remove material evenly. You can't grind .001 off either. You still have bearing thickness and housing bore to play with. What type engine are you building? How will it be used, RPM range? Most of what I have built over the years have been racers. Always turning over 8000 RPM. Now I'm building slow turning torque units to pull highway gears. The A+ blocks are line honed after line bore leaving a straight hole and very nice finish. I'm working on closing up clearance and going to thinner oils. If your machine shop has the tools (and should) measure line bore, use ball mike to check bearing shells and mike shaft then do the math. Working 25 years at a machine shop I never sold plasti gauge. It can confuse someone not knowing how to use it. Find out what your clearance is then figure what next. I would dress the soft face of your thrust maybe .003 for street more for race depending on RPM to avoid pinching the thrust as crank flexes. Steve (CTR)

Sorry, lack of information.
It's for competition use, 1.625" s sized big ends, mahle bearing shells. I tried to do a dry build to check piston deck height and ended up with witness marks on a few shells. The rods are not new and cannot for the life of me see anything or feel anything that could pinch the shells.
Obviously I oiled the shells before setting the crank in, but not the back of the shells.

Not looking good for Blyton here, a huge compression problem has reared it's head too

Oh, I thought I would need to dress the back of the thrusts so that the bearing surface wouldn't be contaminated?
Would it be feasible to put new thrusts on the clutch side but used ones on the other side? I have straight cut drops so surely the crank won't be subject to so many side loadings.

wantafaster1 wrote:Oh, I thought I would need to dress the back of the thrusts so that the bearing surface wouldn't be contaminated?
Would it be feasible to put new thrusts on the clutch side but used ones on the other side? I have straight cut drops so surely the crank won't be subject to so many side loadings.

I have done that without problems. Perhaps not ideally by the book, but practical.

Some of the custom crank manufacturers do leave the cranks a little oversize, so you can have it sized to your exact requirements, though, usually it's more like 0.005" over.

I'd suggest giving what it sounds like you want to do with it, giving it more rather than less clearance.

CooperTune wrote: You can't grind .001 off either.

On a Crank Grinding Machine, my word you can. Our Crank Man can take as little as 0.0001" off, but feels more comfortable working down to 0.0002" at the low end. Mind you, there are even better machines than this old one around these days.

Googling for technical info here, but cannot find what the conrod bore or mains housing bore should measure. Interestingly the tolerance on cranks is plus, so 1.6258 and 2.0007 is correct actually.

Sorry, I should have added there to let your crank grinder man know if you are working to A or later A+ sizes. They are different.

For A Series,

the Main Tunnels are 2.1460" to 2.1465"

the Con Rod Tunnels (small journal) are 1.7705" to 1.7710"

Great information, many thanks. Yes its pre A plus.

All the tunnels and rods checked out fine.

The centre main strap is at fault, its not flat. Another thing with the strap is the bolts are squashing into it, so I can't see how it will be properly torqued down?

Sorry should have been more clear. I don't know that you can take a crank off the shelf chuck it up in the machine and remove .001 . While in the machine the operator can do anything he knows how to. Grinding .009 or .011 is not the problem. Sometimes stresses from grinding or heat treating will make a shaft less than true. If you have someone who can go from floor to .001 you are lucky. Stroke would be a lot of fun also. Shop I retired from sold their big grinder and is looking to get out of crank grinding all together. Sending it all out rather than pay a grinder to stand there all day. Steve (CTR)

Shop I retired from sold their big grinder and is looking to get out of crank grinding all together. Sending it all out rather than pay a grinder to stand there all day.

This seems to be the norm for many experienced, small engine shops everywhere.

wantafaster1 wrote:The centre main strap is at fault, its not flat.
Another thing with the strap is the bolts are squashing into it,
so I can't see how it will be properly torqued down?

I once had the opposite problem...

Many years ago I paid "Avonbar Racing" of Weighbridge, Surrey,
to do me a cap and strap. It looked like they'd spun the cap on
a lathe and it ended up with a 4thou dip in the middle. So instead
of providing EXTRA support, it was probably worse than the standard
offering.

This got fixed on a surface grinder, the strap and the cap...
Losing a few thou to clean up where the bolts go should not be
a problem either...

My Dad's "trick" was to grind everything flat, then preload the
assembly by about a thou by inserting a "Rizla" in the middle
between the strap and the cap.

Ian