Base gasket thickness

[FFRDave]

There are three thicknesses of base gaskets, .15, .30, & .50, and any combo of them.

I have read on the forum a long time ago that these gaskets can be mixed and matched for different powerband effects.

Can we revisit this question ??

I do most of my riding at high RPM in the sand dunes (90% of time). There is another place with some big steep hills with thick grippy soil, but I only ride there occasionaly (10% of time). I want try some forest trail riding next summer if I can get to it.

What base gasket set up would be best for my High RPM Sand Dune jumping ??

 

[GMP]

The effect is very subtle. The thinner base gasket(s) add compression and alter port timing slightly for more low/mid. The 250s usually come with a .5 and .3 together. I usually install a single .3 at the first rebuild. What I would do in your case is keep the .5/.3 and have the head cut to correct the squish which is usually really far off with this thick a base gasket combo.

 

[FFRDave]

My gasket

I only had one gasket, and I dont know which it was. I only used one gasket on my rebuild too. I dont know which one it was either. There is a little sticking out around the edge, I guess I could go measure it.

I did have my Squishband adjusted. That in combo with the LTR needle and jet combo and the LTR PV cover have really made the GG go.

This has been my first GG rebuild and it went nicely. I had some questions along the way, but now I am totaly comfortable.

 

[GMP]

The .3mm gaskets are green, .5mm grey. Its important to use the same gasket that you had your squish band adjusted for, as the squish clearance is tighter now. If you had the head cut based on measurements using the .5mm, and now you used a .3mm, you could be a little tight and your compression will be a little higher. The other way around and you just loose a little compression and add .2mm of squish clearance, negating some of the head mods benefits.

Thats one reason stock bikes are set up so loose in the squish, to account for tolerances and fuel variables.

 

[FFRDave]

My old stock gasket was kind of a dark grey and the new one is a very pail green.

From what you say, (if pail green is 'green') I might have lost .2 (mm?)of squish and my compression will be a little higher now. I started the bike last night and heat cycled it and it seemed fine. I did notice that I had to use a little more fource to kick it over, but that could just be the new rings.

If I remember correctly, the squish-band adjustment was more of a mod to the inner part of the head. The outer rim of the head was not changed. This means that the piston is closer around the edge (well, really everywhere) of the head by .2 (mm?).
I can tolerate a small change in performance, but parts smashing into eachother would be a disaster .
There was a dark ring around the top of the cylinder that was about (measuring by eye and memory) an 1/8th' to 3/16s" wide, or it was maybe 5mm or 6 mm. Now I guess it will be .2 less.

I need to drag my old gasket out of the trash and take some measurements.

What effect should I expect from higher compression ?

 

[GMP]

You went from a .5mm to a .3mm. Assuming you took all your measurements with the .5mm and had Ron cut your head for pump gas, you may now will be on the edge. I'd be looking for signs of detonation especially riding hard in sand. What was your final squish clearance with the .5mm gasket? Compression? Compression now? I don't think you want to be less than 1mm squish on a 250, and 200 PSI is approaching the limit for pump gas. I'd email Ron and ask him. If its OK and doesn't ping I'll bet it runs good off the bottom.

 

[FFRDave]

Confirmed !!!

Ok, I got it !!

I only had the one grey gasket (0.30mm) on there when I tore it down. I did not have two (0.30 + 0.15= 0.45).

With my piston at X= Max height (with a single 0.3mm gasket) as I brought it home from the showroon when I bought it new. My cylinder sat at y height.

Now I have removed that 0.3 mm grey gasket and traded it for a thicker 0.50mm gasket (thicker by 0.20mm).
Now, my cylinder is at height= y+0.20mm because my new gasket is 0.20mm thicker, and my max piston height is still at X (as it was when I brought it home from the dealership) with neglagable rod stretch and bearing wear.

My stroke has not changed, but my combustion chamber is 0.20mm larger than it was. This should actualy drive my compression DOWN, not up. I suspect that the gasket has been compressed (squashed) a little (dare I say 0.10mm ? ) by the pressure exerted by tightening the four base nuts.

It seems to me that I should have plenty of room.

Does this sound right ???

I know I had only one grey gasket when I tore it down for rebuild. GoFasters has confirmed that they sold me the 0.50mm gasket and that is the one I installed on Sunday.

The bore on my GG is 66.4mm, or (6.64cm).
If stroke is increased by 0.20mm or (0.02cm), then

6.64cm X pi X 0.02cm = 0.417cm cubed = 0.417 cc I dont think that is very much of a difference. Including gasket squash, its even less of a difference.

 

[Boomhauer]

Guys the new base gaskets have changed in colors.

I copied this from a post that Berky Boy posted sometime back.

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

The new base gaskets are as follows

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

 

[GMP]

If you got a .5mm to replace a .5mm then your fine. I have a gasket kit thats older I guess, as the .5mm is dark grey and the .3mm is green. Its a good idea to measure your compression anyway for referance.

 

[FFRDave]

Sometimes intergity is all we got....

Brace yourselvs, your gonna see a first for the internet.

I was wrong, mistaken, in error....

I went to the shop trash can and dug till I found my old bse gasket. I thought it had gotten torn when I removed it, but I discovered that one layer.... One layer !!! Wait, theres TWO Layers !? No, tow gaskets very stuck together. A dark green one and a grey one as I peeeled them apart.

So as I was so adement before about only having one single grey gasket, I discovered that there really was two there.

This means that it was 0.45mm thick and now its 0.50mm thick. Thats even better because its even closer than what I thought I had before.