Advice on Gudgeon Pins required ?

Why not ask Thunder Engineering?

The clearance issue could be with the pistons or the Rods.  I have heard of these problems before. A tiny difference in the tolerances most likely. I had a similar issue when fitting used 99 pins into new HEP 650 pistons. Overcame it by polishing the pins. Worked well for the last 25+years.      Everything is made with tolerances but it can work against you.  I was once a very young junior surveyor sent to a Major steel framed Hospital project. I soon determined (with my trusty100ft steel tape) that the building works would grind to a stop due to the steel being too short.by a few mm. The top brass all laughed at me and said the steel was within tolerance. It was. But all on the minus side. It ground to a halt and thousands was spent making new wall modules. Did I get any thanks?,or even a pat on the back?.

I had the same problem, a new heptolite piston fitted fine but the pin for a new Gandini and JP would not, I bought a hand reamer in the end and reamed it slightly. The new pistons seem to have fractionally bigger pins 

The gudgeon pins should be freely floating in the small end of the conrod.  My original Hepolite pistons gave the correct clearance in the Thunder rods.  It's likely that the gudgeon pins for the new pistons would be OK in high-mileage old rods - the eyes would have worn a little.

Paul

Some engines are designed with the conrod gripping the Gudgeon pin and the ends of this pin turning in the piston bosses.

Obviously the best solution would be to get the rods re-sized although this may make for a looser fit next time you change makes of piston !. If the motor is in the frame I would be tempted to re-size the pin after taping off the area that works in the piston.It would still be a drive fit to get assembled but then would work fine. Another BOB bodge!, but it would work .We are only talking of half a thou or so.