Tuning 650 Skimming the head

[Vikingsen]

I bought a old 650 engine and gearbox, the plan is to restore it and make it run as powerful it can, without going crazy on tuning. Aka buying new tuning-parts. My plan is to have the head skimmed and put in and 0,5mm kobber gasket.
My question: Is there a "sweet spot" for head skimming, how many mm. should be grinded off?

 

[dirkdeman]

I wouldn't go too crazy, especially with a 0,5mm gasket (that's already (0,7mm thinner than the stock gasket!). You can remove 1 to 1,2mm of material from the head, but this also erases the gas ring (in case of a head gasket failure this ensures that exhaust fumes are vented into the engine bay through a hollow bolt, instead of exiting in the cabin which is dangerous!) Also, I feel that raising the compression alone won't make a huge difference. Tuning these engines is more of a package deal: a bigger carb, sports exhaust, more agressive camshaft, some mild porting and, yes, raising the compression slightly.

The 650cc engine isn't quick, but I don't find it incredibly lacking. If you're budget constrained I would opt for a nice revision (honing, skimming, new set of pistons, new seals/rubbers) which is not too hard to do it yourself (except for skimming/honing of course). With a nice, refreshed engine the Fiat will have no trouble in city traffic!

 

[Mike1alike]

how many mm. should be grinded off?

Not 1mm! The purpose of grinding is to take off the stucked parts of the old gasket. To clean the surface. And to make it even, of course, but there shouldn't be any curved zones. So it's best to start with numbers like 0.1, 0.2 mm on the grinder machine if you put it on those. And see how it cleans up. Ideally, you don't take anything out of the cylinder head itself, just the 'dirt' stucked on it.

 

[Vikingsen]

I wouldn't go too crazy, especially with a 0,5mm gasket (that's already (0,7mm thinner than the stock gasket!). You can remove 1 to 1,2mm of material from the head, but this also erases the gas ring (in case of a head gasket failure this ensures that exhaust fumes are vented into the engine bay through a hollow bolt, instead of exiting in the cabin which is dangerous!) Also, I feel that raising the compression alone won't make a huge difference. Tuning these engines is more of a package deal: a bigger carb, sports exhaust, more agressive camshaft, some mild porting and, yes, raising the compression slightly.

The 650cc engine isn't quick, but I don't find it incredibly lacking. If you're budget constrained I would opt for a nice revision (honing, skimming, new set of pistons, new seals/rubbers) which is not too hard to do it yourself (except for skimming/honing of course). With a nice, refreshed engine the Fiat will have no trouble in city traffic!

Thanks. I have no plan to grind the gas ring away. I’m really not on a budget—I’ll buy what I need. It’s more about the urge to make a 650 run as well and as powerfully as it can without tuning it to the extreme.

I will definitely do some porting and polishing, and I already have a sport exhaust. I’m not sure I’ll use it, though. For me, it’s about the driving experience and the sound of the engine is a part off that, where loud doesn’t automatically mean better.

This engine rebuild is for my Autobianchi cabriolet. I just want a better driving experience and not feel like I have to run at full RPM on a small 500 cc engine just to keep up with traffic.