Thanks for the replies all - appreciated. Al, worship::worship I'll run through your points one by one if I may as we agree on much. I have been lying abed running the facts through the old grey cells and have a few points of note. I noticed the proximity of the marks to stud location and did wonder, but, have to discount this as I had the stud holes enlarged very slightly and I have a 1mm steel support plate on the spigot to help with location, strengthen the joint and give a better piston to barrel height and C/R. Not sure about the Piston 'expansion-control' all I know is they were very expensive and direct from a top Italian maker!! My engine man is an old hand and was happy with the piston/bore clearance after rebore, he will be checking the pistons/rings and bores anyway. I'm only happy that the damn thing didn't completely seize and grenade on me, at least the situation is recoverable He was happy with the plugs and the colour of the same, he considers them correct for the application.
NOW :worship::worship: much respect Al, as you have touched on something that I am wondering about and that is the cooling system. On my engine the fan housing is from a 500 modified to fit onto a Panda 30 head on the left side (viewed from the rear) using an alloy fan. The engine fins are all in place, BUT the side plates (front and back) are missing and I wonder if this is causing excessive heat A standard Panda 30 thermostat housing is fitted to the head on the right. I do have a modified 500 thermostat housing that will be fitted post rebuild - this does have the side plates. Peter, the pistons are in the correct way. I'll leave it all with my man and get back with info when available
Ian.
Ian,
Your kind words of praise are making me blush like I haven't done since I was a teenager (a very loonng time ago!). I'm just throwing out ideas for you to consider and maybe discuss with your engine man.
I'm not sure what can be done to improve/uprate the cooling system on an air-cooled Fiat engine.
On a modified water or should I say liquid-cooled engine, it's easy to monitor coolant temperature via a gauge and make changes if excessive temp. becomes a problem, e.g. larger radiator, electric fan, increase system pressure, higher-flow water pump, different coolant etc.
On an air-cooled engine, it's not so easy. In the '70's? Suzuki introduced 'Ram-Air' cooling, basically a slanted cover above the cylinder head fins supposed to scoop and force extra air over the fins and supposedly improve cooling.
On your Fiat engine I suppose it comes down to ensuring that all cowling,
side-plates and baffles are in place to ensure that air is being directed to where it's needed.
I've sometimes wondered about engines that have been converted from a dynamo to alternator and the pulley size has been reduced thereby rotating the generator and cooling fan faster for a given engine speed. I know very little about fan design but wonder if a point is reached where no increase in air movement is achieved beyond a certain speed, similar to what I believe can happen in fluid pumps (air is also a fluid). Perhaps some other members have some knowledge of this subject and could advise us.
Maybe there's a more effective design of fan or fan blades available or a way to get more air in and more air out of the cowling.
I wonder also, what previous tuners (Abarth, etc.) of Fiat air-cooled engines may have done to improve cooling. Maybe check what baffles, refinements etc. are used on other makes of air-cooled car engines - V.W., N.S.U., Citroen
for possible ideas.
What is a little puzzling is that your engine ran quite well for some time after the rebuild. I trust you didn't simply give it too much welly too soon. I'm sure you're too experienced to have done this!
As others have said, maybe recheck that the ignition timing is correct on both cylinders. As you no doubt know, retarded ign. timing can cause overheating...
I don't think that the fuel mixture/carb is the problem for the same reason that you stated plus only one cylinder was affected. The difference in appearance between the two cylinder combustion chambers/piston tops might be explained by one cylinder running hotter than the other. It also suggests it had been doing it for some time. You're right in saying that running a richer mixture should reduce running temperature, lawnmowers often use this trick especially side-valve ones, remember them?
Best of luck with the (next) rebuild!
Al.