Rebuilding bottom end crank

Pottus

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Jul 21, 2014
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Ok well I had some issues with my engine with the cylinder being damaged a piece of metal broke off of the transfer port and ruined the cylinder. I noticed when I put my finger in the connecting rod and pushed my bike there was a clicking noise and and a bit of metal on the fly wheel so I decided I had to do a complete tear down and clean it out.

I noticed that the cheap china bearings were not smooth I have already bought high quality replacement bearings. Now another issue is was the needle bearing on the bottom end crank broken and needs to be changed but I'm not sure how to get the crank wheel apart any suggestions? I know that I need to pop the pin but it's really in there methods?
 
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buy a new crank...


the crank needs to be heated up to about 150 degrees C, then pressed apart. the service will cost more than a new crank.

and it requires a special mandrel to hold the two halves in line when they re-press the thing. notice how theres two small holes just around the shaft itself? ahuh. two guide pins go in there when they press them, so the halves will line up again. ANY inaccuracy, no matter how small, and it will just destroy the crank bearings, or the crankcases, or both. and the whole assembly needs to be heated up a fair bit to do it without damage (AKA shrink-fit)

and it has to be pressed back together to within a thou or two of the original spacing...


simply not worth it :) not impossible, just not WORTH it.

at this point, a new engine is only a few dollars more usually... ;)
 
It's heartening to see you offering such sensible advise HeadSmess: to keep engine rpm under 5,000 rpm to ensure good engine reliability.

Was it "you" that made a video showing how reliable your bike is?

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if you gear it right and do mods to improve low end there's nothing wrong with only revving to 5200. I'd love to have an engine that makes peak power at about 3000 rpm
 
actually, it was a video to show it went fast. so it was a bit of a stuff up :giggle:

still, my crank didnt seize...didnt blow up, didnt have major catastrophic failures, and well...it was a good reason to get off my butt and finally make a proper bracket for the exhaust like ive been meaning to for...mmm...a year now? :giggle:

im failing to see how a broken exhaust bracket has any relevance to the performance of my small poxy lil engine?

oh it was struggling... i pedalled to get it started. thats now a crime. had to carefully predict all of those...um, gear changes to maintain the carefully gained 1/2mph...despite it being a single speed, and running a 36T on a 700c type wheel!


or a flat tyre due to hitting a POTHOLE? because skinny racer tyres have SO MUCH rubber to protect from sudden impacts, of course...


maybe it was shrapnel from my grenading engine that caused the puncture? :wacko: the pothole being a figment of my imagination?


question...wheres your video of your bike doing anywhere near that speed, when its not dragging ten trailers up a 45 degree slope? im waiting.... or maybe....maybe you just talk talk and talk some more :rolleyes: you also have to do it in top gear thankyou, dont pedal, and take off like that. both you and the engine in top gear. remember, you need 36T on a 27 inch wheel... or its cheating :p


i did take your advice last night... finally fitted a rear view mirror, so no freakin headchecks anymore!
 
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if you gear it right and do mods to improve low end there's nothing wrong with only revving to 5200. I'd love to have an engine that makes peak power at about 3000 rpm

two strokes have no bottom end, well documented. theres a reason MOST are rated for around 7000 rpm +
 
still, my crank didnt seize...didnt blow up, didnt have major catastrophic failures,

None of these things happened because your bike wasn't reliable enough to travel any distance long enough for those things to happen. Parts were falling off your bike after not more than about 2 miles, which is a fortunate thing in the prevention of blowing your crank.

Maybe it's best that your bike is built to such precise levels of unreliability.
 
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