400 MC5 cylinder base gasket

[robesetz]

Hello all,
  The old cylinder base gasket I took off my 400 tonight was 1mm thick, should I use the same size or have you guys had better results using a combination of the base gaskets? 5 came in the kit ranging from .2mm-1mm. Any suggestions? Thanks.

ROB
76' 400 MC5

[OUCWBOY]

Rob,
You need to use whatever will give you the same cylinder height. It is best to use the same 1mm gasket.

Donny Smith

[tomale]

Donny is right... To make sure that it is right you need to install the gasket and then install the cylinder. using some nuts and washers secure the cylinder to the center cases. forget about the head for now. raise the piston up to TDC and check the deck height. if it is wrong you will need to add or subract the amount of gaskets... warning, just because you measured the old gasket does not mean that the new one will net you the same results, The old one is already compressed but the new one is not...The result will be that the deck height will be too high and you will need to add another thinner gasket. It is a trial and error deal so take your time and do it right. Also, check your timing afterwards... It is possible that the old gasket could be wrong and if you brought the engine up to correct specs... it could have caused the timing to be wrong in the process.
You might want to consider what kind of riding you are going to do.. It it is Motocross then you want it to be set at the higher setting but it could be set lower for the woods.. This is the real beauty of a KTM motor... Check your spec sheet and see if there is not two setting for your motor.. I know this is alot of work but it will be worth it.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[tlanders]

I always put the head on, torque it down and stick some 1/16" diameter solder through the spark plug hole so that it extends to the edge of the cylinder in the squish area. Then I turn the engine over with my hand, compressing the solder. THen pull it out and measure the thickness of the squished solder. This will be the actual squish clearance. I forget offhand what it should be for a 400, somewhere in the 0.050" range I think. I never have had much luck getting repeatable measurements trying to measure deck height, my dial caliper is thicker than the piston to cylinder clearance.

Teddy

[tomale]

Teddy that is an interesting way to set it up, I just finished putting a top end on the 75 400 and I think I will try the solder trick to see where I am.... I went down to a local Auto parts store... Shucks.... and they has an nice digital caliper for 15 bucks, it reads both inches an M.M. I have used it to convert one to the other... As it turns out... the little tang on the end that is used to measure depths just fits. I had been looking for something like that but everything that I had found up to that point was 50 bucks or more. I have a plastic set of calipers too but I alway worry that the weather is going to effect the readings I'm getting.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[wildman]

I always used a set of bent wire gages used for setting spark plugs. That way I could set it right on top of the piston ring and feel if it came to the top. I don't know if anyone even sells that type of gage set anymore. Your squish clearance should take into account the deck height AND the head gasket thickness. I would set it the way Penton manual recommends. JMHO.    Wildman

1975 250 Cross Country, 1974 175 Jackpiner, 1975 125

[tlanders]

The deck height is an attempt to provide the correct squish area clearance at top dead center. The squish clearance is designed to keep the piston from hitting the head and help control the burn of the gas as it proceeds from the center of the piston towards the perimeter of the piston. The designers of the engine have decided on a squish clearance for their engine taking into account all the criteria (porting, compression ratio, power, fuel octane, timing, etc.) for what they are trying to accomplish. The pistons we buy today may have a slightly different shape on the top from the Mahle ones of yesterday. That is why the safest way to make sure it is set up right is to measure the actual squish clearance with the head gasket in place and the head torqued down instead of the theoretical clearance that you may get from a deck height measurement. It's a little more work, but I feel safe when I'm done.

Teddy

[Merlin]

..........sorry,the cylinder head has no influence on the ignition timing,ignition timing is strictly the relationship of the distance of the firing point of the stator prior to the piston reaching top dead center...........

[tlanders]

Merlin, what I am trying to say is if the shape of the piston or head is changed or the deck height is too small making the squish area clearance too small, it may increase the compression ratio enough to create preignition/pinging and then you would have to retard the timing to make it stop pinging and eating holes in your piston. Or you could use a slower burning/higher octane fuel and not change the timing. I only mentioned timing above because the engine designer has taken into account the compression ratio, fuel octane and timing in his design. If you change one of these three things, and in this case we are talking about deck height/squish area clerance, it will affect the compression ratio and if this changes, you may have to change the timing or octane of the fuel to keep it from pinging. Thank you for your comment, I am sorry I didn't make myself clear.

Teddy