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AArrgggh!! More problems - possible piston heat seize when idling setting carbs


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Is that bottom oil ring smashed? Sad looking stuff. oh Dnepr that's why. bead blast the [sanded 800-1200] piston not too much in the ring lands new rings gapped and hone them up real good [stone or better] might try no second oil ring I've even heard of people sawing off the piston at the oil ring land all together. no sense putting money in a last chance effort on them barrels they are just old industrial machines built to 1950 CCCP standards. don't forget to pour a little gas in the ports to see if they leak into the combustion chamber if so lap them in real good and last or first check for cracks between valves. good luck! all of us Dnepr owners need it. and Happy New Year! 12 more months we get to work on these... works of beauty or whores take your pick. I Love them both.

 

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Hello, Did you try using new fuel? Old flat gasoline or fuel that someone has added too much alcohol{drier} to can also cause a overheat condition. High timing will also cause this though. And of course as others have already stated, lean conditions or vacuum leaks are the most common reason for overheating. Though engine mis assembly is a definite possibility, Im not sure it would effect a seizure from overheating within 2-3 minutes.For giggles and kicks you might try running it off a separate fuel source apart from the spigot valves on the tank. Propane is a good way to check for leaks without so much of the risk of a fire.Valves being too tight could also cause a internal vacuum leak. As they get hot they stay open and basically cause a open combustion chamber.A compression test would show this straight forth.Good luck!!

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Better pistons can be found on Aliexpress and Ebay. Look for the 3 ring type for Chang Jiang CJ750. Not expensive and faaaaaaaar better than the originals. Drastically reduced oil consumption.

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When your engine stops (seized?) can it still be cranked over with the start lever? If yes, it is not seized.

I am wondering if it really heats up and seizes, or stops from other causes as discussed above.

Happy New Year, all.

RussN

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Hello, Did you try using new fuel? Old flat gasoline or fuel that someone has added too much alcohol{drier} to can also cause a overheat condition. High timing will also cause this though. And of course as others have already stated, lean conditions or vacuum leaks are the most common reason for overheating. Though engine mis assembly is a definite possibility, Im not sure it would effect a seizure from overheating within 2-3 minutes.For giggles and kicks you might try running it off a separate fuel source apart from the spigot valves on the tank. Propane is a good way to check for leaks without so much of the risk of a fire.Valves being too tight could also cause a internal vacuum leak. As they get hot they stay open and basically cause a open combustion chamber.A compression test would show this straight forth.Good luck!! P.S. not having seen the pistons before hand..........youve got to start there! Get rid of those dogs!

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  • 2 weeks later...
On 1/1/2021 at 2:26 PM, Serious Black said:

Better pistons can be found on Aliexpress and Ebay. Look for the 3 ring type for Chang Jiang CJ750. Not expensive and faaaaaaaar better than the originals. Drastically reduced oil consumption.

Hi 

will these pistons fit in a K750?

I have a 63 'that needs new ones.

Screenshot_20210113-110333.png

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  • 2 months later...

Hi Guys.

I'm really sorry for the delay in replying to this after you all left such good responses. Thanks for that.

Others stuff took over but I did have a look at this a couple of times. I checked the ring gaps and they seemed a little tight so I opened them up.

However, it was still getting hot.

To cut a long story short, I took the barrels and pistons to a local machine shop.

When cold, all measuremnts seem to work and tolerences for the barrels and pistons seemd fine but... the machine shop guy poured boiling water on the piston (100 deg C, obviously : ) and the piston expanded so much it was larger than the bore. I'm also pretty sure it gets much hotter than 100 deg c when running!!

Here are his measurements:

Orig Bore size:

78.50mm = 3.0905"

Actual Bore:

3.0905" / 3.0910"

Piston:

78.43mm = 3.088"

After boiling water: 3.0916" (bigger than the bore size)

 

These seem to be original pistons. On page 1 I posted a picture of the piston top. They have the P2 size stamped on the top then below what looks like 'OTK 2'

What's going on here? The expansion seems huge and obviously is my problem. Are the pistons too big initally for the bore? Are these cheaper pistons where the metal isn't good, expansion wise.

When I bought the bike, I'd be told the engine had been rebuilt and the oil was crystal clear so don't think it had ever been run.

I don't undertand how these pistons would ever work with that much change under heat, unless the cold tolerance was really, really loose?

Luckily the machine shop guy says the barrels are in good condition. It's just the pistons that aren't great.

 

Any suggestions? Different (smaller) size pistons needed or same size but better quality / lower expansion?

Thanks in advance if you have the time to reply.

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