Compression distance difference on pistons
 

I had my head sent off to R&B for the head mod. Before we sent the head we measured every dimension off the head. The head came back and all of the cuts that were made on the head should have put the minimum squish at 1.2mm instead the squish was 1.9mm and the only change that I made was I switched from a vertex piston to a athena. I never even thought to measure the compression distance differences between the two pistons. Just something to think about if your doing the head mode in conjuction with replacing the topend.

Hi Adrian,
Can you use one thin base gasket to adjust deck height and try to "get it back"? It will also shift port timing slightly. What are you using for base gaskets now? As a reminder for myself - since I am old and forgetful - the base gasket options are .5mm (dark grey), .3mm (green) and .15mm(light brown).

jeff

Adrian

The squish height on the GG 250/300 usually measures around 2,2 mm as delivered from the factory.

It's also kind of tricky to get the squish height measurement right and I've found that it's best to measure at four points on the piston to avoid making the piston rock/tip in the cylinder. I put solder at 12, 3, 6 and 9 o'clock to make sure that I get an even measurement. You may want to recheck the squish height to be sure.

GG bikes are delivered with two base gaskets, 0,5 and 0,15 mm. I used to run my old bike with just the 0,15 mm gasket. This is not recommend but I never had any problems.

Kent

Ron specifies measuring the squish on the line of the wrist pin, both sides. I suspect this is to minimize the error from the piston rocking in the bore. The suggested 1-1.2mm should safely accomodate any such rocking. I can see piston brand changes screwing things up unless you know they are the same. I did everything with one .3mm base gasket and a new Vertex and stuck with Vertex.

By measuring four points at the same time you are more likely to get an perfect measurement. If you measure at two points along the wrist pin you may "miss" the line and allow the piston to rock slightly and alter the measurement.

Maybe I'm just being difficult but it certainly is a good idea to recheck the measurement to be sure you got it right the first time.

Also, like GMP says, sticking with the same brand of piston, and checking the squish evereytime you change pistons, is a good idea.

Kent

Jeff,

The new gaskets that you would be getting from GoFasters or who ever for the .5mm is not gray anymore. They are a light green color. From what I remember they (GasGas) changed manufactures and the base gaskets changed colors also.:confused:

I am sure Berky Boy or somebody else that deals with GG parts can comment on all of the new sizes and colors of the gaskets. I had to deal with this when Girard and I shipped our heads off to Ron at R&B. I ordered a new .5 gasket and it came in the new green color.

I posted up on here making sure I did not get the wrong gasket and somebody responded and gave out all of the new info. Let me see if I can find that post.

Okay I found it over on the ole Talking Smack Forum. Berky Boy posted this info up.

The new basegaskets are as follows

.15mm Brown
.3mm Grey / Green
.5mm Light Green