jakepeewee
Hi all,
I am still slowly getting on with building my 888 engine with 95 mm bore and 748R ported heads and carrillo conrods.
Last Friday checked the squish on the vertical cylinder. I estimated it would be 0.9 mm, but measured 0.75 mm. A little too tight for my tasting.
The 0.75 mm is without fitting a base gasket. I know there were 0.2 mm base gaskets made for the 916SP but they are discontinued. Not sure if I can get my hands on these. 0.3 mm thickness is readily available but will give 1.05 mm of squish, a little more than I prefer.
Anybody got any ideas how I can get to the approx 0.9 mm? Sources for the 0.2 mm base gaskets?
Or am I making this too big of a deal and should I just go with the bigger squish?
On a side note:
I did measure the squish on my motor before starting this tuning project.
It was about 0.8 mm (OEM 888 SP3 engine with 95mm bore). Didn't baby her and didn't kiss the heads, pheww! But it was too tight for my tasting, so wanted to add 0.1mm. I machined the pistons with 1.3 mm and machined the base of the cylinders with 1.2mm.
So that's why I expected the 0.9 mm of squish. But, in hindsight, I also exchanged the conrods (now carrillo's) and the crankcase (now S4). I feel the crankcase is the most likely suspect?
And I did measure the squish of 0.75 mm without the piston rings fitted. Can I expect the number to be lower when the rings are installed?
Your wisdom and expertise is appreciated!
Regards,
Jacco
griff851
Ah the dilemma. Been there. You could do an number of things. If I were you I'd get a custom made base gasket, they are more expensive but instant fix. Gasket making places usually have various thickness materials that will do the job. Or get handy with an Exacto knife and and by the gasket material you want from a bearing shop, they usually have paper and klingerite or some such. It's just a gasket. It's no lie, you can use a cornflakes box. The ink imprints. 😄
You could also shave the barrel bottom, but that's pretty extreme and will cost more than the custom gasket.
Cometic do head gaskets they might have something that you can work with.
Or you can not get too bent up about it and run the higher squish. I have 1.1 on the 888, flat tops, Carrillo rods and 748 drop came heads. It gives me 12.2:1 and goes well enough.
If you just doing road work remember, compression go be you more power, but don't forget your poor old starter motor. It varies the can for your ideas.
Mr.R
Cometic can supply different thickness base gaskets, I spoke with them on the phone when I wanted something from outside their ordinary /stock kit and they were very helpful.
Anything over 1mm and the squish isn't really doing it's job, I aim for 0.8mm when building Desmoquatro motors.
I also measure the squish with the rings fitted.
Steve R
marvin
I have had the local sign shop cut out stensils for painting. I would think that they could also cut a gasket out of the appropriate thickness paper ???
Marvin
jakepeewee
Thanks for all your input!
I've read that people fear 0.8 mm is too tight, but it did work out for me. Happy to read Steve aims for 0.8 mm. That would mean I only need to gain 0.05 mm.
I think I am going to redo the measurement with the rings fitted.
I feel that the squish might be a little bigger with rings fitted.
And I did use the old head gaskets. When fitting new ones I might gain the 0.05 mm to get 0.8 mm.
Thanks again!
Jacco
griff851
Gasket crush at about 50Nm is about 0.03 with a 0.4 gasket if that helps.
Also belts, I have to use ST4 belts and had to change rollers.
jakepeewee
Great info, thanks Griff!
I think it's going to be ok without an additional base gasket.
I also have the bigger gold rollers to get the belts tensioned.
Why the ST4 belts? What's different about these belts?
Jacco
