4bl (3mb0?) cylinder with a 3bn head?
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Hi everyone.
I'm a new DT-R owner and not so knowledged on the topic.
I was wondering whether a 4BL (possibly 3MB0?) cylinder will work on a 88' DTR with 3BN cylinder head and 3BN engine?The seller says it's 4BL cylinder from '97 bike. Are those cylinders nikasil plated?
I've been offered one at a reasonable price and i'm considering buying it.
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@Bavarian You do have to be careful on the very early DTRs that originally ran a 3BN cylinder. They have slightly larger bores and therefore shorter strokes. Running a 3MB barrel on a 3BN crank will result in the stroke falling short. Thus lost in performance. So you'll want to confirm your barrel requirements.
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What cylinder is currently on the engine? If it’s a 3BN then that is the only cylinder you have can fit without adding a later crank...
It doesn’t matter what the engine and head have stamped on them, just the current barrel that is running on the bike... -
Thank you very much for the advice.
So that means I can't simply install a 3MB cylinder on a 3BN engine and I would rather need to replace the crankshaft, connecting rod, cylinder and the head.
Is the piston size same for both 3MB and 3BN? -
@Bavarian if you have the early 3db model then no you can't. Does your engine number begin 3db?
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No, only very early 1988 models have the BN barrel & crank. Most BN engines are fitted with 3MB barrels...
What do you have fitted to the engine at the moment?...
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You’re gonna need to put a later crank in then, if you want to run anything other than a 3bn barrel...
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@Bavarian are you sure it says 3DD and not 3DB? Because only the first year dt125r was 3DB and after that it was 3RN1, 3RN2, 3RN3 etc up to 1996. The early 3DB had a bore of 56.4mm and stroke 50.0mm. after that they were bore 56.0mm and stroke 50.7mm so if you have the early one you'd think it would be quite hard to come by for a used one as it's a one year only item, but that same early cylinder of 56.4/50.0 was used in the tzr125 up to 1996 so that would make it a lot easier to find searching for tzr125 1988-96 cylinder.
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@SpookDog tell me please, I know the bore and stroke are slightly different sizes but what would be the consequences of fitting the later barrel to the early crank? Other than maybe changing performance slightly would it cause any harm?
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You would only be down on compression. The real problem would be putting a 3bn barrel on a later engine because the stroke of the crank/piston would be longer than the barrel and smack into the head...
The 3bn barrel wasn’t even a whole year model was it? My 1988 DTR125 has a 3mb barrel and crank...
Didn’t the TZR use 3mb barrels? I know I’ve used the TZR pistons in my 3mb Dtr as oversized. I’m not well versed in the history...
I wonder it the ‘R’ was for revised? Definitely wasn’t for race!...
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@Bavarian I just looked your VIN up on Yamaha France and your bike is 100% an early 3DD1 model (sold as a 3BN in the UK); 56.4 x 50.0mm bore and stroke, VM26SS round slide carb and different electrics including a separate YPVS controller on top of the airbox.
So if you were going to fit a later crankshaft and cylinder you might end up needing a TM28SS flatslide carb and different stator, loom and CDI unit to fully convert it. Is your original cylinder serviceable?
Take a look here: https://www.yamaha-motor.eu/fr/fr/service-maintenance/parts-catalogue/#/
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@SpookDog the Haynes manual says the tzr used the 56.4/50 3bn up to 1996. I did once read that the r was for replica, being a replica of the tz race bikes hence tz replica.
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I would like to add my 2 cent's.I have 3MB and 2RH(=3BN) cylinders,I've measured them off the bike and on the 3BN00 cases from my 4BL bike,and everything is the same,I think on '88 DT's and early TZR the less 0.7mm is on the crankcase side not the cylinder..
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@MadGyver That's really useful to know actually, I just assumed it would be from the cylinder...
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So, ignore the previous conception that 3bn and 3mb aren’t compatible? It makes no difference what goes on what? If that’s right it’s definitely good to know...
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@spookdog well not compatible on a DTR at least...
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@MadGyver obviously the .7mm difference in stroke would be acquired by altering the crankshaft as that's the only possible way to alter the stroke, but you're saying they both use the exact same cylinder dimensions? Did you measure the port heights because that is where we'd see these possible differences?
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@markus-w I had put the cylinder of a TZR125 2RH on my DTR,complete with piston using the DT's stock base gasket,did not mount the head.I wanted to take measurements to make a custom gasket in case I was going the TZR way and not the Athena170. Everything aligned like the stock,piston bdp was same level with the ports floor and at tdp was level with the cylinder.Put it out and started measuring the height of the cylinder and was the same with my 3MB and the Athena. Didn't do a full port map.
@Calum I didn't have 2RH crankcases to do the reverse measuring.I assumed that the 0.7 is from the cases.
I had the belief like everyone that was from the cylinder. -
Thanks everyone! That's very useful information for me.
@MadGyver so does that mean 3MB will possibly work on 3BN engine with shorter stroke?
Perhaps the difference on the stroke comes from the connecting rod? Could that 0.7mm difference in stroke cause a significant performance drop?My barrel still has some life in it although it has been resleeved and sleeve quality doesn't look that good. But I want to buy another one for spare and to play with
It seems it's quite difficult to find a 3BN one.
