Deck height - RM250T

[Graeme M]

OK, popped the barrel off. No evidence that the bottom of the cylinder has been machined, or at least if it has been it was long ago. All looks quite normal. Top of cylinder looks fine too but there is a fine pattern of parallel lines across the whole mating surface. Whether that is original factory machining or not I have no idea.

The piston protrudes almost exactly 1mm above the cylinder deck. The ring groove is 2.6mm below the top edge, so the ring is about 1.6mm below the deck at TDC. I can just see the ring locating pin's top edge. The top edge of the piston aligns with the bottom of the transfers and the exhaust port at BDC.

Thoughts?

[Viper79]

Have you measured the stroke? That will tell you if it’s the rod or crank that have been modified..

[JohnnyO]

Measuring the stroke won't tell you if a longer rod's been fitted, the stroke stays the same. You can't tell if the barrel's been machines by looking at it, you need to measure it against a std one. The top and bottom of the barrel are machined to spec at the factory so it's no point looking for machining marks.
From what you've said so far with the top of the piston lining up with the bottom of the exhaust and transfer ports it's sounding like the top of the barrel has been machined.

[paul]

take the barrell off and see if there the right part number on the rod

[oldfart]

While the barrel is off check these measurements against your one ( taken from a NOS barrel still in box ) Rm 250 T
total length of sleeve 134.32mm
bottom of sleeve to gasket surface 17.34mm .
  *Rod will not have a part # stamped on it.

[Hoony]

sounds like incorrect rod to me.

this would be the only time any bloke is disappointed to have a longer one.

[JohnnyO]

If it is the wrong rod you just need a spacer plate cut from aluminium sheet to raise the barrel the same amount as the extra length of the rod so that the top of the piston is flush with the top of the cylinder.

[Graeme M]

Thanks for the numbers Oldfart, now we know where the problem is. Bottom of skirt to surface: 17.34mm check. Total length of sleeve: 133mm...

Someone's machined the top of the cylinder. So, just need a thicker head gasket and we're good to go huh?

I measured the base gasket at .47mm, so I think two base gaskets would be good to just lift that barrel a tad to give a bit closer to correct deck height before I calculate the right thickness head gasket. Is that likely to upset port timing to much? Bear in mind the things been pretty seriously ported so who knows what the timing is now. I don't care that much, as long as it goes really.

[Lozza]

Have 1, 1.25,1.5, 1.75 and 2mm gasket paper here and it's tough as billy goats knees. Using 2 gaskets is asking for trouble. As I keep saying machine the head to get 1-0.95mm squish, if the bottom of the transfers and timing edge of the piston are level at BDC that cylinder is in the correct position.

What are the port heights from TDC to when the exhaust and transfers open? You can make much more mid and top end HP with lower ports 

[GMC]

Now that we know the top of the cylinder has been reduced it’s easy to advise that  the best thing to do is to effectively put it back again.

Either machine the head or make some special head gaskets.

If you machine the head and you one day want to buy another barrel then you will need another head as well.

If you make new gaskets then make several to make future top end jobs easier.

I wouldn’t recommend adding 2 base gaskets either, some bikes blow base gaskets out quite easily as it is.
Also by raising the barrel you are raising the exhaust port which will make the engine peakier, I assume you want to avoid this?

# Theory 1
Perhaps the old piston may have been machined to fit, lowering the crown of the piston would effectively raise the exhaust port which would make it more peaky giving you the impression it had been ported.

# Theory 2
Perhaps the top of the barrel had been machined so thicker base gaskets could be used to raise the ports but somewhere through different ownerships this info was lost.

[Graeme M]

Theory #2 is interesing Geoff. The barrel has been seriously ported, ever port has had work tho I am no judge of whether it was done by someone who knew what they were doing. Peakiness is of no concern to me as the bike is primarily a dirt track/grass track ride. I will never again go near an MX track! All I really want is for it to go passably well. I like the idea of using a 1mm base gasket (stock measures at about .47mm, but that's after having been on the bike), and getting a head gasket made. However as Lozza observes some machining may be required as head gaskets are probably only able to be made from 1, 1.25, 1.5 etc sheet.

[JohnnyO]

You're probably better off machining the squish on the head if your barrel still has plenty of life left in it and you don't intend buying another, then you can run std head and base gaskets. Fitting a thick base gasket and raising the barrel (exhaust port) will make it harder to ride, even on dirt track unless you're a Pro, especially with it being already ported now.

[suzuki27]

What about a second hand barrel and go back to stock etc. My barrel was stock with only some polishing and went quick.

[suzuki59]

I can vouch for suzuki27 statement,his old bike was ridden at warp speed over here in the recent mr vmx series by a guy called Nick Prescott -it flew!

[Tomas]

There is nice RM250 cylinder for reasonable price on e-bay. reed valve is bolted on.

http://www.ebay.com/itm/SUZUKI-VINTAGE-MOTOCROSS-RM250-RM-250-CYLINDER-WITH-REEDS-1980-80-AT-68MM-/281098136069?pt=Motorcycles_Parts_Accessories&hash=item4172c12a05&vxp=mtr

head machining an custom gasket will probably cost just as much.
hope it helps