Vmax540
VIP Member
Ok, now nobody laugh..... at least not outloud ! 
Is squish simply the combustion area when the piston is T.D.C. ? If a engine is bored out to 60 thousands over would this have any effect on Squish or Compression ? Normally how much can you mill off a head before you can't safely run pump gas ? Thanks for putting up with a rookie..... !
Is squish simply the combustion area when the piston is T.D.C. ? If a engine is bored out to 60 thousands over would this have any effect on Squish or Compression ? Normally how much can you mill off a head before you can't safely run pump gas ? Thanks for putting up with a rookie..... !
bluewho
Active member
hey whos laughing but it is a little boring no?ha if you go to the faq forum 2nd page and 5 up from the bottom it cant be explaned any better than that.good reading.
Canuck
New member
Squish if the area between the piston and the outer ring of the cylinder head at TDC, it generally in the area of .060" - .080"
Boring .060" won't change the squish but it will raise the compression slightly because you are now squeezing more volume into the head.
There isn't any set amount that you can take off a head, it depends on the sled and in many cases you will need to recut the squish band.
Boring .060" won't change the squish but it will raise the compression slightly because you are now squeezing more volume into the head.
There isn't any set amount that you can take off a head, it depends on the sled and in many cases you will need to recut the squish band.
do a search on mrviper700 he has a great post on the subject. you may have to widen your search to last year.
search on compression, squish band and his name
search on compression, squish band and his name
Vmax540
VIP Member
Thanks guys !
Vmax540
VIP Member
I knew that...........yea right............ ! Wow good reading ! ! !
The squish band is the area in which most people dont take the time to set up correctly and this is where you will have problems. If your going to do a head mod on a sled, you need to know what the stock squish is set at. This can be done in a easy fashion of simply using a bit of lead solder (.080" or larger)bent into a 90 degree "L", its important to have the solder touch the sides of the bore and you can feel the smooth side when inserted thru the sparkplug hole. You want to check the squish at the left and right side of the cylinder, this prevents the piston from rocking back and forth on the wrist pin and will give you a false reading, just stay on top of the wrist pin and youll be fine. Each engine can be differnt, there is machine work tolerances when engines are built, so the reading you get from 1 will not mean its the same in the others, deck clearance,head relief , piston top will all play a role in the reading. In most Yamaha sled cases, you will want a good .052-.055" to be considered safe pump gas range. The 2000-02 srx usually have .060-.063" stock, the 98-99 srx were a bit wider. So what you want to do is take your stock reading and subtract the safe range from it, this will give you the correct amount to mill off the head. You can mill significantly more then that,**BUT**, you will have to machine the squishband angle and "rechamber the head" to do so, its not something the average guy is going to do, and really isnt neccessary unless its a special built purpose engine, like ice drags, grass,etc. The stock squishband is around 12-14 degrees, and by changing to a steeper angle like 17-19degrees, you can run more compression with out having detonation, the reason is this, the stock angle is very flat and by milling the head , it sits even closer to the piston at TDC then before, the mixture becomes trapped in the squishband at TDC, when the plug fires, it will only burn the mixture in the center, and the trapped mixture out in the band becomes stagnent and builds heat, it then fires from heat(like a diesel), this is the detonation, and it makes little indents in the edges of the piston, like a small pick hammer pounding on it. By changing the angle to a steeper one, it helps to force the mixture to the center of the haed and promote better flame travel on the top of the piston crown. This enables you to run higher compression without the risk of detonation, but again, this is not just a quick easy flat mill of the head, and not really much gain for the "trail sled", was just tryin to give you a idea of why and where. The overall meaning of this post is that more milled off is not always better unless you do the entire job correctly.
The squish band is the area in which most people dont take the time to set up correctly and this is where you will have problems. If your going to do a head mod on a sled, you need to know what the stock squish is set at. This can be done in a easy fashion of simply using a bit of lead solder (.080" or larger)bent into a 90 degree "L", its important to have the solder touch the sides of the bore and you can feel the smooth side when inserted thru the sparkplug hole. You want to check the squish at the left and right side of the cylinder, this prevents the piston from rocking back and forth on the wrist pin and will give you a false reading, just stay on top of the wrist pin and youll be fine. Each engine can be differnt, there is machine work tolerances when engines are built, so the reading you get from 1 will not mean its the same in the others, deck clearance,head relief , piston top will all play a role in the reading. In most Yamaha sled cases, you will want a good .052-.055" to be considered safe pump gas range. The 2000-02 srx usually have .060-.063" stock, the 98-99 srx were a bit wider. So what you want to do is take your stock reading and subtract the safe range from it, this will give you the correct amount to mill off the head. You can mill significantly more then that,**BUT**, you will have to machine the squishband angle and "rechamber the head" to do so, its not something the average guy is going to do, and really isnt neccessary unless its a special built purpose engine, like ice drags, grass,etc. The stock squishband is around 12-14 degrees, and by changing to a steeper angle like 17-19degrees, you can run more compression with out having detonation, the reason is this, the stock angle is very flat and by milling the head , it sits even closer to the piston at TDC then before, the mixture becomes trapped in the squishband at TDC, when the plug fires, it will only burn the mixture in the center, and the trapped mixture out in the band becomes stagnent and builds heat, it then fires from heat(like a diesel), this is the detonation, and it makes little indents in the edges of the piston, like a small pick hammer pounding on it. By changing the angle to a steeper one, it helps to force the mixture to the center of the haed and promote better flame travel on the top of the piston crown. This enables you to run higher compression without the risk of detonation, but again, this is not just a quick easy flat mill of the head, and not really much gain for the "trail sled", was just tryin to give you a idea of why and where. The overall meaning of this post is that more milled off is not always better unless you do the entire job correctly.