Hi All
This may be of interest to those restoring 4F OHC machines. The alloy pickup areas on the original pistons from my project bike are very clear, and underpins Otto's approach of relieving the pistons in a thumbprint sized area below the bottom ring. Quite a swage of material.
There's a lot of skirt area wasted there. and the rod is slightly bent. Also with a solid skirt piston. Less ovality and a bit of extra taper would have helped there.. Gets more heat out of the Piston with better (more surface) contact. Often once a piston has grabbed, the area shrinks where that has happened. Filing pistons is an art and you need to check where the actual contact is occurring and what clearance you have at various Points. Nev
It could be a defective piston, but it might also be an installation error. The 4F engine actually has a slight V-configuration; the pistons have their wrist pins offset slightly to one side. If you measure the piston, you will find that the pin bore is slightly off-center. This design allows the piston to transition more smoothly at Top Dead Center. Simply rotating the piston 180 degrees might be enough to stop the seizing.
Several years ago I did some research into pistons with offset small ends, because I was considering using some GPM pistons with offset pins, in my Mk1.
What I discovered was that the offset tends to push the piston flat against the bore on the up stroke, resulting in slightly better heat transfer and reduced piston slap as it goes over TDC.
You can check if your pistons have this offset by measuring with a caliper. If you find that the measurement from the pin hole to one side of the piston is different to the other side; mark an arrow on the piston crown pointing from the centre to the side with the shorter measurement; this is the thrust side of the piston, and should be in contact with the "major thrust side" of the cylinder bore.
Of course I may have misunderstood everything that I read. I would advise that you search; major thrust side of a cylinder bore; and come to your own conclusions.
Here is drawing I made at the time.
Interesting photos, Cam. The pickup being so localised, and sitting low on the skirt like that, does point to a clearance or contact-pattern problem rather than the whole piston being too tight.
On the offset pin question, worth checking before you fit anything: caliper the pin bore to each side of the skirt, as Chris described, and mark the crown so you know which way round it goes. If the original pistons were fitted 180 degrees out, the loading would shift to the wrong side of the skirt, and that could give exactly this sort of local grab.
I'd also do what Nev says and blue the skirt up and turn it in the bore by hand, so you can see where it's actually bearing before deciding how much to relieve. Then check the con rod for straightness, since a slight bend would load one edge of the skirt and produce the same pattern.
