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Recomended piston to wall clearance

fastjeff

fastjeff

Gold Medal Contributor
Having an early triple bored out .030 over and need to tell my boring dude what clearance I want.

Looked in my genuine Merc manual and couldn't find it. Anyone have a spec?

Thanks,

Jeff
 
All the triples was 2.875 bore so add .030 to it and thats your finished bore size......….2.905 for cast piston and .002 more for Seizeco piston
 
You should have your pistons in hand before boring and honing the cylinders. The piston should be numbered and the bores machined to the piston sizes. Pistons are like snowflakes, no two are exactly the same.
 
I have measured numerous O/S outboard pistons.----------Modern factory makes parts " all the same "----As stated bore to the o/s of the piston.
 
.030 " clearance! That sound way too much. We used to set our street motors at about .007 or so.

Surprising that I couldn't find that dimension in the Merc atalog.

Jeff
 
I just did. (Duh! Mondays...)

Obviously, the idea is to take the recommended stock bore and add .030. That's not the way I'm used to having a block bored, however. The machinist usually takes the piston's OD and the widest point and adds 'X' thousands to the bore.

Jeff
 
This sort of thing is now not done the way it was 40 years ago.-----Computers and factory tools have changed / improved almost every year.
 
Clearance is .003 to be technical as piston is widest at shirt.....2.872. .....2.872(skirt) - 2.875 (finish size)= .003..... plus all this is in factory manual
 
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