The RD400 drag bike

[Duffy1964]

Oh that’s a shame Al, my RD400 had a partial seizure last time out, but I haven’t investigated it yet. However there is a guy called Mally on the RD400 forum hat had issues similar to mine and it turned out to be aftermarket pistons that expanded too much as it was poor quality. So if you find the right size pistons it might be worth checking if they are of a decent quality.
Hope you get it sorted in times


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So i now know to avoid the Mitaka brand

Exactly mate


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[fabiostar]

Thats a proper bummer Al . kinda strange to see the back break up as the windows look like they havnt been open up.. i take it from the DT175 thing its been overbored?
im not sure about your pistons but iv seen some that have a little tang hangs down on the bottom of the rear skirt between the windows and iv always removed it as it can catch on the inlet port causing chaos,, i learnt the hard way with those

[AMCQ46]

Yes it’s over bored to get it up to 436cc apparently.

Have tracked the model & year of the 2A7 piston to 1977 `DT175 .

Think the are std size, so just need to establish the best aftermarket option for good quality

[AMCQ46]

Today I stripped the bottom end, washed the crank and both top and bottom cases to get rid of the rest of the broken piston bits.









And then put it all back together and back into the frame.

I have worked out it need 1977 era DT175 pistons, but waiting for a chat with the bloke who tuned it before I order anything.


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[Duffy1964]

Great progress Al


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[sirch345]

A good job you ran the engine up when you did.
Good luck in getting the right piston's Al

Chris.

[fabiostar]

No time wasting there Al

[AMCQ46]

No time wasting there Al

Well that might not be true! I posted the same photos on the RD forum and one of the guys spotted the oil seal on the clutch side of the crank is fitted the wrong way round in my disassemby photos (the castellated part should face the bearing aparantly. I obviously put it back together the same way it was when i took it apart! GRRR

Seal wrong

so now I have to decide if it will be an issue in the short term and can I wait and fix it next time I am in there…….as I am sure this wont be the first time I have to split it to clean bits of piston out of the crankcase!

To my way of thinking the seal has to stop oil coming in from the gearbox side and stop air pressure (+ve and -ve) from the crank side and if the lip shape is fairly symmetrical, then it has to work both ways round, its not as if 2 stroke crank case pressure is that high that it is critical which side the spring is.

@fabiostar, can you post a photo of your crank seal before you fit it?

[fabiostar]

Al mines the other way round as in yours is the wrong way , god knows what difference it makes but urs is back to front.

the little castlations are to keep the seal away from the bearing face, or so iv always been told

[AMCQ46]

Back in bits again waiting for the new seals

As I have a few days to wait i had a more detailed look at the crankcases.

I had spotted that there are a couple of the M6 crankcase threads near the back of the gearbox are starting to fail, so I will drill and helicoil them.
More interestingly, I also noticed wearmarks from the rotation of crank on the inside of the cases, more investigation showed that the crank has been welded up at the big ends, which is a good thing for a tuned engine…. Except they hadn’t bothered to grind back the welds, so that was what had been touching the cases.

so I masked up all the bearings and ground the welds back, then washed it all again.

also of note is that one big end is hollow and the other is solid, so the cranks are obviously a mix and match of parts from various generations of RD400. Not ideal for balance, but it will have to do!

[AMCQ46]

Oh and while I am commenting on bodges that I keep finding, here is a good one.

When I took the LH piston off, there was 2 circlips fitted on the inner side of the small end!!

That was a timebomb waiting to wreck the bore when it worked its way out

[sirch345]

Oh and while I am commenting on bodges that I keep finding, here is a good one.

When I took the LH piston off, there was 2 circlips fitted on the inner side of the small end!!

That was a timebomb waiting to wreck the bore when it worked its way out

A good job you found that Al

Chris.

[fabiostar]

Holy jesus that was proper lucky to find those clips

[Duffy1964]

Great spot Al, that could have been costly


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[tony.mon]

Holy jesus that was proper lucky to find those clips

Better two than none.
If there was room for two, is the gudgeon pin shorter than it should be- is one just a spacer?