AIRCOOLEDRDCLUB FORUM

Duffy , have you done many miles on yours? I have had a few replies to the Mitaka pistons and there use on another thread on here, with a couple of members who have them fitted and done a good few miles without any issues ( albeit they are for one of them more noisy) . Thanks ( sorry Colin don’t want to go jack the thread )

Hi Mally, I brought it this year from a guy who said it had a rebore by the previous owner but he was a bit vague about it. I sorted some wiring issues and had only done a couple of miles on it at less than 5,000 rpm.
I decided to give it a run to Box Hill along the M25 and it didn’t seem to pull past 7,000 rpm and didn’t have the expected power band I was hoping for. I dropped down a gear and felt it tighten up, so I swiftly pulled the clutch in and coasted to the hard shoulder. I left it about 15 minutes to cool down and it started ok without any nasty noises. I then rode it to Box Hill and back(around 50 miles) but didn’t go over 4-5000 rpm and it seemed ok.
I did however notice two stroke oil around the right hand cylinder. I haven’t ridden it since,and haven’t inspected it either.
My initial thoughts were that the oil pump was ok at lower revs but wasn’t putting enough into the cylinders at higher revs, but this was just a guess without any inspection as I found a receipt for an oil pump with the paperwork.


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I offer a full restoration service on the pumps.
You mention the correct shims fitted, this means the correct ones to achieve the correct minimum stroke, is this the case?
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That’s what I am hoping to get is some idea of what pistons owners have fitted ( other than original or Mitaka ) mainly for the 250D but for other models to get a steer on what is available and what works . Noticed the Mitaka listing on their website says at the top 250, 250DX and LC but the chart at the bottom only lists for LC , hmmm 🤔.

Don't forget, a badly rebored rattly engine is just as likely to seize by breaking something. When I bought my 250B, it had 350 barrels on it, which had badly seized. It was well oversize and I bought Mitaka pistons and had it rebored. It goes ok, but is so noisy. In the end I put it back to a 250 using genuine Yamaha pistons and it is so much quieter. I'm lucky with the B model, as there are a lot of pistons out there, and there is no way I would use Mitaka watercooled pistons in it while I can get genuine pistons. Your not so lucky!

I guess what people are advising is, if your engine is within Yamaha spec, why use pistons for a watercooled engine if you can buy genuine pistons, especially an RD250C for example. How can you tell how much the engine rattled with noisy expansion chambers on the bike, and lets face it, 1500miles isn't very far is it! about a month and a halfs riding back when they were used properly. At the end of the day, it's your bike and your engine so do what you want, it is just that people that have used and repaired these bikes for 40+ years advise aircooled pistons in aircooled bikes. As Fleetwood Mac once said, "You Can Go Your Own Way" if you don't mind a rattly engine, thats fine.