Author Topic: Rebuild of a RD 250 engine for racing  (Read 410 times)

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Offline Drehmoment

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Rebuild of a RD 250 engine for racing
« on: October 19, 2021, 01:31:37 PM »
Hello Everybody,

i ruined my 250 engine in my last (and first since 15 years) race because I did not open both fuel valves. yes yes I know... Lets not talk about it...

Now I want to rebuild the one I killed and maybe build a second as spare.

I hope to find some knowledge here about some questions I have regarding the racing engine.

Currently I run 2R8 cylinders which are modified to accept TZ exhausts. These exhausts you see in the picture where used by me simply beacause I did not have any other and could not afford anything else. Now the engine runs good and gives good power, but I think it is quite high in the rpm range. the bike starts to pull quite early and revs to the moon, but I was quite astonished to see that after the first three trainings sessions the piston looked really bad and all the honemarks where already gone. The pistons I used where these:
https://www.scheuerlein-motorentechnik.de/motorrad/yamaha/200-299-ccm/rd250-modelle/rd250-luftgekuehlt/yamaha-rd250-kolben-kit-mos2-uebermass-1-00

So one of my thoughts was to go with an exhaust which is abit longer and shifts the powerband more towards the lower range. Jim Lomas and others say they race the 350 exhausts on their 250ies and it works well.  I guess a 350 exhaust is a bit longer and will maybe perform in such a way. He also sells a TZ250 exhaust, but I guess this would not improve the situation. What do you think? Or maybe this is just nonsense and I should just find good quality pistons?

That leads me to my second question: Which piston to use?
I heard about the DT125 Meteor piston being used and also the Woessner forged one which Ed Erlenbach used on his landspeed machine (which I could not find for sale). But the meteor pistons starts at 56mm which would be a shame to bore my 55mm cylinders to.
I read through the piston catalogue of meteor, and there are other pistons available which are very close to the measurements of a RD piston, but are probably made for nickasil plating. Which I don't know if it matters. Wössner for example says that their piston rings can be used in all platings /castings except hardchrome.

So that makes me think if it maybe is a good idea to try to send the cast cylinder to get it plated.... Will it work?

Offline 5port

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Re: Rebuild of a RD 250 engine for racing
« Reply #1 on: October 19, 2021, 04:07:44 PM »
Difficult to comment on the piston condition.  Uwe ("german" on this forum) knows a lot about 250 racing.  :)  Same goes for the exhausts.  The expansion pipes really should match your engine characteristics, so a "250" pipe might be designed for powerband at different rpm, anything from medium to very high rpm.  You could design a pipe to give a nice power at lower rpm, Mick Abbey has built race pipes for the RD350 that give high power at medium rpm (about 8,500).  However, the maximum power from a typical racing RD250 will be at much higher rpm and will need different pipes.
You might find some pipes that suit your engine but, I think it can be a guessing game, unless you have them properly built to work in a particular way by an expert.    I am using some LC350 race pipes on my Track RD350 and they are working well.  However, I think it is likely that I will have to have some special pipes made to match in the future. 
Cheers

5port 
5port

Offline Drehmoment

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Re: Rebuild of a RD 250 engine for racing
« Reply #2 on: October 19, 2021, 07:49:23 PM »
Thanks 5port for your comment.
Investing in a custom made exhaust is probably very expensive and I even don't know anybody who builds them in my proximity.

Here are some screenshots from the meteor catalogue attached.

Which is the goto piston everybody uses? Did someone consider or een did a plating on their steel liners?


Offline german

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Re: Rebuild of a RD 250 engine for racing
« Reply #3 on: October 20, 2021, 09:21:15 AM »
Hi Martin,
just saw this thread.
I have sent you a PM via the german forum a few minutes ago.
The piston I refered about there is one of these Meteor pistons. I will investigate which model was used by my friend a couple of years ago. He will come to my home early November as he will ride my Siroko TZ next year.
And as written in the PM. What you mean exactly with badly worn pistons. Any pics?
Regards Uwe
Classic Racers do not hide the ashes, they keep the fire burning
Yamaha RD250, RD400, Yapol TZ250B, Daspa TZ350, FZ750/1000, Honda RS125, Jackson Special 250, Höpfner-TD3, Hejira PE250, Broadstock F350 Sidecar, Siroko TZ250

Offline Drehmoment

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Re: Rebuild of a RD 250 engine for racing
« Reply #4 on: October 20, 2021, 09:28:14 AM »
Thanks Uwe! Much appreciated.
I did not take pictures unfortunately back then. But I will pull the cylinders tonight and post some pictures.

Offline Drehmoment

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Re: Rebuild of a RD 250 engine for racing
« Reply #5 on: October 20, 2021, 04:01:05 PM »
Here are eine Pictures.

The Last two are only for entertainment from the right cylinder which starved.

Offline Drehmoment

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Re: Rebuild of a RD 250 engine for racing
« Reply #6 on: October 20, 2021, 07:56:42 PM »

So one way to go could be like this:
Send the barrels to powerseal in the Netherlands.
Let them be bored to 56mm and plated with nicasil.
It's quite expensive at 310€ each plus boring.
And then use the 55.95mm meteor piston which are race proven.