Race Engine Forum UK

Just was wondering what the ultimate overbore of Fiat/Lancia DOHC blocks are. I was looking at my first edition of Guy's book last night and it seems that the block is limited by bore centers, case studies never quite getting beyond 2.1-2.2 liter range. Am I correct in thinking that you can not go beyond an incremental bore.

For context at 2.0l, Abarth's engines were at 88x80 BxS, while the Fiat block was at 84x90.

Keith Walker

Bore centers yes but most important is gasket fire ring diameter.

All current gaskets are 85mm - one can tolerate a modest amount of overlap but the pistons must be set up so as not to hit the overhanding fire ring and that overlap reduces the contact area of the fire ring (the bit that has to seal against the combustion). 86mm is the practical limit really.

G

What if you have custom gaskets made up?

well, sure and Goetze used to make 86mm bore ones for Abarth. They don't now although Spesso probably do - if you order £2000 worth.

G

I can order custom made gaskets from a specialist shop in the Netherlands. Not exactly cheap, but I can get a set of two for my 3.1 bored Alfa V6 for substantially less than that £2000.

Any idea what kind of bore you could go for safely in the Fiat DOHC (naturally aspirated)? There must be a limit where the cylinder walls get too thin or the width of the fire rings in the gasket will be too small.

Spesso have a large min order qty hence the £2000 or so it was last time I ever rang them.

As I said above (and it's all I want to say) ... 86mm

Can you just take my word for it? I've done so many times. There will always be a sound engineering justification for the things I cite but that doesn't always mean I want to sit here and slavishly write about it.

G

I didn't need a dissertation, just a quick answer to a quick question, thanks Guy!

In Finland, where we have this cheap auto class called jokamiesluokka (i don't know the name in English, sorry), almost every TC is bored to 85,5 mm to apply Supertech big dome pistons. They use standard headgasket and make a chamfer in to pistons to avoid hitting fire ring.