Deck height - RM250T

[Lozza]

Squish should be 1% of stroke 1.3mm is not even working, and 40 thou is a mm  Don't worry about the old piston, length of the rod or cylinder etc etc etc, just work with what you have.

[Graeme M]

Good plan Lozza, it's what I'll do. Thanks fore the thoughts re squish - I was clearly well off the mark. That's why I am not an engine builder I guess...   

[whitey 43]

I got caught out with a YZ80 i did up for my son...Some "Tuner" had fitted a short stroke rod from a 84-85 model (from memory) then machined the base off the cylinder. Whilst apart, they re cc'd the head to approximately 1 Lt. So in the end when nothing fitted, I called a wrecker who measured a couple of cylinders in stock.
Some tuners are brilliant & some are just wackos!
I would call some werckers if you want to confirm whats been done to cause your issue...

[GMC]

Don't worry about the old piston, length of the rod or cylinder etc etc etc, just work with what you have.

I disagree, with statements like this...

[quote ]
One curious thing about the bike is that from when I bought it (and I have done no more than maybe 2 hours on it), it has always run strangely - it will not rev freely, sounding like it is strangled or something. Yet on the bike it revs out OK, though without any noticeable 'hit'. I am wondering if the piston has always been close to the head and the new piston etc has just slightly exacerbated that situation. The bike itself is a bitza of sorts and I have no idea of its history.
[/quote]

I would be checking out what has been changed otherwise you may be forever chasing your tail with running / tuning issues

[Lozza]

 Time you spend checking this and that is stopping you from sorting what you have. Bum dynos are not very reliable

[OverTheHill]

Bought some gasket paper 18 months ago from Repco [in nz] which was, by memory, 0.8mm & 1.6mm thick. Theory at the time [think i had one] was 490k was sucking in base gaskets so made a 1.6 gasket slightly over hanging the outer edge & the base sort of bit into the paper when compressed & never did it again due to the slightly un-squashed overhang [so far], didn't affect the running at all [slightly lower com etc.]. What i'm saying is--should be thick gasket paper out there if you want to raise it up in one hit rather than multiple gaskets. Myself i'd probably sit it at bdc, look at the ports & go-ok take a mm of the top of the piston if you can get away with it [ie top ring too close etc]. Not sure without looking if the T is a long stroke 68mm or still yamaha 70x64+stroke but bought a really good looking [& working] "Namura" Brand piston kit off Ebay for my RMX250 [1992] 68mm bore--have a look at them. Rm Thanksx is case reed so no holes in skirt but like i say, 68mm. Anyway, another 'food for thought' idea.

[OverTheHill]

Sorry if i'm getting of the beaten track a bit but, of course, from a 'not wanting to rev' point of view just make doubly sure the pipe is clean right through. I had an IT something [465 i think] bought to me in desperation years ago after 'everything had been tried' according to the owner but still had the restrictive original tail muffler on which was blocked in the spark arresting little [jet engine looking] things, but my main point being it had built up in the tail pipe forward of the muffler so much it continued to strangle the top end. Easy fix & the easy bit is when someone says [i've done everything & lists them] you just go to what's 'not done' & they think you magic [just lucky], although sounded pretty obvious in the running at the time i must say.

[Andrew L]

Super crap autos has 0.4, 0.8, 1.6mm gasket paper 230mm wide x 1m long.

[Graeme M]

Can you use that gasket paper? I figured it was just for oil jointing surfaces, not for the heat of a base gasket?

[crossedup2]

The gasket paper will do the job as long as you take your time cutting it.

Good luck with it Graeme.

Peter B

[Graeme M]

Thanks Peter. I also found a place to turn me up a new head gasket if need be to get that squish right, so with any luck it'll be a goer in a couple of weeks.

[Montynut]

I am sure someone will correct me but if you have the piston poking 1-1.5mm out the top of the cylinder just putting a thick head gasket on or machining the head will not correct the seriously detuned port timing. Putting a 1.6mm base gasket will give you a seriously unstable crankcase to cylinder joint, I mean 1.6mm gasket paper is not very tuff stuff to bolt your cylinder to after all it would allow movement which is not good. If the piston is that far out of the bore the rings would be very close to being exposed also not good. All the solutions suggested are heading for a very very messy pile of twisted and shattered parts I think. It seems to me that you have something seriously wrong here.

Either the cylinder is incorrect or has been machined for some reason either top or bottom. More likely the incorrect conrod has been fitted maybe from a PE or RM-A (the A had a different stroke I think). There must have been an incorrect piston fitted before this rebuild for it to run.

It seems to me that you have to find out what is wrong not try to engineer around an obvious serious mismatch of parts. Just my thoughts

[Stan S]

Graeme,
I strongly agree with GMC and Montynut,, you need to get to the bottom of this problem.

Stan.

[Graeme M]

I will be having another look at it tomorrow as well as pop the cylinder off and see what I can see. I'll report back what I find...

[OverTheHill]

just one more thought, you could make an alloy base plate & just use something like 'loctite jointing compound' [or something] either side of it, it'll seal if the surfaces are in reasonably good nick. We all want to know the outcome now!!, good luck.