A little Piston work..

griff · Nov 1, 2008

Dont try this at home.... unless you have a mill at home... and you have machinist experience.... and you know what youre doing..... LOL

These are the pistons from my '86 GSX-R750. I had to lower the compression, and since I'm already using a +base gasket, the pistons were the next logical thing. A quick call to Wiseco to confirm my calculations, and I was good to go. The default for the kit is 13:1, of which I determined California pump-gas doesnt cut it. .030" will knock it down 2 points. I went for .027 off all 4 pistons, and they all have <.001 difference between them now.

Took me about an hour to index my mill, and another hour setup on the 1st piston to make sure it was perfect. The other 3 took about 20 mins each after that.

Thought some of you gear-heads might get a kick out of it.....

dave76 · Nov 1, 2008

Nice, doin the end mill there. Yeah 13.1 on an air cooled motor is a bit high, water cooled engines seem to be able to handle it better. I always thought the SACS Gixxer ran best with 11:5.1 on the street?? I can vouch being stuck in traffic in the summer with my 92 Gixxer 750 was hell on the motor as well as you.lol

griff · Nov 1, 2008

dave76 said:
Nice, doin the end mill there. Yeah 13.1 on an air cooled motor is a bit high, water cooled engines seem to be able to handle it better. I always thought the SACS Gixxer ran best with 11:5.1 on the street?? I can vouch being stuck in traffic in the summer with my 92 Gixxer 750 was hell on the motor as well as you.lol

I know guy's running the 13:1/771cc config with pump gas, and theyre basically OK with 93-95 octane.

The problem for me is that my config is a little more radical. I have .018 off the head, and .004 off the jugs, with a .022-over base gasket. So stock height, but a much tighter head chamber (narrow squish band, etc).

California gas is 91 octane just isnt enough with my setup, I might be able to get away with it still with 93-95 octane, but its not available here. My only choice would be to mix race gas, and thats not an option since I cant fill up on a trip somewhere.

Take'n these piston down will put it n the 11 to 11.5 range, and I can even run +5* advance, and it'll run on 91 octane all day and be happy.

jasonp · Nov 1, 2008

What is odd is that the closer you run the piston to the head the cooler the engine will run and be less detonation prone. Sometimes though you just can run that kind of cylinder pressure with the fuel you are running.

griff · Nov 4, 2008

Geezus... found out that the initial specs Wiseco gave me were WRONG. Idiots... Then they couldnt tell me what the reduction would be.

Yet another reason to stick with JE. Anyways...

I got the right specs, and a friend and I worked out the theoretical numbers. I ended up milling off a lot more than whats in the pics I posted before. Total milled off ended up at .087", which will bring the CR down to the mid to high 11 range.

This is pre buffing and clean-up:

floridaredneck · Nov 4, 2008

Make one for the craptor. I'll be the test dummy. Move the wrist pin hole down a bit then make a spacer that goes between the cylinder and the cases and give it a longer stroke.

blonka · Nov 6, 2008

That wont make a longer stroke.

floridaredneck · Nov 6, 2008

You sure?

yamarider · Nov 6, 2008

Definitely not. Moving the wrist pin lower and creating a spacer would just counteract each other. There's no way to get a longer stroke without changing the crank.