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MG MGB Technical - Decarbonising Pistons


Any advice on the best way to decarbonise pistons while their in the block, and generally clean the top of the bores and the block.

I have the head off the engine at the moment and the pistons have alot of carbon deposits on them.

Also were can you buy Payen gaskets in the uk?

Peter S


Make sure you don't remove the carbon deposits at the edge of the pistons. A good idea is if you have an old piston ring put this inside the bore over the piston and then concentrate your cleaning inside the ring.
Try MGOC for the Payen gasket.
Sealy do decarbonising brush sets.

J White

Peter, I'm not sure cleaning the tops of the pistons in place will be a good idea. I can visualize all sorts of very hard carbon material falling down the edges and scratching the bores, spoiling the seals. Instead, run a 'decarbonizer' in the fuel tank when you get your engine back together. In the states, we use 'Seafoam' which is basically Naptha or white spirits, and a few other ingrediants. About a pint to your tank would do it, I would imagine.

good luck,
Dave Braun

Peter, I went into the local motor factors here in Ayrshire needing a head gasket set for my wife's Fiat Punto. I said to the guy did he stock Payen gaskets and he said no, but he could get them. He then said why did I want Payen particularly, and I was just opening my mouth to reply, when he answered for me "Because they're the best".

So try a local independent motor factor. Bet they can get them, even if they don't keep them. I find these places excellent, helpful and good prices. They aren't fazed by old cars and have catalogues bursting out of all corners of the shop.

I agree with the comments about cleaning the tops of the pistons. The carbon deposit actually helps seal against burning oil. Take it off and you might see your oil consumption go up.

Mike Howlett

Stuff a length of rope or cord between the piston
top and the bore wall. This'll keep out any carbon
from falling between the piston and bore wall.

Brush on (do not spray) some kitchen oven
cleaner and let set.

Scrape off with a sharpened, flat wood stick
(or a putty knife held at a very shallow angle).

Remove rope.
Daniel Wong

I have always uses a blunt scraper and just left about 3/16" around the edge of the piston. Doing this with the piston at TDC. If you are concerned about coke getting down the bores you could put some grease on them before you bring the piston up to seal them, and just move the pistonm down then wipe it off again when you have finished. I've never had a problem. I did look 4 a pic but can only find the shocking state of my valves in 2005. I no longer use ocatane booster.

Stan Best

Moss sell Payen gasket sets. Also Chris Betson at Octarine Services sells them. About 18 for the set last time I bought one.

67 BGT
I D Cameron

Thanks for all the responses,

I'll probably clean the centres only and leave a ring of carbon around the outside.

How easy is it to remove the pistons, replace all the rings and refit (I'm quite happy working on engines but have never done this before). The engine is currently on a stand, so is it a case of removing the sump, disconnecting the rods and sliding the pistons out? Should I give the bores a very light hone to help seat the new rings, although with all the internals still in place I guess cleaning the bore after honing might be a problem?

Peter S

I also use the Payen gasket on my roadster and ordered it from Moss.
Everybody told me they do not need to be retorqued after installation as they are made of a laminated resin compount. After some @ two K miles there was oil in the cooling system and i retorqued the head nuts again, wondering how much the gasked has seated meanwhile.
After the retorque there was no further oil leak into the coolant.

What are your experiences with the Payen?

BTW, I never have had any problems with the OEM head gaskets before when having them put into water one day before insallation and retorquing according to the workshop manual.



When I decoked my pistons I had a shop vac type vacuum cleaner running with the hose positioned at the bore. As pieces of carbon were scraped off they would be sucked into the hose. It actually worked better than I thought it would. I also used the grease ring at the pistons TDC.

Ralph. I have always recommended re-torquing the cylinder head after initial start up, after 500 miles and at 1,000 miles. By the 1K mile point, things seem to have stabilized and the system holds the proper torque. I have been using the resin type gasket for the last six years. Good gasket, but, re-torquing the cylinder head is a must.

Les Bengtson

I've de-coked lots of cars and old bikes and never had a problem with removing all the carbon from the crown. If you leave some the carbon edges will get hot and cause pinking. The carbon that seals (if any - new engines don't have it) is that formed around the lands between and above the top ring, and, in older engines, in the ring groove itself (taking up groove wear which causes pumping). You won't be getting at that, so just clean off the tops completely. Carbon bits get blown out and won't damage bores. As a point of interest BMW had a coke problem a few years back and used to blast inlets clean with the head on using walnut shells. Use a sharpened stick of solder or aluminium for a scraper.

