I understand this is a bit of a cheat to raise compression and some race cars have had this done despite it being against the rules. I've got access to a lathe and ally mig kit, can I just fill it with weld and turn it to a nice smooth finish, or is there quite a lot more to it.

I would have thought that knowing exactly how much weld has been added would be important to know what the new CR would be. I can think of 2 ways of doing this, weighing the piston before and after, or cutting an exact amount of mig wire from the reel, and then welding until it runs out. I would think the first thing to do is work out what CR you want and what volume of alu needs to be added. Plus any calculations would be screwed up if you get enough penetration for the weld to protrude on the underside of the crown.

My rough plan would be to get both pistons dimensionally identical. Once they're the same size I'd adjust the weight of them by making the heaviest one lighter by removing material from underneath. Without sounding like a right numpty, what makes one side of a standard piston the front? I've never really looked closely at them.

Or I could just either buy some or put up with that's there already.

Um you mean the arrow on the crown that points to the exhaust port? I assume the valve cut out is sightly larger on the inlet side to match the larger valve head. I'll just go and look...

Yep, very slight difference in shape between the front and back of the crown.

Russell,
the gudgeon pin is offset slightly, to counter the effect of the piston 'tipping' at TDC.
Get the arrow the wrong way round and the rattling can be rather annoying...

As regards the CR, if you start with 9.0:1 pistons and shorten the barrels, you'd probably need to use 98 octane and also retard the ignition from its ideal setting.
Raising the CR by too much can be counter-productive...

ken

Why don't you shorten the barrels

(in your Lathe) instant more compression??????

Or Am I wrong?????

I will shorten the barrels anyway, I've just heard of this being done on race cars and wondered if it was easy ish to do and worthwhile. I don't mind using 98 octane and so on, although the more I think about the accuracy required I probably won't bother...

Russell,
dig out the plasticine and check that squish band...
http://www.chevyhiperformance.com/tech/ ... nce_guide/

ken

not forgetting how much distortion may occur to the piston

you could volume the crown by the "ureka" method of measuring how much the level of a liquid spills out the pint glass when the piston is dipped untill the bar that was through the gudgeon pin hole rested on top of the lip of the glass - or something like that...