A forum for owners of Yamaha TTR250 trail and enduro bikes!

There are quite a few web sites that give CR measurement details. 

This one is fairly comprehensive - CLICKY - and gives formulae and diagrams that help.

I've never bothered too much and am satisfied if my engine fires up and works after any work I've done.

Martyn

Thanks Chaps,

That calculation is exactly what I have been doing - right in to the nitty detail of gasket thickness etc. There's no need to measure from BDC - swept volume is trusted by the given throw of the crankshaft --> 59.6mm. It's merely the combustion (clearance) volume that needs measuring. The method is by using calibration fluid and measuring by volume (ml's or cc's). This can be done with a completely built top end by sealing the top piston land with grease or by measuring part by part using a flat Perspex plate sealed with grease. The engine built method is a) blind b) prone to trapped air c) best done with engine out so it can be rocked to remove the air

OK so I've spotted my school boy mistake - the micrometer leant to me is imperial - I was reading it wrong, oh back to my Apprentice days

So a correction on the standard head gasket compressed thickness is actually 0.58mm at the flame ring (not 1.55mm!) So if anyone is interested the standard squish I make about 0.58+0.27 (to 0.28) = 0.85mm --> That is not a lot and is a clear indication of why piston contact can easily occur through a worn little end, built up carbon or excessive piston slap (worn bore). Another point - a conrod failure mode is always in extension as on the unloaded stroke (exhaust) the mass of the piston tries to accelerate away from the conrod. Squish is usually reduced by removed material from the barrel (not the head) --> I have seen enough here to take no risk, besides I know I have a worn little end. #futurerebuild

Method 1 I obviously had trapped air so I am scrapping this. Method 2 with the corrected gasket thickness and I think I leaked calibration fluid when measuring the piston valve pockets. So the new calculation: -

252.88cc swept volume
24cc combustion chamber in head
0.8cc & 0.6cc approx. for valve cut outs in piston
Gasket is 0.58mm with a 74mm opening making 2.49cc
Piston decking height (crown of piston to top face of barrel) is 0.27mm at 73.5mm bore making 1.14cc
Total clearance volume therefore 29.04cc making the compression ratio 9.71:1

Given measurement errors I am going to live with that. I agree with you Kaos about the standard CR not being what it says on the tin. I echo this by saying that I work for a major Diesel Engine Manufaturer and even on our brand new concept engines we only measure the piston decking (topping) and the volume of the combustion chamber parts (piston bowl and valve pockets). The rest is all theoretical. TBH to an engine manufacturer it's not really that important as true cylinder pressures are read whilst the engines are running to establish the limitating factors.

I won't bore you anymore but I am here if anyone wants to try a bit of light tuning in the future. I'll attach some piccy's shortly of the work I've been doing

I forgot to attach this photo from when I first got this engine apart. You can clearly see where the piston has been contacting. I had an excessive build up of Carbon on the cylinder head in that spot - when I bought the bike the cam timing was all over the show, timed incorrectly and with a stretched chain, plus with aftermarket exhaust and filter but no jet changes, so I've probably been running a bad mixture.

TTRfan after this evening I kind of agree with you...

Measuring the combustion volume in a built up engine in a frame always gives a low measurement for me despite chocking the front wheel up and ratchet strapping the back down to try and make the engine 'horizontal' - to get all of the air out. I am somewhat 1.5cc's away from where I need to be. The compression ratio is allegedly high but I don't believe it.

But also this new Wiseco piston has a crown that rises so it's pretty much impossible to measure calculate the new volumes at piece part level. So, I don't really know what compression ratio this alleged 10.5:1 will give ! I can see that rise in the crown may almost be counteracted by the enlarged valve pockets but without making an imprint and measuring I can only assume that the enlarges crown is designed to raise the compression.

However what I do know is that I measured squish and gasket thicknesses. I conclude that the Cometic head gaskets sold on totallyttr are 0.96mm thick compared to the Yamaha MLS at 0.58mm. The base gasket is only about 0.02mm thicker and seems to be of a harder material. What does this mean?

Well it actually means that all that performance gain you were expecting from this piston may not be so, whilst using these cometic gaskets. Because I measured the squish as 1.27mm on the inlet side whereas with the standard pistons and gaskets it was 0.86mm. Conclusion: 1. I can't really establish what the true compression ratio is, 2. I wouldn't use the cometic head gasket with this piston as it reduces the squish. Check out Wikipedia if you want to learn more but less than 1mm is a good place to be. 3. I would say for the level of hp these bikes create, if one doesn't pay attention to detail for this type of upgrade as to setting the compression and squish correctly then you may as well not bother and just stay completely standard. The power gain you 'feel' could be just through a new bore & rings, 4. I would trust that Wiseco have designed at 10.5:1 for use with standard gaskets by measuring a standard piston as a reference - I can't see for every piston they develop that they have a bike in the workshop and spend hours playing with it....

One thing for sure, this is a "safe" build

Alternative solutions - I have been told by a friend that you can use a dremel to open up a standard MLS gasket and this is what drag race engine builders often do. Unfortunately for me I am against the clock and I need to get this build completed (no Yamaha gaskets on Ebay) so it's going back together using the cometic gaskets. I have just completed the 6Sigma carb upgrade including 150 jet and need to do the airbox mod all to match the FMF and filter I added last year.

One additional comment on a fantastic post about this Wiseco piston I spotted on the forum - mentioning the increased squish areas. Unfortunately these increased areas go under the combustion chamber in the head so could they really be considered as extra squish?

Of course, I could be eating humble pie in a few days time when I get her running! I will report!e

Stewy,

Looking good there.

My bike didn't need to have the head skimmed, I asked for higher compression than what the wiseco kit offered. I asked for more compression but with safe Piston to Valve clearances.
My TTR doesn't see any serious road miles its just a play bike for bush trails
I can't comment on how the numbers were worked out, but the motor was built professionally in a workshop. He came up with 10.5 : 1 as the final compression with the wiseco 10.5 +2mm piston AFTER shaving the head.

As for the end result, it is remarkable. The exhaust note is different, it sounds more purposeful. As soon I eased the clutch out the first time I could tell it had heaps more torque. The motor is a lot stronger every where.
I want to put about 500 km on it and I'll return it to the shop for a dyno run.

leigh wrote:

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Great info, excuse my ignorance, why does a standard head gasket need to be modified to use?

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 The thickness of a head gasket will either rise or lower the whole head depending on the gasket thickness. This will affect the compression ratio.

If the stock gasket is thicker than a replacement then the ratio would be lower.

If it was thinner then the ratio would be higher.

As the comedian used to say, "though not by much"

Martyn