Surprises when lifting the 55 KH 500 Twin cylinder head

[MartinL]

I'm new to British motorcycles and am in the process of dismantling and inspecting the Ariel I bought in the summer. A lot of things here are different from what I know from my Boxer. It starts with the different types of screws.
Today I removed the rocker arm brackets and dismantled the cylinder head.
I have already read in the manual of my KH 500 Twin that the engines tend to carbonize and that the cylinder heads should be decarbonized regularly, but I didn't expect to come across a carbon mine.
Where does the tendency of these engines to carbonize come from? How much carbon can be normal and when it might be caused by an problem?

I also did not expect a piston with an oversize that I have not seen on offer at any dealer so far. Does anyone here know the pistons with + 0.80 oversize? That would probably be the 4th oversize for the cylinder. Could that be the case?

[dave.owen]

Could they be triumph pistons?

[Roger Gwynn]

De-carbonising used to be a regular chore but modern oils and fuels have stopped all that amusement. I have never seen such carbonation before, which is nothing to do with old technology but rather to do with something seriously wrong with the engine. Has it been run on scrap wood? What are the numbers from inside the pistons? +.080" pistons have been made for a few popular models that have thick enough cylinders so they may be Triumph ones, as Dave says. The KH is not really popular enough.

[nevhunter]

Pretty sure JP Pistons AU will supply Pistons and rings made to special oversizes where you might hone that bore and go up ONLY +003" (as an example. Don't rebore metal out as by the time you set it up, you might have to take another +020" out. The pistons might be OK for more miles as they are. Check the top groove Wear. Check valve seat wear also. (Pocketing). Might need new inserts. Best put in with liquid nitrogen. Like with Porsches). Nev

[nevhunter]

Be SURE to clean the crankshaft oil galleries out and do remove the Plugs. Crack test the Crankshaft first and consider a new set of conrods as alloy ones fatigue and that bike has done Lots of Kilometers. Do a dynamic balance on it as many of these engines vibrate more than they have to. Nev

[MartinL]

Could they be triumph pistons?

No idea how to find out, Dave.

[MartinL]

Be SURE to clean the crankshaft oil galleries out and do remove the Plugs. Crack test the Crankshaft first and consider a new set of conrods as alloy ones fatigue and that bike has done Lots of Kilometers. Do a dynamic balance on it as many of these engines vibrate more than they have to. Nev

With „cleaning the oil galleries“ you mean what mentioned in a manual by this?:
„ In the case of a complete overhaul, before fitting new bearings, it is advisable to clean out the oilways in the crankshaft by unscrewing two large plugs and the relief valve and thoroughly washing with clean petrol.“

I will definitely check the connecting rods for cracks with a paint test system.

What do you mean with dynamic balance? Balancing the crankshaft together with the flywheel?

[dave.owen]

Take piston off and check the number inside

[nevhunter]

Alloy rods fatigue more and that can't be reasonably checked by non destructive methods. Detonation loads up engine innards. Use colder plugs with High Compression, high speed or sidecar work. Make sure there's not a lot of carbon build up.
I had a look 500cc twin cylinders I have and the extensions below the Lower mounting flange should cope with +080. Clean carbon carefully from the ring grooves and check wear particularly in the top one. Clean holes where the Push rods go. Putting the head back on can be a little fiddly re the Push rods. Nev

[Karol Burger]

I've never seen an engine so heavily carbonized. I think you might be burning oil, or you have a very poor mixture. Under normal conditions, you can't accumulate that much carbon. For comparison, the photo shows the KG engine head that we removed due to a damaged gasket after 10,000 miles (I have no idea how many miles the previous owner had driven). I bought the motorcycle already refurbished.