RD400 Pistons - WTF

Discussion in '2 smokers' started by Bloodweiser, Jun 4, 2012.

  1. Bloodweiser

    Bloodweiser honestly

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    So after concentrating on my bottom end which has been like forever,
    I finally broke out dial gauge and crap to figure out what to do about my pistons.
    1 looks great, 1 had a seizure at some point but was free when I got it.

    Anyhow, I can't believe it took me so long to notice this:

    [​IMG]

    I've got 2 different pistons!
    And different sizes to boot! :huh

    The one on the left measures 2.545ish, which an estimate on that thousandths digit. It has 383 stamped on the inside wrist pin boss.

    The one on the right measures 2.55 dead. Numbered 458 on the inside wrist pin boss. (I think anyway, that 4 is a little sloppy).

    I'm thrown off on several fronts.
    The difference between the 2 is not .25mm which relates to available piston sizes, but almost half of that. WTF?
    Is/was this common practice to just bore 1 cyl? Is that even what happened?

    I mean, I reckon to get a set of 65.00mm pistons and bore them both just so they match.
    Would still like to hear if anyone has a clue what's going on here
    #1
  2. wfopete

    wfopete Suffer Fools; Gladly!

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    What you have there is a 1st Generation Yamaha power valve!

    A collectors item fer' sure, don't change it! :D

    I believe that was the set up Kenny Roberts was going to run before they talked him into riding the TZ750 Flat tracker.

    Or someone was really hard up for parts when they put that motor together.
    #2
  3. tripodtiger

    tripodtiger I used to be SCRay

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    Where is the rest of your story?:lurk

    From: a fellow RD400 re-builder.

    One of the first things I found when I started cleaning the oily, dirty, spider infested parts resting in a plastic ice cream container was a piston with the skirt broken off.:eek1

    I left it at that point for a few months. Bit scared what else I may find.

    Anyway, I'm getting back into it. The frame, swing arm and steering head are done. I've bought a set of period looking new shocks and am waiting on bearings etc so I can put the first few pieces together. Nothing terribly exciting at this stage.

    So, long way of saying, I want to read what you've done, what you had to change and what you've learnt.....:ear
    #3
  4. Bloodweiser

    Bloodweiser honestly

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    ^Check out my sig, Bulletproof vs. bloodweiser. :deal

    Once I get this top end settled, it shouldn't be long before we have a runner:evil
    #4
  5. backroadbob.com

    backroadbob.com Roads Scholar

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    No it's not common practice, but there's no reason to redo a cylinder thats OK.
    You may have pistons for two different models or someone modified a stock piston for higher performance.
    That was common practice. The scored piston is exactly how we used to cut them in the 70s on racebikes. The numbers may be piston weights. What year is it? Was it raced?
    #5
  6. Bloodweiser

    Bloodweiser honestly

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    It's a 76. I doubt it was ever raced. Bought it off the original owner who seemed very reserved. Bike was stock anyhow.
    Are you saying to keep the good piston and just replace the scored one? The cyl looks just like that piston.
    #6
  7. anonny

    anonny What could go wrong?

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    Seeing how your aim was to do it as cheap as possible... why not do the bad cylinder .25 over and RTF :evil
    #7
  8. Bloodweiser

    Bloodweiser honestly

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    Wouldn't that throw the balance off?
    I'm into doing things cheap,
    But not sloppy.

    Already put $590 into the crank,
    Would feel silly trying to save $100 that way.
    #8
  9. FloridaSteve

    FloridaSteve Long timer

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    If you've gone through the trouble and expense of redbuilding the crank it would be foolish to skimp on the top end. Get a matched set of ART's or Pro-X pistons in there and have it bored correctly. With conservative jetting and todays oils you'll be set for a lot of miles.

    2 cents.
    #9
  10. Bloodweiser

    Bloodweiser honestly

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    That's how I feel Steve.
    I just really don't understand the discrepancy between the diameters of the 2 pistons.
    They only come in .25 mm sizes,
    So how does one only measure half that?

    Could it be a difference between forged or cast?
    I thought I remember that the expansion rates are different and maybe the come in weird sizes to compensate?
    #10
  11. FloridaSteve

    FloridaSteve Long timer

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    I have no idea what the piston on the left is. There could be a lot of reasons for the slight differences in sizes though including slight differences in manufacturing. Forge vs cast etc. Looks like someone just made something they had work in a pinch after a seizure. I've seen some wildly mismatched bores and piston on some old 350's and 400's that ran just fine. I even pulled apart an old H1 triple that had 3 different size bores. You just never know. People will sometimes just keep seizing one side without ever really getting to the source of the problem (often an air leak) and keep boring that side larger.

    With that crank rebuilt it looks like you're on your way to a smooth running ride. Keep em coming.
    #11
  12. Shocktower

    Shocktower Long timer

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    Dead on :clap, get a fresh topend since you have already done the crank JMHO :wink:
    #12