1oldbanjo Posted January 31, 2011 Report Share Posted January 31, 2011 My 74 RL250 was getting a bit noisy so I took the piston and cylinder off to see how things looked. The crank looks fine and I cannot feel any play in the crank or rod bearings. The piston had a crack on both sides that runs straight up from the piston pin - but the crack is tiny and doesn't go too far up the piston. I cleaned the piston and cylinder and a 0.009" feeler gauge slips in easily and a 0.010" is snug - I doubt if I could get a 0.011" gauge in without a good amount of force. The piston is the stock 70mm size. I ran a hone in the cyclinder and it is obvious there is some uneven wear as there were high and low spots as indicated by the uneven honing marks. I have a new piston and the clearance is very close to what the used one measures - it is about 0.009". What is the maximum clearance that is allowed and is 0.010" the probable cause of my noisy bike? Quote Link to comment Share on other sites More sharing options...
copemech Posted February 1, 2011 Report Share Posted February 1, 2011 Jeez that sounds loose! Best I recall, air cooled 2T is set up a bit looser than the watercooled bikes , but not too much. I am thinking the clearance on a new piston should be about .002in, wear limits about .006 or a rebore is needed(if it is steel)and first oversize piston. PM Jon,(JSE)to confirm my impressions if he does not pick up on this thread. Quote Link to comment Share on other sites More sharing options...
beamish owners club Posted February 1, 2011 Report Share Posted February 1, 2011 My 74 RL250 was getting a bit noisy so I took the piston and cylinder off to see how things looked. The crank looks fine and I cannot feel any play in the crank or rod bearings. The piston had a crack on both sides that runs straight up from the piston pin - but the crack is tiny and doesn't go too far up the piston. I cleaned the piston and cylinder and a 0.009" feeler gauge slips in easily and a 0.010" is snug - I doubt if I could get a 0.011" gauge in without a good amount of force. The piston is the stock 70mm size. I ran a hone in the cyclinder and it is obvious there is some uneven wear as there were high and low spots as indicated by the uneven honing marks. I have a new piston and the clearance is very close to what the used one measures - it is about 0.009". What is the maximum clearance that is allowed and is 0.010" the probable cause of my noisy bike? Is that measurement at the top of the piston or at the skirt? 0.010" = 0.4mm, the 2 stroke piston is tapered slightly so the skirt is in contact with the bore at the bottom and the rings for a seal at the top. Is there a 'step' in the bore right at the top? Quote Link to comment Share on other sites More sharing options...
1oldbanjo Posted February 1, 2011 Author Report Share Posted February 1, 2011 Is that measurement at the top of the piston or at the skirt? 0.010" = 0.4mm, the 2 stroke piston is tapered slightly so the skirt is in contact with the bore at the bottom and the rings for a seal at the top. Is there a 'step' in the bore right at the top? The feeler gauge I used was as long as the piston, and the largest measurement I could get was with the piston pushed down about an inch from the top of the bore - the feeler guage was measuring along the cylinder wall just above the exhaust port. Quote Link to comment Share on other sites More sharing options...
monty_jon Posted February 1, 2011 Report Share Posted February 1, 2011 The feeler gauge I used was as long as the piston, and the largest measurement I could get was with the piston pushed down about an inch from the top of the bore - the feeler guage was measuring along the cylinder wall just above the exhaust port. Don Quote Link to comment Share on other sites More sharing options...
beamish owners club Posted February 1, 2011 Report Share Posted February 1, 2011 Is there a 'step' in the bore at the top? If you have cracks in your piston I would bin it anyway! Quote Link to comment Share on other sites More sharing options...
jse Posted February 1, 2011 Report Share Posted February 1, 2011 Jeez that sounds loose! Best I recall, air cooled 2T is set up a bit looser than the watercooled bikes , but not too much. I am thinking the clearance on a new piston should be about .002in, wear limits about .006 or a rebore is needed(if it is steel)and first oversize piston. PM Jon,(JSE)to confirm my impressions if he does not pick up on this thread. Mark, Just picked up on this and will give some info that might help (hopefully). .009" is way out of spec. and the stress cracks are a result. You are very lucky as this kind of clearance usually results in skirt collapse and a very messy crankcase. Assuming an iron/steel liner (not plated), from the new piston clearance at .009" it is apparent you will probably have to go to an oversize piston and re-bore, not a hone, as the cylinder is tapered and you will usually not get the walls straight even with a rigid hone. A dial bore indicator will give you a direct readout of cylinder wear, if you have one. In the newer type watercooled engines the piston clearance is in the area of .0015/.002" with a forged piston. I'm guessing that the RL is a cast piston and the air cooled engines operate at a wider bore distortion/temp range, so the clearance will be a little more (even with the lesser expansion rate of the cast, compared to forged, piston) probably in the .0022/.0024" (.055/.060mm) range (the Suzuki guys will know the correct clearance, I'm sure). Two-stroke pistons are usually tapered and cam-ground to expand to appropriate cross section under operating temperature. 2T pistons are exposed to unusual temperature influences, unlike a 4T piston, with intake/transfer/exhaust ports having much different charge temperatures. Piston skirt clearance is measured at the lower 15mm of the piston skirt on the thrust surface, which is the front or back, not at the pin side, which will give a larger measurement and not the one you want. Jon Quote Link to comment Share on other sites More sharing options...
copemech Posted February 2, 2011 Report Share Posted February 2, 2011 So there you go, OK iI was generalizing here and Jon caught me at being off by approx .0003 or so. He can be such a pain sometimes! But long made short, if you can pass .009 between the skirt and the bore within normal piston travel, there is an issue. There are much better ways of measureing all this, but basically what you have said means it needs to go to a reputable shop with a boreing bar and a decent tech. There should be someone local. Good news also is the RL and enduro pistons were the same as I recall, so oversize should be readily available in factory of aftermarket I would think, and not too pricy! Quote Link to comment Share on other sites More sharing options...
