wrist pin siezed in piston and caused engine failurewrist pin siezed in piston and caused engine failureMaybe someone can shed some light on what happened here...This bike sat for a few years before i bought it. I got the bike running and drove it for a very sort period. While installing an electronic ingition here's happened: The engine seemed to heat up fast and came to a quick stop. After a complete engine tear down i found that it seems as though the pistion pin was siezed into the connecting rod and the piston. Thus not allowing the pistion to be free floating on the wrist pin/connectng rod. It was almost impossible to remove this pistion from the connecting rod. It took the other wrist pin and a hammer with lots of muscle to remove the wrist pin from the connecting rod. This bike had plenty of oil and was running and ideling fine when this happened. The pistons are stamped 0.50 on the top and cb77 on one side and the letters ART on the other. I would be most appreciative if anyone could shed some light on what may have caused this or what i should look for during reassembly to assure this will not happen again.
another picThis picture shows the sccaring on the wrist pin where the connection rod attaches
This looks a lot like a problem I had on my CB72 a couple of years ago on the L/H cylinder.
Again when stripped it looked like the wrist-pin ( gudegeon pin ) had seized but I couldn't decide what happened first .. piston partly seizing, causing small end to nip up or the other way round. Anyway one thing I did find afterwards was that the crank big end was shot; I thought it was OK when I'd put the motor together about 1000 miles before but when I took it to a specialist he said both big ends were U/S and I had to get the crank rebuilt. The oil pump had seemed OK when cheked as per Bill Silver's book ( marks, clearance etc. ) but I managed to get a new one and fitted that for the rebuild. Also checked the pump output before starting it up ( its on one of the threads here ). The lubrication to the lttle end seems to rely on splash from the sump so maybe the oil level was a bit low ? I read in a Cycle reprint about the early 350cc Webco kits having a drillway put in to take oil from the gallery at the front of the barrels into the cylinder to cover that part of the bore which ( they said ) was never exposed to this spray because the piston skirts were so long. I go along with it all being down to a lack of oil somewwre but I'd check the big ends as well before getting a rebore and putting it together again, and definitely check the pump output. Had you cleaned the oil filter out after you got the bike ? good luck Phil piston failureThis is common on harleys also as a result of 2 things, first is an intake leak leaning the engine down, and second is ignition timing off causing a lean condition. Could also be from lack of oil, but sure looks like lean condition. When you go to rebuild it, make sure you put new orings on carbs and intakes, I always smear a super light coat of high temp silicone on both sides of the intakes, to ensure its leak free. Also make sure your timing is right on the money. If thats way off, it will also cause a lean condition. Carbs are also very important to make sure they are adjusted just right with each other. I would say the piston got so hot in the bore it kinda melted the pin to the piston and seized it up in the rod. You may want to consider going oversize on the rod pins if you reuse the rods. Any play at all will cause a rattle in the engine. Been there done that. Hope this helps. Nutt
Hi Nutt
I've read about Honda doing O/S wrist pins ( strange word ) but had no experience of them. Understood they were to be used when the small end bush wore beyoind spec but if you used them would you have to open out the holes in the piston to match, i.e. could using an O/S pin mean the clearance between the piston and pin is too small ? Just a thought. Phil Thanks for the information guys. When this happened I had just installed a new electronic ignition. I had run the bike a few days earlier with no problems. On the day of the melt down it had only run on the bike stand for only a few minutes. It started to heat up and did push a little oil out the head gasket. The timing may be a little off but the bike has not seen more then 3500 rpm since the new ignition. I am having a hard time telling if the pistion heated up then siezed or the wrist pin siezed causing the piston to heat up and fail. The valves on the head look good and i do not see any hot spots or heating marks on the pistions. I may have to go with oversized pins because the connecting rod is very scared. Thanks for the ideas
AAron
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