K-Series - Piston to bore specs seem way off.

[Fordiesel69 271]

See attached pic.

 

The book says style "a" pistons should be .007 to .010 which in my opinion seems very high.  The last 12HP I had bored to 10 over, the shop left the clearance at .003 and it worked fine.  I just took a measurement of my newest project engine and I have a .007" bore to piston space and it is already a 10 over.  In my opinion it feels ok, in which the play is enough I would like to have it bored, but could leave it if it would save money.

 

Opinions on what the best clearance should be using a style "a".

[MarkPalmer 81]

As long as the meausrement is within spec, you'll be fine.  Clearance at .003 is a bit tight, this clearance is room for the piston to expand when it gets hot. How did you measure the piston to bore clearance?   

 

-Mark-

[Fordiesel69 271]

Harbor freight snap guages, with a friends digital caliper.  I know micrometers are better but the caliper is decent enough.

 

So you think that much clearance is OK enough to not make noise?

[MarkPalmer 81]

It sounds fine, as long as it wasn't measured with feeler gauges as so many often do.  At the lower end of the tolerance, it shouldn't have any piston slap.  It's a judgement call but if the bore looks good, measures within tolerance and isn't all out of round, and if the piston to bore clearance is OK and new ring end gaps are under .020, I would probably forego getting it bored over again until next time around.  The last (and only) K361 I ever heard running at a show was kind of noisy just by its OHV design, you could definitely hear the ticking as opposed to the regular L head K engines. 

 

-Mark-

[WH nut 553]

I would call it good.