An idea I had earlier. (Squish and short comp pistons)
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Ok, I have buggered about grinding my own heads and it wasn't really in the ballpark I wanted, I looked at decompression spacers, didn't like the idea of 2 head gaskets but reckoned it was a good idea to match the bores of the gasket to the piston bowls, thus retaining squish. So, I have a 1380 block and I'm looking to drop the compression ratio. Due to principles rather than finances now, I'd rather run cheap(er) pistons and spend the money elsewhere. The whole head gasket thing worries me and on a 73.5mm bore, I'd rather have some more meat. So, whilst I'm at it, why not go a weeny bit further and use Simca 1200 pistons with the short height? That's got to be a bad idea as a 74mm bore has to be worse. I like the idea of running more boost, hey, if you're thinking about it, you may as well try and get a bit more. So I was sat out on the doorstep and it came to me.

I'd been thinking about it the wrong way, I wondered if somehow I could get the block bored 73.5mm from the bottom and leave some more meat at the top (unbored), this would necessitate having hacked up pistons and probably an angry machinist, not to mention things like not using a torque plate for the head, well I suppose you could bolt it on anyway.

So, getting some short comp height 73.5mm pistons (6mm short?) getting the machinist to bore down about 5.5mm a bit wider, say about 20thou over 73.5. So, that would make space for some "rings" which would be accurately machined a gnats willy over 73.5mm +20, these would interference fit into the grooves (after being put in dry ice for a bit) and the inners would be matched to the piston bowl/head. So, I got thinking a bit more and wondered whether this was beyond the capabilities of my shitty old lathe. Probably so. I gave it a bit more of a think and wondered about which materials to use. Basically fit these things, get the blocked skimmed by a nano-amount and then you're ready to rock. By choosing a material with a weeny bit higher expansion coefficient than cast iron, when warmed, the rings would exert even more force against the head, creating a better seal. Sure, they would shrink away a touch but like running castor oil, let it warm up first.

IMO, this would allow you to use big bores, cheap pistons, have head gasket reliability and get the desired CR. To make matters more interesting, the inside of the ring could be relieved a tad to add even more chamber space (couple of cc perhaps).

I reckon it's worth asking you lot what you think.....

Stu

Bugger off, I'm getting there.

How do you fit the pistons from the bottom of the bore,they won't fit that way crank bearing webs are in the way

Metro turbo weekend driver,Mini turbo in the making again!

Dangerous makes a valid point.

Unless he inserts the "rings" after putting the pistons in.

I can see the logic, but why not just get the big dish pistons and use a 25cc head?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ahh, there's the snag. Fitting the rings afterwards would be difficult as the extra bored bit would have to be done mega accurately. I suppose they could be made to interference fit but I don't like that. The whole point was to save a load of money, perhaps do the job better.

I need to get a decent milling machine :(

Bugger off, I'm getting there.

You could never have rings go through there with the proper ring gap. You'll have to have excessive gap to make it happen even if the pistons goes through. Also, you cannot fit the two center pistons from the bottom.

Interesting idea to say the least. Not really worth a damn but an idea anyway. I don´t get it, why do you make things so difficult ? I am running 1380 with Omega 11 cc pistons pretty much flush with the deck. The head is close to 35 cc so I am running CR around 7,8:1. IIRC Stuart Gurr has cut heads close to 40 cc (success rate unknown...). I have successfully cut 2, and failed on one.

The inserted rings would probably screw up the mixture flow and flame fronts etc big time.

You're probably right, I would say that it would act like a big head chamber rather than a big piston dish. In the instance of combustion, I'd say that the pressure was exerted more in a downwards way on the piston more effectively than a dished piston. Not sure how much theoretical bollocks this would be, but if the surface area is less, you would have less heat rejection and perhaps a weeny bit more welly produced. I doubt a significant amount though.

Can't remember if it was sheepspeed or someone, but I saw a very nice head (again, compromised combustion chamber shape???) which had just been set up in a mill and the chambers were circular. That looked very good and probably easy to do. Me being a bit of a tight bastard decides that I have a lovely 36/32 head which has weeny (not sure but V small) chambers. I'm looking at ways of using that without compromising squish. On principle, I


before my shed fire, I had just finished grinding a blower head, that was about 35cc and took me bloody ages. I'd rather try some other means of attaining the ends.

Just out of curiosity, for a 73.5 engine, normal stroke, what sort of bumping clearance would you give?

I'm convinced it's a good idea. Simca 1200 pistons are forged for starters, have a short compression height and with my spangly ring things would possibly increase reliability. It's a departure from convention but I suppose you could fit the rings after pistons/crank, fill the clearance up with vaseline and then mount the whole thing up for a gnats willy skim. I don't think any of it is that difficult.

I'll have a play with the calculations and come up with how worth it, it is. After fully knowing virtually every atom on that head I ground, I'd rather whack a couple of rings in.

EDIT:- not sure that's in the right order, I had to cut and paste it while the computer pulled a wobbly.

Edited by Hedgemonkey on 16th Oct, 2006.

Bugger off, I'm getting there.

so how much are the simca pistons?

Not sure, but they're the sort of things you can pick up at an autojumble for f/a

Bugger off, I'm getting there.

Not sure, but they're the sort of things you can pick up at an autojumble for f/a

Bugger off, I'm getting there.