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Piston Stuff

Piston and chamber ceramic coatings

No, you can't just spray VHT on them. It will not survive the environment. It's not just hot, but there is combustion too! ;) It will burn off. Besides, VHT just doesn't insulate well enough to make a difference.

The coatings (like Vizard talks about) are readily available in the US. I don't know about availability in the UK. They are ceramic-based. The latest generation of these coatings are blends to yield acceptable overall properties.

The best insulating ceramics are typically thermal shock sensitive. They tend to fail after a number of thermal cycles (startup, run, shutdown). They were/are good for racing applications where continued, every day use is not required. The ceramics that are most tolerant of thermal shock do not insulate as well. As I said, the latest are blends to get the best of both.

The application of these materials is by plasma spray. Plasma is a state after gas as you continue to go up in temperature. (solid -> liquid -> gas -> plasma). The plasma (several thousand degrees) is shot at the relatively cool piston or head. The plasma freezes all the way down to solid and bonds to the host.

For some ceramic blends, a very thin layer of a third ceramic is applied first to improve bonding.

Piston insertion

The gaps in the rings should be spaced equally around the piston - i.e. for 3 rings 120 degree apart etc.

To insert, the pistons are put in through the top of the block, the compressor should be tight on the piston, with a little of the
compressor past the base of the piston. Make sure there is a bit of oil on everything to help it get in. Then sit the compressor on the top of the bore (I found on standard engines that this was very slightly chamfered - probably to assist the rings to enter the bore?) and tap the crown of the piston with the shaft of a hammer, keeping loadsa pressure on the whole assembly - if you lose contact with the block you've had it.

Best of luck - been there, done that, mashed the oil control ring :)

Piston ring compression


Took me a while too, and I kept taking them out because SOME people kept moaning that I had to get the gaps inline with the crank and 180' out. I found a ring would pop out under the compressor if I didn't tap the compressor down all round every 5 seconds or so. Go slow, and get the compressor really tight. Keep it HARD down on the block face and tap round the edge of the piston dish in a circular pattern with a rubber er... tapping thing (I used the handle on my ratchet).

Piston rings, broken, possible causes

Things that break rings:

1. Ridge not reamed or bored out when fitting new rings. If you re-ring the old pistons without using a ridge-reamer or hone on the bore-tops, the top rings will probably break when they hit the ridge, a step worn in the bore at the top of the top ring travel.

2. Engine not sufficiently revved during break-in period. Even if the bores are newly-cut and pistons and rings are new, if the engine is not revved to its operating limits every once in a while during break-in there's a good possibility the ridge worn in the bore where the rings seat will be too low. Then when you rev it a lot higher, the con-rods will stretch a microscopic amount and the top ring will hit the ridge and break.

3. New rings in a tapered bore. A tapered bore, worn more toward the bottom than the top, will break new rings. When the bore gets out of spec it should be re-bored and
honed and new oversize pistons and rings should be fitted.

4. Clumsy fitting. This breaks more rings than anything else. ;)

5. Running too rich can wash the bores and reduce ring lubrication.

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