77 RD400 L Cyl Detonated To Death, Suggestions?

[Clem710]

Blew up the left side while seemingly running normally, blown thru the first ring and wasnt running too bad like this, hopefully the pic shows up.  Quickhand touch test on fins was pretty close during the ride, L was maybe slightly hotter, maybe.

I'm guessing airleak or timing but timing was late if anything on a quick check and I didnt find anything obvious on teardown. Rt cylinder 100% fine, piston dark but no signifcant carbon.  Tiny hole clogged but thats about it.

12k mi or so, never been apart, OEM 64mm piston, think we cleaned the carbs at the gap a few years back when Paul had just gotten his bike and we did the SV coil thing.  Originally had diode problems and siezed calipers, otherwise actually started and ran pretty decent, was stored semi well, PO bought for the wheels and never moved them to a 350.

Likely ran it with fuel off on the sidestand before draining tank but did find no gas in left bowl and was suprised to find a good bit in the right, did them on different days and didnt think I was going to pour a bowl of gas on my hands but did.  Tested petcock and float operation, all normal. No screen in the petcock so found some powdered tank rust in the bowls.

Foamie Uni likely going in the stock airbox, thinking 125/130 mains with 30 pilot. 115/25 was in it but airscrew liked 1/2 out only. Hopefully going to leakdown test, definately going through the timing well, maybe I can find my light instead of just setting it slightly late static.

Thanks for reading and thanks in advance for anything I could be missing, hope u like the pic, seemed like a pretty good one:)  Cyl actually not destroyed, dont know how that ring got out cleanly.

[rodneya]

That looks more like something was bashing around in there.
Possible the PO grenaded the motor and did a bandaid repair job to get it sold.

[Hawaii-Mike]

We can get a better idea if you include a picture of the exhaust side of the piston, to see if it's melted.

[m in sc]

and under the dowm, underside of piston. best way to see if it was actual overheating.  I would t say air leak, that looks timing related. interesting its all around the perimeter.

[bitzz]

Never been apart?
So you mean there is 45 year old seals in it?

Left is always the first to go because of that GREAT BIG alternator rotor hanging off the end of the crank.
... or your crank is out of phase. Set the left at exactly TDC, rotate the motor and see if the right is at EXACTLY 180 degrees.

But my money is on seals.
And you have a TON of burnt oil on the crown... what's up with that? Let's see the bottom of the piston.

[Clem710]

Thanks for the responses gents, sorry to keep you waiting, was sending off my first attempt in Vermont for a different machine.

In, out and under and slightly smaller size, never used postimages b4, shiny stuff underneath more sticky than oily, not thick at all.

Thats not oil on the crown topside, just melted aluminium. Yes, likely 45 year old seals, and was actually tires until a few years ago, black and round:)  I'll peep the crank phase when I work on it again, R head loose but not off so pretty easy.  Unless it moved, It should be where the built it.  Seals are always a concern but it exhibited no unusual behavior.

Was running totally well and good until I thought I heard a funny clank(likely the ring down the pipe) and lost a bit of power, was somewhat concerned but my attention was on keeping it between the trees. Generally cover the clutch anyway when going quickly.

There were definately things bashing around, namely the rings, did some nice work on the head as well:)  Blame is squarely on me, probably done 4-6k on it. 

Very suprised at the lack of damage and that it ran reasonably well for probably 20 miles after the clank. 1 kick bike that morning after sitting probably 7 months, had 50/50 aviation fuel and no ethanol in it with a spash of castor for storage.  Tank has not been treated ever and has surface rust so it stays full or empty and fogged. 

After the incident and with the piston as you see, was still just a 4 kick bike.  Knew something was bad when I found the plug closed up.

Pin was about as tight as they are new, didnt even have to get behind it with a hammer, nice parts Yamaha:)

[m in sc]

yeah.. id have to agree. changing my bet to seals.

[Clem710]

Considering your experience, 100% will do a leakdown and not just throw it together, promise.  Been meaning to build a rig for that for quite a few years.  Could switch to four strokes but that doesnt seem likely to happen:)

Did notice a drip under it as well walking past it tonight....oh well, add it to the list, still totally worth the effort.

Going to have an experienced eye evaluate the cylinder and do what needs to be done. Should only be 1 up at worst imho.

Glad to click around and see the Grom is still healthy and thanks again for the ride, keep me on the top of the list for when you are done with it:)  Sucks that we cant buy stuff like that brand new, it really is a great bike.

Finally, thanks to you and the mods for keeping the forum alive, was disappointed to miss the gap but should get down there with some old race friends early fall, will represent the 2 stoke brotherhood and spread the word

[teazer]

back in the day I bought up every RD400 motor I could get my hands on (still not a lot) and i found several with 1.0 to 2.0mm oversize on the left and stock on the right. The left side runs in air (not oil like the clutch side) and with that heavy rotor, tends to leak.

Thank goodness they are so easy to strip and rebuild.

BTW, are the crank halves not splined and pressed together, so they should be phased correctly unless there were manufacturing errors?

Drip is most likely from the shift shaft seal or neutral switch.