gt250 ramair oddity

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dollydog
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Suzuki 2-Strokes: several gt250 ramairs

gt250 ramair oddity

Post by dollydog »

i recently stripped a ramair crank down to replace the seals etc and found these conrods in it. this is a generic photo, but my conrods are exactly the same. they are from a mid 80's yamaha yz125. anybody ever seen this before? i certainly haven't and i've stripped hundreds of gt250 engines. they seem a lot more substantial than the standard gt250 rods, so i thought it might be a common proddy racing trick? anybody know? :)
cheers, dd.
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GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
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jabcb
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Re: gt250 ramair oddity

Post by jabcb »

Your pic link didn’t work.

But that’s interesting.
T250 & GT250 has 54 x 54 bore & stroke
T350 has 61 x 54
GT380 has 54 x 54
YZ125 has 54 x 54.5 (for 2004: http://www.bikez.com/motorcycles/yamaha_yz_125_2004.php)
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dollydog
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Suzuki 2-Strokes: several gt250 ramairs

Re: gt250 ramair oddity

Post by dollydog »

what link? i get the photo straight up on the post - odd :(
i did some googling about and noticed the yz was 54.5mm stroke. perhaps the big end on the yz uses different bearings? just guessing here, and if i scratch my head hard enough i seem to remember this engine [a gt250k] having 2 head gaskets each side, which seemed a bit daft to me at the time. i measured a gt250k conrod end to end and according to my little metal ruler it was 140mm and so was the yz rod and as near as i could tell the centre of the big end eye to the centre of the little end eye was the same on both sorts of rods. hmm :D thanks for the reply anyway :)
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
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jabcb
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Re: gt250 ramair oddity

Post by jabcb »

The second head gasket likely was intended to deal with the additional 0.5mm connecting rod length.
I’m certainly no expert on this, but I’m inclined to think that it would be better to use multiple cylinder base gaskets.

The pistons in the bike might not be for the GT250K.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
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dollydog
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Suzuki 2-Strokes: several gt250 ramairs

Re: gt250 ramair oddity

Post by dollydog »

hi jab, they were definitely gt250k pistons on a +0.5mm. they were scored to hell on one side, as were the barrels. looked like one circlip each side had come out - somebody using old circlips or didn't put the new ones in properly :( threw the pistons in the scrap bin and put the barrels aside for a rebore, as the exhaust threads were good [for a change] :D
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
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Alan H
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Re: gt250 ramair oddity

Post by Alan H »

Shirley, conrod length wouldn't affect stroke, it would be crank tbrowthat gave the extra 0.5mm? The piston would only move the same distance, but slightly further up the barrel, hence thicker head gaskets. Mike Yeadon had a crank done and the company fitted (I think) KTM 125 rods which were 0.5mm longer and caused 'conflict' (read 'hitting!') between piston crown and head.
Are the ports or pistons modified on the engine as they would be operating slightly differently if the pistons were 'higher' in the bores at any crank angle compared to standard rods? Thicker base gaskets would put that to 'standard' as jab suggests.
Last edited by Alan H on Sat Jun 09, 2018 1:25 pm, edited 2 times in total.
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dollydog
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Suzuki 2-Strokes: several gt250 ramairs

Re: gt250 ramair oddity

Post by dollydog »

hi alan, i measured the rods against each other with my battery powered length doohicky :D and they were very close, something like 140.232mm against 140.234mm so i'd say they were the same length. the 16mm little ends fit perfectly in both, the big end roller and crankpin fit perfectly. i mean for all i know they could be an aftermarket product specifically for the t250 and early gt250? unfortunately there's no markings on them anywhere. the pistons were scrapped years ago and i must have 5 sets of barrels with scores in them, so i couldn't really check on the ports. the only reason i asked this question is because i was going to use them on MY NEW PROJECT :D btw, don't call me shirley :D :D :D
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Craig380
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Re: gt250 ramair oddity

Post by Craig380 »

Sounds like those rods are a good swap for OEM Suzuki, 0.002mm difference in length is probably well within Suzuki's production-line tolerances ...

As Mike said, the YZ Yam will get the extra 0.5mm stroke from the position of the big-end pin relative to the crank centre.
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dollydog
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Suzuki 2-Strokes: several gt250 ramairs

Re: gt250 ramair oddity

Post by dollydog »

hi craig, i think you mean alan, not mike :) but i think he's right. i just can't remember when this engine came to me. i keep records of all my engine numbers, size of bore, size of little end, but not where i bought them from. bit late now after 40 years :D
anyway, here's a photo of my rods, the yz type and a normal 250k.
cheers, dd.
Image
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Craig380
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Re: gt250 ramair oddity

Post by Craig380 »

Apart from the extra machining around the big-end eye, they look identical ... that extra machining is probably good for big-end lubrication too :up:
1976 GT380 - wounded by me, and sold on
2006 SV650S - killed by a patch of diesel and a kerb in Feb 2019
2017 SV650 AL7 - naked and unashamed
dollydog
Yeah Man, the Interstate
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Suzuki 2-Strokes: several gt250 ramairs

Re: gt250 ramair oddity

Post by dollydog »

hi craig, the big end is a lot heftier, slightly wider and much thicker. when i get back up the shed i'll weigh them for comparison. i'll edit this post when i've done it.
cheers, dd.
right, i weighed both.
yz type are 494 grams each
gt type are 420 grams each.
so quite a difference :)
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Vintageman
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Re: gt250 ramair oddity

Post by Vintageman »

DD,

What is the Part # for the YZ125 rod if you know? What year yz125. If that truly fits a GT250K/T350 that is a sweat find... getting hard to find the early GT250 Con Rod.
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