Piston-Cylinder tolerances

[cvlngineer]

The manual I have does not mention anything (that I have found) about the tolerance of the clearance between the piston and the cylinder wall. Could someone please post this information?<br /><br />The motor is a 1975 Johnson 70 hp (inline 3 cylinder engine)<br /><br />Thanks,<br />cvlngineer

 

[G DANE]

Re: Piston-Cylinder tolerances

The clearance always seems chocking to mee - especially on the pre 1987 models. I guess the pistons must expand a lot. If your compression was OK before - dont worry. The big clearance is the reason why these models rattle some in iddle specially when cold. A mechanic used to automotive motors wont beleive it !!

 

[papasage]

Re: Piston-Cylinder tolerances

i have a 50 hp 1974 when i got a new piston for it it wouldnot go in the cylinder untill i put oil on it without the rings on it.it has run now for about eigh years.

 

[Hooty]

Re: Piston-Cylinder tolerances

I can't believe your manual doesn't have it. Which manual do you have??<br /><br />c/6<br /><br />Hooty

 

[Dhadley]

Re: Piston-Cylinder tolerances

Max cylinder wear is .004". I would bet youre close if not past that now if its still standard bore.<br /><br />Cam ground tapered pistons are confusing to measure at best even with good equipment. Use a dial bore gauge to have the cylinders measured. That will tell you all you need to know.<br /><br />Good luck!

 

[Walker]

Re: Piston-Cylinder tolerances

My book says .0030 but if I remember right Wiseco calls for .0040 for their pistons.

 

[cvlngineer]

Re: Piston-Cylinder tolerances

Thanks for the information folks. I will post the measurements that I took in a while.<br /><br />cvlngineer

 

[Forktail]

Re: Piston-Cylinder tolerances

It all depends on the piston. Forged pistons usually require more clearance than cast pistons because they expand more when heated.<br /><br />Aftermarket piston makers will include the clearance specifications along with the pistons.

 

[ledgefinder]

Re: Piston-Cylinder tolerances

Seloc manual has piston-cylinder clearance for 1975-78 70hp at 0.0055" max, 0.0035" min. Others on this thread have way more experience than me with this issue. I am only posting because some of the max clearances listed seem HUGE - for example, 0.0075" max, 0.0055" min for the V4s from 1978 on. These clearances feel huge to me, also, when I wiggle a piston top. <br /><br />BTW, the Seloc numbers check with the OMC factory manual for the two cases where I have the manuals, so as far as I can tell they may well be correct.