Piston Pin to Small end bearing clearance question
 

Hi,

I am measuring clearances before assembly and I'm getting a little confused with the small end clearances for a '78sc 3.0l motor and hope get it clarified.

Wayne's book (pg 205) states:

Rod bushing / Piston Pin Diameter 22.020 - 22.033mm and;
Piston pin to bushing clearance 0.020 - 0.037mm with wear limit 0.055mm

What I am getting:

Rods - Pauter: small-end bush diameter 22.01164 (0.8666" from spec sheet)

Piston Pin 22.0025mm measured

Clearance .00914mm

Also the Pauter rods have less width in the small end i.e. 22.86mm compared to the 24.8 - 25.0mm in the book. Is this correct?


Cheers

Gert

Typically the rod bushings are honed to the correct clearance for the pins. I suspect these need to be honed. I'd send them to a capable machine shop for evaluation/honing.

-Andy

That is correct. It should be .01mm or a little less than .0005" inch. If it is a .001" inch you will hear the wrist pin knock.

Cool, thanks for the confirmation. I will have it measured to make doubly sure.

Cheers

Gert

pin fit
 

For performance/race clear. we set them up .0006- .0008 in. on a 22/23mm pin, for street use .0004 -.0005 in. any tighter and there is little room for lubrication. the same goes for the fit in the piston. the other important part of this is to have it checked on a proper fixture like a Sunnen rod machine where the total travel of the dial guage is less than .010, (not something that can be checked with a snap guage and a V. caliper)
Mike Bruns JBRacing.com

Mike, thanks for the explanation, I will call you to discuss further .

Cheers

Gert

i like a light finger push to move the pin in the bushing. not a hard push, and not so loose that it falls out with it's own weight.

John,

This is where its at for the small end bush, the pin fit to the piston is a little tighter and requires more effort to move the pin in the bore.

a semi hard push to move the pin into the piston is good. nice when you can still do it with your thumb or just light taps on your pin install tool. i like a slightly firm pin bushing fit in the rod when new, because the microscopic roughness from the honing job gets smoothed out after running and clearances increase a tad bit.

Thanks for all the inputs,

As future data point for someone using the same combination (Wossner pistons / Pauter rods), both companies tech reps indicated between 0.0006 - 0.0008" for both piston and small-end bore to wrist pin clearance.

I will go middle of the road .0007" on both.

Cheers

Gert