6 replies to this topic

[Beeaj]

I ordered a load of parts from Gas Gas UK including a cylinder base gasket. I have just had a look at a parts list which shows there are four sizes available, I assume that Gas`Gas will have sent me the correct size for my TXT 250 pro??
Also does it matter which way up the gasket sits (printed side up / down and should it be fitted dry or lubricated in any way.
Thanks
Beeaj
(thank god for this forum!)

[dombush]

Hi

couple of answers:-

• It doesnt matter which way up the gasket goes if it is a straight paper gasket
  
• Always check that the tranfer ports line up with the gasket cut outs when dropped over the crankcase studs. this can determine the way that the gasket is fitted.
  
• I have never applied lubricant to gaskets because it makes them stick when you take it apart next time. Some people put RTV silicone on them but i wouldnt.
  
• The reason that there are 4 gaskets is to achieve the correct piston to Cyl head clearance (squish), this is required to deal with production tolerances
  
• look at the gasgas manual to see what the squish clearance for your model is
  
• There are a couple of ways to measure squish, the one i use is squashing some soft solder. I assemble the barrel/piston/head and insert some 2mm solder through the spark plug hole, then turn the motor over on the kickstart. You then pull the solder out and measure the "squished" solder.
  
• typical squish clearances are between 1.0 and 2.0mm. A number of factors determine this but generally larger engines have bigger squish clearances

if all this is double dutch, you are probably best just measuring the gasket you took off and getting the same again. The danger of getting it wrong is probably not great, the worst being you change the characteristics of the engines power (too abrupt or the reverse)

Dom

[Beeaj]

dombush, on 05 February 2011 - 12:03 PM, said:

Hi

couple of answers:-

• It doesnt matter which way up the gasket goes if it is a straight paper gasket
  
• Always check that the tranfer ports line up with the gasket cut outs when dropped over the crankcase studs. this can determine the way that the gasket is fitted.
  
• I have never applied lubricant to gaskets because it makes them stick when you take it apart next time. Some people put RTV silicone on them but i wouldnt.
  
• The reason that there are 4 gaskets is to achieve the correct piston to Cyl head clearance (squish), this is required to deal with production tolerances
  
• look at the gasgas manual to see what the squish clearance for your model is
  
• There are a couple of ways to measure squish, the one i use is squashing some soft solder. I assemble the barrel/piston/head and insert some 2mm solder through the spark plug hole, then turn the motor over on the kickstart. You then pull the solder out and measure the "squished" solder.
  
• typical squish clearances are between 1.0 and 2.0mm. A number of factors determine this but generally larger engines have bigger squish clearances

if all this is double dutch, you are probably best just measuring the gasket you took off and getting the same again. The danger of getting it wrong is probably not great, the worst being you change the characteristics of the engines power (too abrupt or the reverse)

Dom

Thanks Dom for replying so quickly with this info
Mike (Beeaj)

Edited by Beeaj, 05 February 2011 - 12:29 PM.

[JSE]

dombush, on 05 February 2011 - 12:03 PM, said:

[*]It doesnt matter which way up the gasket goes if it is a straight paper gasket
[*]Always check that the tranfer ports line up with the gasket cut outs when dropped over the crankcase studs. this can determine the way that the gasket is fitted.
[*]I have never applied lubricant to gaskets because it makes them stick when you take it apart next time. Some people put RTV silicone on them but i wouldnt.
[*]The reason that there are 4 gaskets is to achieve the correct piston to Cyl head clearance (squish), this is required to deal with production tolerances
[*]look at the gasgas manual to see what the squish clearance for your model is
[*]There are a couple of ways to measure squish, the one i use is squashing some soft solder. I assemble the barrel/piston/head and insert some 2mm solder through the spark plug hole, then turn the motor over on the kickstart. You then pull the solder out and measure the "squished" solder.
[*]typical squish clearances are between 1.0 and 2.0mm. A number of factors determine this but generally larger engines have bigger squish clearances

Dom,
Good description and that's pretty much how I do it too. I usually use a wrench on the flywheel fastener to turn the crank over and shoot for a clearance of 1mm (.039"). I don't remember a squish clearance being mentioned in any manuals but Dom's 1mm/2mm range is what I would use. If you use the old gasket thickness as a guide, I'd use about 20% thicker on the new gasket to account for compression.

Same "trick" as a re-torque on the head fasteners. Heat cycle the engine a couple of times and re-check the cylinder base nuts just to make sure you don't get future leakage on the outside gasket sealing surface of the transfer port, which is fairly narrow.

Jon

[ormplus]

thicker the base gasket, smother to ride.will slightly lower compression + port timing.fit gasket dry.

[dombush]

had a root around....found this on Jim Snells excellent trialpartsUSA website

It sums up some of the details discussed.

My link

hope this helps

Dom

[Beeaj]

Thanks all great advice