Base gasket

[trapezeartist]

As Boris has stopped me riding, I decided to investigate the piston slap noise emanating from my 250 Evo. The bore is a bit scruffy but no serious problems. It will go to Langcourt for replating, honing and matching to a new piston.

I'm just trying to identify the thickness of the base gasket that has just come off. It measures 0.5mm, but of course it has spent the last 4 years squashed under the barrel so it may be thinner than nominal. The only marking on it that gives a clue is a "40" printed in one place. Could this mean 40 thou ie 1.0mm? Unfortunately I didn't measure the piston height while stripping it so I don't know how close to spec it was before. Which gasket thickness should I order? I'm not a scrooge (despite what my wife says ?  ) but I would really like to get it right first time as I'm dreading compressing the rings correctly and guiding it all into the barrel, and I don't want to have to do it twice.

[MIckyT]

yes depending on how you want it to run 1 X 0.5mm base gasket if you want bike to run crisp and sweet off the bottom bit more lively  but can go up to 1.5mm i think which is standard (but stand to be corrected) 

mind you they are docile like that and pretty slow reving (standard)

Base gasket thickness makes a big difference on to performance on the EVO 

 

Edited November 12, 2020 by MIckyT

[reggie]

Base gaskets come in the following thicknesses      0.2mm           0.3mm                0.5mm                 and               1.0mm

[collyolly]

I would be very surprised if it crushed down from 1mm to 0.5mm, is the whole gasket in contact or is there some left outside the joint you could measure.

Also it is an Italian bike, made in Italy, in Europe, so why would they use ‘thou’ on their gasket material unless of course they got some cheap ‘new old stock’ made by  ‘Klingers’ back in the 1970’s ?

[trapezeartist]

9 hours ago, collyolly said:

I would be very surprised if it crushed down from 1mm to 0.5mm, is the whole gasket in contact or is there some left outside the joint you could measure.

Also it is an Italian bike, made in Italy, in Europe, so why would they use ‘thou’ on their gasket material unless of course they got some cheap ‘new old stock’ made by  ‘Klingers’ back in the 1970’s ?

I agree with you on both points. I wouldn't expect the gasket to crush to half it's thickness, but I would expect some reduction and the only thickness above 0.5 is 1.0.

The point about the '40' being the thickness in thou was grasping at straws. There is absolutely no other way of identifying it.

As I have time, I think I'll reassemble with the old gasket and measure the squish. Then order and fit the appropriate new gasket.

[huski]

It must be a 0.5 gasket,surprising it didn't compress more though.You could go thinner if you want it slightly pokier but it won't make it really powerful with the thinnest 

[burns1989]

One thing to keep in mind is it will also change your port hights so a thicker gasket will advance porting moving the power up abit and a thinner gasket will lower the ports will if I'm wrong im sure some one will correct me , it will give you abit more power down low . But realistically you probably wont notice much difference 

[trapezeartist]

6 hours ago, burns1989 said:

One thing to keep in mind is it will also change your port hights so a thicker gasket will advance porting moving the power up abit and a thinner gasket will lower the ports will if I'm wrong im sure some one will correct me , it will give you abit more power down low . But realistically you probably wont notice much difference 

A thick gasket raises the port heights but also lowers the compression ratio and increases the squish clearance. I’m generally happy with the engine characteristics as they are (were) so don’t really want to move away from Beta’s recommended settings.

[pindie]

I put a 1mm base gasket in my 300 Evo. Best thing I ever did. Before it had a super thin one and it was a handful for me. With the 1mm the sting has gone and it hooks up far better and is much easier to ride. 

[trapezeartist]

So I settled on using a 0.5mm base gasket (subject to measuring when it's all installed) and then I find there are no pistons to be had anywhere in the country.???

[MIckyT]

18 hours ago, trapezeartist said:

So I settled on using a 0.5mm base gasket (subject to measuring when it's all installed) and then I find there are no pistons to be had anywhere in the country.???

Trials and Tribulation ltd have 250 evo piston kit in stock size c 

[trapezeartist]

10 hours ago, MIckyT said:

Trials and Tribulation ltd have 250 evo piston kit in stock size c 

I see it now. Not keen on using a C but if that’s all I can get. Thanks. I’ll ring them tomorrow.

[trapezeartist]

After a phone call to Beta UK, I believe I have the only B size piston in the country heading my way. Great service from them, and also South West Trials.