jse Posted February 2, 2011 Report Share Posted February 2, 2011 So there you go, OK iI was generalizing here and Jon caught me at being off by approx .0003 or so. He can be such a pain sometimes! But long made short, if you can pass .009 between the skirt and the bore within normal piston travel, there is an issue. There are much better ways of measureing all this, but basically what you have said means it needs to go to a reputable shop with a boreing bar and a decent tech. There should be someone local. Good news also is the RL and enduro pistons were the same as I recall, so oversize should be readily available in factory of aftermarket I would think, and not too pricy! I was only guessing, Copey, and you are correct, the best thing would be to have a good tech guy look at it. It's always a tough game to talk about mechanical measurements, as .009" on a piston may be fine as measured in one place and not-so-fine in another. I think we can all agree that cracks anywhere on a piston are cause for replacement. Jon Quote Link to comment Share on other sites More sharing options...
1oldbanjo Posted February 2, 2011 Author Report Share Posted February 2, 2011 Thanks for everyones input. I have a new piston that is standard size and the measurements for the new piston versus the old one are only about ).001" tighter with the new one. I don't have a factory manual and wanted to know at what point I was due for an oversize piston and rebore of the cylinder, and it sounds like I am there. I plan on sending the cylinder to B&J Racing for boring and they are very familiar with the RL250 and should be able to get it right. Here is a photo of a change I have made to the clutch. I bought a TM250 clutch cover and stripped the black paint and polished the case. I installed a TM250/400 clutch basket cover with the cogged release actuator and installed a TM250/400 clutch cable. I removed the center push rod and spiral clutch mechanism from the magneto side of the engine and plugged the oil seal. I have not ridden the bike yet at the cylinder is still off - but the clutch pull is far easier and much smoother now. I can't post pictures as I don't have any place to link the pictures to - but here is an eBay add with pictures and I am selling a second polished case that I had. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=320649337217&viewitem=&sspagename=STRK%3AMESELX%3AIT Quote Link to comment Share on other sites More sharing options...
jse Posted February 2, 2011 Report Share Posted February 2, 2011 Thanks for everyones input. I have a new piston that is standard size and the measurements for the new piston versus the old one are only about ).001" tighter with the new one. I don't have a factory manual and wanted to know at what point I was due for an oversize piston and rebore of the cylinder, and it sounds like I am there. I plan on sending the cylinder to B&J Racing for boring and they are very familiar with the RL250 and should be able to get it right. Here is a photo of a change I have made to the clutch. I bought a TM250 clutch cover and stripped the black paint and polished the case. I installed a TM250/400 clutch basket cover with the cogged release actuator and installed a TM250/400 clutch cable. I removed the center push rod and spiral clutch mechanism from the magneto side of the engine and plugged the oil seal. I have not ridden the bike yet at the cylinder is still off - but the clutch pull is far easier and much smoother now. I can't post pictures as I don't have any place to link the pictures to - but here is an eBay add with pictures and I am selling a second polished case that I had. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=320649337217&viewitem=&sspagename=STRK%3AMESELX%3AIT Beautiful cover. Bob Ginder at B&J will know exactly what to do, he is very well-versed in all-things-vintage. Jon Quote Link to comment Share on other sites More sharing options...
copemech Posted February 3, 2011 Report Share Posted February 3, 2011 I was only guessing, Copey, and you are correct, the best thing would be to have a good tech guy look at it. It's always a tough game to talk about mechanical measurements, as .009" on a piston may be fine as measured in one place and not-so-fine in another. I think we can all agree that cracks anywhere on a piston are cause for replacement. Jon Say NO to Crack! Quote Link to comment Share on other sites More sharing options...
1oldbanjo Posted February 21, 2011 Author Report Share Posted February 21, 2011 (edited) I hate stories without a happy ending.....so here are the results. I bought a new Suzuki Piston, rings and gaskets. The new piston had about 0.002' clearance when measured at the skirt with the piston upside down - and the ring end gap was about 0.016". It all went together well and it runs fine. It now makes less noise - but I guess with all those fins it will always make more noise than a water cooled bike. The original rubber spacers that go into the fins were hard and brittle and I have not replaced them as it just doesn't seem like $ 40 that would be well spent - maybe if I watch eBay long enough I can find an affordable set some day. Thanks to everyone that gave me advise - my initial measurements were just too big as I was measuring the top of the piston....and not the skirt. Edited February 22, 2011 by 1oldbanjo Quote Link to comment Share on other sites More sharing options...
jse Posted February 22, 2011 Report Share Posted February 22, 2011 It now makes less noise - but I guess with all those fins it will always make more noise than a water cooled bike. The original rubber spacers that go into the fins were hard and brittle and I have not replaced them as it just doesn't seem like $ 40 that would be well spent - maybe if I watch eBay long enough I cand find an affordable set some day. If you don't mind not having the OEM fin inserts, you can use new rubber fuel line cut in 2-3" lengths and forced in between the fins. If you are craftsman-like and use the appropriate OD line (which you can get in black or grey for polished fins), it can look quite good. Jon Quote Link to comment Share on other sites More sharing options...
jse Posted February 23, 2011 Report Share Posted February 23, 2011 If you don't mind not having the OEM fin inserts, you can use new rubber fuel line cut in 2-3" lengths and forced in between the fins. If you are craftsman-like and use the appropriate OD line (which you can get in black or grey for polished fins), it can look quite good. Jon Here's what I meant. Quote Link to comment Share on other sites More sharing options...
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