Gasket for true compression

[thebig450es]

Quick question ,What is the best gasket or way to get the true compression? Im just finishing up my list for my build to see if im in budget. Thanks

[CJM]

XR400, see the last question here: http://www.atvriders.com/articles/askmickey1.html

[400exmxrider15]

Originally posted by CJM
XR400, see the last question here: http://www.atvriders.com/articles/askmickey1.html
So saying that, maybe I should get a 400ex gasket kit? I'm replacing the top end, so i wanna do it right the first time.....

[thebig450es]

Thanks, i just want to make sure im getting all the compression i can , i just need to find it in 88 mm

[CJM]

Originally posted by 400exmxrider15
So saying that, maybe I should get a 400ex gasket kit? I'm replacing the top end, so i wanna do it right the first time.....
Its your call, you can run the OEm gasket, cometic gaskets, XR400 gaskets. Me, I have the best cometic gaskets you can get-thier ESL kit. Was a bit expensive around 75 bucks, but since I have a worked motor I wanted to take no chances.


Originally posted by thebig450es
Thanks, i just want to make sure im getting all the compression i can , i just need to find it in 88 mm

You can run a stock gasket, piston doesnt touch it at all. I have no idea why they even bother making larger sizes b/c it doesnt come anywhere near the sleeve.

[400exmxrider15]

I dont wanna hijack this, but what gasket do I use for a true 12.5:1 compression? i've heard using regular oem gasket with that high comp you only get like 9.5:1?

[CJM]

Originally posted by 400exmxrider15
I dont wanna hijack this, but what gasket do I use for a true 12.5:1 compression? i've heard using regular oem gasket with that high comp you only get like 9.5:1?

The only way to truly get the comp is to follow Micky Dunlops advice:


Thank you for your question. I prefer to build a motor that pulls quick and hard through the RPM range and the piston with higher compression will build more cylinder pressure which develops torque and will rev quicker and run faster through the RPM range. On most after-market pistons, the pin to deck height is the same as stock. You should have a minimum of .040 clearance between the piston and head with everything torqued down. Right now the piston is about .043 too low and you are not getting a true compression ratio as advertised by most piston companies. For instance, a 440 11:1 flat top piston advertised is really only about 9.75:1 if you measured the volume correctly. In order to get the deck heights better, I use Honda HT silicone seal on the cylinder base and an XR400 gasket if you are running an 80 over piston or smaller. This will get you close to what your deck height should be. Keeping the squish clearance down to .040 gives a better quench area around the outside of the piston keeping it cool and less chance for detonation. You will be able to run an 11:1 motor on pump gas in most cases and have a more efficient flame travel and quicker burn rate across the top of the piston. I have read on the forums here how everyone thinks a 416 is faster and revs quicker than a 440 kit with the same compression. I think most people are not running as high a compression ratio with the 440 kits as they think they are and when you put a cam with it, you end up with lower cylinder pressure making the machine rev slower than the higher compression 416 motor. Just my thoughts anyway. Without talking to each individual, it's hard to figure out why people think the 416 is faster than the 440. Any time you put a longer duration cam in, you have less time to build cylinder pressure. This is why it's important to put a higher compression piston in when you install a longer duration cam. You can get more torque the higher you go on the compression ration and your cam will give you the mid-range to top-end without losing low-end.

[crownandmonster]

Originally posted by CJM
The only way to truly get the comp is to follow Micky Dunlops advice:




Or get your jug decked a couple thousandths. Same idea except you still use stock gaskets.

[CJM]

Originally posted by crownandmonster
Or get your jug decked a couple thousandths. Same idea except you still use stock gaskets.

Forgot about that, also a good idea :)

[crownandmonster]

Originally posted by CJM
Forgot about that, also a good idea :)


If it were me, I would take both the head and jug to the machine shop and have them take a little off both. Just to help ensure they are nice and flat and not warped.