Deck height - RM250T

[Graeme M]

I have a strange issue with my RM250T - the new piston contacts the head. On checking, I can see that the piston top edge is about 1mm proud of the cylinder gasket surface. With a single standard head gasket the piston contacts the head, with two gaskets the squish clearance is 1.37mm. I have no idea what the standard clearance should be, but 1.37mm would be in the ballpark. A head gasket is 1mm thick. So, right there, it looks like the 1mm that the piston clears the cylinder by is the problem. I am just trying to confirm if in fact the piston edge on an RM250T should be flush with the cylinder surface. Anyone?

[JohnnyO]

The piston shouldn't protrude past the top of the cylinder, either you have the wrong piston or the cylinder has been machined.

[SON]

Piston is NOS Genuine Suzuki Part of the right model

[GMC]

Did you keep the old piston?
If so then drag it out and compare it to the new one.

[Andrew L]

How thick is the base gasket compared to the original one

[Graeme M]

The obvious question Geoff. Sadly no, I don't have the old piston! But I can say that the new head and base gaskets are OEM, and that the gaskets on the bike when I got it are definitely thinner (ie used) than the new ones. The piston is, as SON notes, the correct piston. The old piston and combustion chamber were seriously beat up as something has been in there while the engine was running.

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.

Anyway, the main thing I wanted to learn is the correct deck height so's i can adjust gaskets etc as necessary.

[TM BILL]

Alpha list an optional thicker head gasket

http://www.alpha-sports.com/suzuki_parts.htm

[Lozza]

Just machine the head rather than run a 2 head gaskets and a ultra wide squish.

[firko]

Just machine the head rather than run a 2 head gaskets and a ultra wide squish.

I might be having another pensioner moment and be missing the obvious here but I read it that Graeme's problem is the piston sitting 1mm proud of the gasket surface. Wouldn't machining the head make the problem worse?

[head]

Does the piston at bottom dead centre (BDC) sit level with bottom of transfers and exhaust ports. If not you may need thicker base gasket to adjust.

[Graeme M]

Yeah Firko, I didn't follow Lozza on that one. Good point Head, I'll check that out. Re ultra wide squish, does anyone know correct squish clearance for the RM? I did some research and found that around 1.3mm (.040") should be a fair middle of the road number for a 250 2T. Is that not correct?

In the absence of any good idea why things are as they are, I want to fiddle base and head gaskets to get correct squish. I needed to know deck height so I had a baseline compared to my situation. But I also need squish clearance.

[GMC]

Pretty certain he means machine out the combustion chamber to allow clearance for the piston.
First thing though is to determine what has changed.
The motor may even have a different rod kit installed

If the bottom of the barrel has been machined then it will be affecting the port heights and thicker base gasket may be the better fix.

Yes it was the obvious question Graeme but even a knocked around piston should still be able to have the deck height measured.

[JohnnyO]

Pretty certain he means machine out the combustion chamber to allow clearance for the piston.
First thing though is to determine what has changed.
The motor may even have a different rod kit installed

If the bottom of the barrel has been machined then it will be affecting the port heights and thicker base gasket may be the better fix.

Yes it was the obvious question Graeme but even a knocked around piston should still be able to have the deck height measured.

Yes all of the above..

[firko]

Pretty certain he means machine out the combustion chamber to allow clearance for the piston.

It seems a bit drastic . There has to be an obvious reason why a piston that's supposed to be the correct item doesn't sit right.....something has changed from standard, my guess is that it's got a longer rod than standard but I'm no Ernst Degner in this department . It's a pity you didn't keep the old piston for reference Graeme.

[JohnnyO]

Like Geoff said, either the bottom of the cylinder has been machined to lower the ports, top of cylinder machined for more compression or a longer rod fitted from another model. I've seen all done before and without knowing the history of the bike anything is possible..
Only way to tell is measure the cylinder against another original one.