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MG Midget and Sprite Technical - Piston polishing.

Note to Prop. You don't need to do this.



Anyway, I have an old sidevalve Morris which seems to suffer from quite an affection for soot. The piston crowns collect the combustion products and it seems to need more frequent de-coking as a result (every couple of thousand miles), so, I was wondering if the pistons would benefit from a mirror finish on the crown to reflect heat and to reduce the chance of coke adhesion. The worry would be that there is a coating on the crowns (anodised?) that would be removed, or, that there would be another reason to avoid changing the surface texture.

Any racers/engine builders/metalurgists out there?


rob thomas

Interesting question, Rob. My first response would be another question; what's causing the coking in the first place? Is the mixture overly rich? And would regular use of a fuel additive - after a good de-coking - reduce the accumulation?

Just curious.

-:G:-
Gryf Ketcherside

Rob,
I have heard that polishing reduces the coke build up, but I don't know about anodizing etc.

I thought the need for frequent decoking was from poorer fuel of yesteryear, and isn't much of an issue today. I forget the figure exactly, but my 59 Triumph 650 book said to decoke something like every 3000 miles. Unless it is running very rich, as Gryf suggests, maybe it is oil rather than fuel causing it.

So, if the coke build up were due to oil rather than fuel, maybe a different oil would coke less?

Charley
C R Huff

I would fear that the soot would than stick to then top of the combustion chamber or valves.
For it has to go somewhere
Onno Könemann

Hi Rob

Some racers blast the crown to make it rough so it carbons up quicker to form a heat barrier and make more bhp.

Peter
P Burgess

ROB. You can clean the soot off the piston heads very quickly.. Take a can of trans fluid and pour into the carb while running the engine. You pour enough to strangle the engine. Let rest overnight and you might have to clean plugs first before starting up. Be certain to put an old potatoe scak over the exhaust because on start-up there will be black gunk coming out the pipe. I did this with a Ford v-8 with twin exhaust and had two black stripes all down the driveway.
Another way to soften the soot up is to use a teakettle of hot water poured down the carb while keeping the engine running by holding the throttle open enough to stay running.
This too will remove a lot of soot.
Sandy
SANDY SANDERS

Your 26 months to late rob,

I had looked into this when I was planning my build, long before the engine blew. There is a company called...

http://www.techlinecoatings.com/hi-performance/index.html

they make various coatings you can apply to engine parts and bake in the oven ... they have a ceramic you can apply to the piston crown that can lower combastion temps and reduce coking as the burnt carbon wont stick...I decide agianst it because like Onno says... the coke has to go somewhere and that can be the combustion chamber or the exhauste valves...but I think its worth a look esp if you coated the head and top of the valves

Prop
Prop

But, that costs money. If polishing it will do the same job then why spend out?
Being a sidevalve engine you can remove the head by undoing 13 nuts and just lifting. If careful you can even re-use the same gasket. When the head is off you can get to the piston crowns and polish or decoke as necessary so it would be easy to do. Now, having looked at some old pistons it appears that they have some sort of anodised coating to the outsides. Is this just a friction reducer / oil collector to keep the flanks lubricated or does it serve a useful purpose on the crowns as well?
rob thomas

It was my understanding back in the 1960s that the main reason for frequent decokes for sidevalve engines (every 2 to 3 thousand miles) was down to (1) poor quality of fuel then (2) poor quality oil.

Certainly with the advent of Duckhams 20/50 (was somewhat more dark green in those days) my Austin 7 (in the early 1970s) went well beyond 5000 miles before it needed another decoke.

Maybe you are using a 1940s period lubrication oil?
Or cylinder compressions have dropped?

Roly
Roly Alcock

Wasn't there someone a while ago who said to put a little braking fluid in the petrol tank to burn of the sediments?
Alex G Matla

This thread was discussed between 17/08/2010 and 18/08/2010

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