Piston Skirt Clearance

'71 750. .020" over. Engine ran like crap, anemic, inconsistent. When I disassembled it, there were there were seizure marks on both pistons. Mostly around the wrist pin. I have always called this type of seizure "cold seizure." It is the result of poor break-in, not letting it warm up before pinning the throttle or simply poor piston clearance (or all of the above).

The bore was actually still in good shape with no scratches, no metal transfer. I measured the piston skirt clearance....a .002" feeler gauge slides smoothly, a .004" could BARELY be coaxed in there but was REALLY tight.

I am going to be using garden variety EMGO replacement pistons. At first glance, opinions about piston skirt clearance appear inconsistent....so I'll just say it: I am going to hone the bore to .005" piston skirt clearance. Anybody see any problems with this?

In my book, that is excessive. 1.5 to 2 thou. Make sure the piston pin is a good fit, oiled, it should be a nice slide oiled.
Not fall through and hit the table.

My new Hepolite + 0.020 850 pistons/rings came with instructions for a 4 thou clearance.
The machine shop did this and I checked and clearance was 4 thou.

When I checked the ring gaps, they were all towards the tighter side of spec but within spec except for one ring that was
9 thou gap instead of 10.

I am not sure if Robert is saying your 5 thou is excessive and should be 1.5 to 2 thou clearance, or he is saying excessive by 1.5 to 2 thou (meaning he suggests 3 thou to 3.5 thou clearance.)

My 850 was my first air cooled engine rebuild and it was all learning for me. But my gut feel is that clearance under 4 thou is too tight. But then there
is the material the piston is made from and its expansion characteristics
Dennis

5 thou will be fine, but if the seizure marks are in the right place to count as 4 corner seizure then also check your jetting too and fuel feed to the carbs.

In the factory manual fresh fit clearance is 4 to 5 thou with a wear tolerance of another 5 thou. Considering the low miles we now all do eating up an extra thou of wear is not an issue.

Four corner seizure
Symptom – The piston has vertical seizure marks at four equally spaced points around the circumference.

Cause – A four corner seizure is caused when the piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced.

Reason – The main reasons for this problem are too quick warm-up, too lean carb jetting (main jet), or a spark plug with too high a heat setting being used

Emgo pistons are made by JCC and are very close copies in sizing and materials to Norton OEM. Their rings are suspect so ask whose rings are fitted.