630 Piston Skirt Clearances

[GlenT]

I just got done going through two sets of honing stones, #100 grit and #240 grit, trying to get more clearance. One of the previous owners had the cylinders bored to .045 and put in new pistons, rings, and rod bearings. The new piston skirts were scored a little on the top (the bottom was okay, I suppose the top ran drier). Anyway, I bought some NOS .045 high-altitude pistons and rings and have been trying to get more clearance so the NOS pistons don"t get scored.

Before honing, the clearance from the bottom of the skirt at right angles to the wrist pin to the cylinder wall (I put the new pistons in upside down and used a feeler gauge) was about .004". After honing, I am at .005 to .006 depending where I am horizontally in the cylinder with the piston. I also put the piston in the correct orientation and measured the skirt at the bottom of the cylinder.

A machinist recommended I go to about .008" clearance, I am at .005 to .006, and I&T says (P. 23) that the "Piston skirt clearance at bottom of skirt right angles to piston pin should be .0046-.0070). So, should I buy some more honing stones or are my clearances enough? I guess I am really asking if the I&T clearances are enough. I know on this forum people say the old 2-cylinders like clearances.

There is some pitting in cylinder #2 just below the 3rd compression ring at TDC about the size of a dime that looks black but the indentation can"t be more than a couple thousandths. It has decreased in size with honing of course. Should I be concerned?

I use the tractor for logging and hauling firewood in the 45 loader, splitting firewood (hydraulics), discing a large garden, and plowing snow. A total of maybe 60 hours a year. I"d like the overhaul to outlast me (I"m 65).

Thanks once again!

 

[RobMD]

Hmm... I'd say 7 thousandths would be a fine number. Sure, Putt putts like clearances, but who wants piston slop? Rings would last a little longer, I suppose if you were right around 7 thou. Better to have 7 thou than 3 or 4 thou.

Those are iron pistons, aren't they? I would think 7 thou would be dead on. Somewhere I heard that putt putts like a tapered bore out towards the bottom.

 

[GlenT]

Rob, the pistons are aluminum.

 

[RobMD]

I'd definitely have the bore 7-8 thou out then... those aluminum pistons expand faster than the cast bores.

 

[Anonymous-0]

New pistons they said .010 clearance for 620 jd .

 

[jd b puller]

Glen, not sure what you're honing with but you better check to make sure your bore doesn't look like an hourglass. You should get ahold of a bore gauge and check it. 2 bangers are pretty forgiving, but if you're going to this much trouble...

 

[GlenT]

620, where does it say .010? Thanks

 

[Anonymous-0]

Bought new pistons from the green magizane add that is there all the time . He told me the piston manufactor called for .010 due to the fact that the pistons are made from modern material . So that what I had my block sized to for .045 pistons .They were suposed to be a m&w copy . Bought they from Jerry Henisch 450 $ .010 clearance between piston and cyclinder wall .