OzVMX Forum
Marque Remarks => Suzuki => Topic started by: Graeme M on April 22, 2013, 08:20:35 pm
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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?
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The piston shouldn't protrude past the top of the cylinder, either you have the wrong piston or the cylinder has been machined.
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Piston is NOS Genuine Suzuki Part of the right model
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Did you keep the old piston?
If so then drag it out and compare it to the new one.
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How thick is the base gasket compared to the original one
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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.
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Alpha list an optional thicker head gasket
http://www.alpha-sports.com/suzuki_parts.htm
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Just machine the head rather than run a 2 head gaskets and a ultra wide squish.
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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? ???
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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.
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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.
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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.
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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..
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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.
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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.
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Squish should be 1% of stroke 1.3mm is not even working, and 40 thou is a mm ;D Don't worry about the old piston, length of the rod or cylinder etc etc etc, just work with what you have.
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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... ;D
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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...
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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
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Time you spend checking this and that is stopping you from sorting what you have. Bum dynos are not very reliable
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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.
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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.
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Super crap autos has 0.4, 0.8, 1.6mm gasket paper 230mm wide x 1m long.
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Can you use that gasket paper? I figured it was just for oil jointing surfaces, not for the heat of a base gasket?
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The gasket paper will do the job as long as you take your time cutting it.
Good luck with it Graeme.
Peter B
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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.
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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
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Graeme,
I strongly agree with GMC and Montynut,, you need to get to the bottom of this problem.
Stan.
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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...
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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.
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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?
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Have you measured the stroke? That will tell you if it’s the rod or crank that have been modified..
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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.
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take the barrell off and see if there the right part number on the rod
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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.
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sounds like incorrect rod to me.
this would be the only time any bloke is disappointed to have a longer one.
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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.
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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.
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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 ;)
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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.
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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.
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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.
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What about a second hand barrel and go back to stock etc. My barrel was stock with only some polishing and went quick.
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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!
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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
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Thats a lot of money for a cyl on last oversize ???
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Sorry than. In description it says 68mm but than it also say 080 which is +2.00mm. you are right TM bill. you never know what you going to get with this e-bay thing.
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Pe 250 T/X are.... in my opinion the same barrel.
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OK... a custom 1.2mm head gasket and two base gaskets later and I have a runner. Rode it at a local club dirt track day on Sunday and was surprised how it went. Took it pretty easy as it needed the piston run in but all seemed good. What is funny is that it has a heavily ported barrel yet the power seems very flat, no big hit anywhere. That said, it was noticeably faster than a YZ250H (which was also being run in) and a couple of MX250s, so it seems to be a decent goer.
All good, it seems!
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Probably pay to keep an eye on the head nuts every few rides.