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Honda CB250 K4 -72 racer mods

13803 Views 80 Replies 10 Participants Last post by  crazypj
I bought the Honda three years ago and have raced it for two seasons now. In 2015 me and my son didn't change it much, only changed silencers in order not to get black flag, and changed shift pattern in normal race practice. Only minor problems, had to weld a broken ignition coil bracket and weld an alloy tank leak. Improved rubbers under tank. Season ended with a problem with exhaust too low. Result nothing more than new foot peg, brake pedal and hand brake lever. Plus some frame welding. Winter spent with changing exhaust and making a GRP diaper (bellypan). Season 2016 it run well except that we got oil on our right knee, could not find out why. This winter bike will get an overhaul and some modifications. Chassi behaves well, previous owners had lengthened swingarm, reinforced swingarm mount. Modified steering stem and usual frame welding. Front disc brake works well and the rear Suzuki wheel no problem. The 30mm Mikuni VM pair perform well.
Thought it could be wise to check the engine so it was overhauled. Fairly good condition, only one broken piston ring and a broken valve spring. The improved camchain and tensioner was in good condition. The (not the fanciest) racecam was in mint condition. Another year I might polish the rockers. Right engine cover changed to a standard one because the oil lines to an external filter didn't fit with the new exhaust. Took some time matching head and carbs to inlets and exhausts.
We decided to change from half to full fairing, so yesterday we made new fairing brackets and fitted the fairing. As nothing ever is as easy that you expect a new tacho mount was needed. New ignition coil mounting parts had to be made.
Except for paintwork the bike is almost raceready.
Problem left to solve is a smart quick detachable upper T-bracket between screen sides and centre steering nut.
Any ideas?
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Poor fit of the wrist pin followed by catastrophic wrist pin spring clip failure?
Then I'm sticking with tight fit on the wrist pin, the steel got hot enough to melt piston,
unless you have reason to believe there was foreign material in there.
Motor slowly dismantling itself, very small pieces at a time commencing with an unhappy gudgeon pin :( offerings to the god of speed.
Looks like it never had a chance to seat the rings,

Ah,Ha! you are missing the clutch retaining ball ;) it must have split in 2 and been left rattling around inside the wrist pin.
After some trials, somewhat higher magnification, opposite side (where circlip could be removed
View attachment 96319
Can you almost push the wrist pin through the hole now with the cir-clip still in place?
Possibly once everything is flaming hot
imho the rules suck whenever they hold back the advancement of a technology and ultimately participation in the sport.
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