Monday, 20 October 2014

Piston disaster

Here is a run down on my problems with a set of cruzin image pistons for a KH400, 1mm o/size.
I'd done a fair bit of asking round, and having heard no comments worse than "They're basic but do the job", decided to buy them. Being as I'm using a set of barrels on max oversize, which has had some use but no serious wear, I didn't really see the point in getting top of the range wossners.

On assembly, everything was meticulously clean and well oiled. I checked the ring end gaps in the bore, and they felt to be a good, free fit in the grooves. For some reason I don't think I checked the actual groove clearance though. Still, on assembly they fitted fine, and showed no sign whatsoever of sticking. In fact, I had to take two barrels off to double check something, and the rings sprung out as soon as they came out the bore, as they should do. Obviously the ring gaps were matched with the pegs too. The rings were fitted the right way up, and in the right grooves.

The bike was kicked over several times as I fitted the carbs and exhausts, all the time it felt free, smooth and with excellent compression.

When assembled, I started the bike and ran it for about 40 seconds-1 minute, just to get a bit of heat in it and start priming the pump. Again, it felt and sounded fine.

I returned to it about 10 minutes later, and tried the kickstart again. It was immediately obvious that a lot of compression had gone, and it felt just a tiny bit gritty. Taking the head off revealed the internals to be covered in an oily grey liquid, which when checked with a magnet proved to have iron filings in. It has a very fine grittiness to it, more like toothpaste or chrome cleaner than grinding paste. Taking the barrels off revealed that all 3 sets of rings are now jammed solid in the grooves, and the crank cases have a little of the grey sludge coating the internals.

I've not been able to remove the pistons yet, they will need warming with a heat gun and I'd like to let the petrol fumes evaporate first. The pistons are scored fairly deeply, the barrels are too but I think a hone with emery should just about reclaim them. When I get the pistons off I can see if it got into the bearings, and also try and see why the rings are now stuck.

Below are some photos

I'm hoping I'll be able to rinse out the cases with plenty of carb cleaner, without having to do a full strip. If that is OK, I'll get the barrels honed, cleaned and new pistons in. Other than that, I'm pretty fed up. Still, pistons aside, the 250 to 400 conversion seems to work pretty well.