Base Gaskets
 

Can anyone tell me the correct proceedure for establishing what thickness of base gasket to use?

Use the .3mm and .015mm, or you could use a single .5mm.

I used the 5mm with a thin bead of Permatex wich comes to approx 15mm.Reason for sealer is so my front 2 studs would not leak.That was a year ago no leaks no problems.

I think the green one is 15mm and the brown one is 3mm.

I think the procedure would be to establish the squish band and, based on that, decide how thick of a base gasket to use. It seems the factory sets them up a bit conservative (thick) and many here have gone to a single .3mm with good results. I think I even read that a single .15 was used but it carries the slight risk of not being thick enough to get a good seal.

Trying to correct the squish with a thinner gasket does not work because the problem is the match of the head to the piston and lack of consistant clearance across the squish band. The thinner gasket will add compression and reduce the clearance, but the effective squish is a function of both the clearance and match to the piston. For example, you could have a decent clearance of 1.5mm near the cyl wall that rapidly tapers off to 3mm+ at the dome, so you see little gain. Thats why the head must be cut.

If your not going to do the head, just drop in a .3mm or .5mm and be done with it, it doesn't make that much difference. The stock squish is sloppy enough not to worry about it. I dropped .5mm from stock(.3mm and .5mm) on my '03 250 and still had close to 2mm at the cyl wall.

I think the colors may have changed, but in my kit, .5mm is grey, .3mm green, and .15mm light brown and obviously the thinnest.

I just got some base gaskets this week. According to my calipers the green one is .5MM, the brown paper one is .15MM. The old one I took off was green, I did not pay attention however to the relation of the piston dome and the port openings with the piston at BDC before taking it apart. When I re-assemble it I will check the results and post them.

That is my experience too.

Like Glenn, I found the stock squish band to be large.

On my '06 EC300 stock I measured 2.2 mm squish near the cylinder wall and 3.1 mm near the edge of the dome.

On my '01 XC300 stock I measured 1.9 mm squish near the cylinder wall and 2.8 mm near the edge of the dome.

Taking out a 0.5 mm base gasket will help a bit. However, to correctly fix the squish, the head needs to be machined.

Also note, when you put in a new piston you need to recheck the squish and adjust base gaskets accordingly. When I put a new Vertex piston in my '06 EC300 I measured the squish band with the new piston. I found the top of the new Vertex piston was 0.2 mm higher than the original piston. I compensated by increasing the thickness of the base gaskets.

Here are the results from my recent top end overhaul. I had the cylinder replated to A specs. and a Vertex piston. I used one green base gasket that measured at 0.5MM. I did not take any precise measurements after assembly but, at TDC the top of the piston was approx. 1.5-2MM below the top of the cylinder. At BDC the top of the piston extended about 1-2MM above the lower opening of the middle exhaust port.