Rich is correct. Not a good idea to leave carbon at all.

I wish somebody would explain to me what the reasoning behind the blunt scraper is; if children are not to be trusted with sharp knives, then they shouldn't be allowed to play with knives at all.

The old rule is that for any edged tool, a dull tool is a dangerous and useless tool. I find that it is far easier and better to use a razor sharp wood chisel to clean carbon off. Otherwise you wind up beating on it, struggling to get it to cut, and doing far more damage to yourself and the parts than you would if the tool were correct.

I scratched a lot of pistons and other things using blunt scrapers, purpose made proprietary scrapers, and other BS, before I ignored the oft repeated nonsense and figured out that sharp tool is good. For dished surfaces, make a radiused scraper out of you chisel. If you ding the edge and cause a burr, fix it immediately with a stone; to start you may want to ease the corners a bit, but experience shows that a sharp corner is very effective in many places. The same chisels are used for gasket scrapers.

Not all that difficult.
You must use a ridge cutter to get rid of the wear ridge at the top of the bore, or you will break rings and possibly pistons. There used to be "Ridge-dodger" rings to eliminate this, but they seem to have disappeared.
You must hone or break the glaze on the walls, or your new rings will not seat.
You must carefully clean the ring grooves in the pistons, WITHOUT scratching the sides of the grooves - this is a critical sealing surface.
Everything must be scrupulously clean on assembly, final clean walls with oil on paper towels until there is no trace of black on a new oiled towel - black is abrasive residue.

And, you must retorque head as Les says, but in my extensive experience, the settling goes on for years. I have posted my procedure many times on these boards - if you can't find it email me, ask for "Boltbabble.doc"

FR Millmore

I actually use an old screwdriver. you sort of "lean" on the carbon and it pings off. I have completly cleaned the pistons in the past without the world coming to an end, but hesitate to recomend it. I have wondered if a bit of carbon does get down the side of the piston, wont the oil flush it out?
Stan Best

The grease in the bore before you bring the piston up prevents any problem; what tiny bit may be left gets blown out when running.
FR Millmore

Thanks for all the replies,

Is it ok to use wire decarbonising brushes (in a drill) to clean the tops of the pistons, or would they be too soft?

Peter S

In the 50's and 60's we heard that if you held the revs around 2,000 and slowly dump water into the intake of the carb or carbs without air filter of course the engine would smoke and carry on then eventually settle in with steam cleaned heads and valves. Has anyone ever heard of this or is it some urban legend. At some level it makes sense although I am afraid to try. What do you think? Gary L. Emes
gary emes

Wire brushes might work, it depends on the carbon - some is soft, some is hard - harder than the pistons. If you have to really lean on it you can damage the pistons.
Remember that the pistons are covered with a layer of aluminum oxide, very hard stuff. So if you don't break through it, the piston is well protected.

The water thing works, best done as a mist under load. This was commonly used as an anti detonation coolant in race cars and aircraft for heavy take-off conditions. My father built such for a 1936 Dodge he had. He found that in constant use, the steam ate wormholes in the pistons, but it was a clean wormhole! I don't know where the tipping point between clean and holey is, possibly a religious consultation is required. I keep forgetting to do water injection tests before dismantling to sort this.

FR Millmore

Is it sensible to remove the pistons for cleaning and re-fit without chaning the rings (compression is spot on so I've no reason to change the rings), should the bores still be honed?

Peter S

Refitting original parts does not require a hone.

Check the ring to groove clearances - rings wear on the under surface leaving a sharp lip - too much wear and the ring starts to pump oil upwards.

Cleaning can be eased by soaking the pistons in petrol overnight - this softens the carbon deposit and allows it to be easily removed.
Chris at Octarine Services

Thanks for all the info.

After all I've decided to take out the pistons and replace all the rings in the process. So now I need a bit of advice on honing to remove the glaze and seat the rings.

I've got a good quality three legged hone and a variable speed drill, and I know what the end finish should look like. Could someone describe the basic process and any handy hints?

Peter S

Follow the instructions I gave above.

Use kerosene or as you call it, paraffin as a honing lubricant - gets the job done quickly.

Pack the area around the crank with paper towels or rags to absorb the abrasive laden lubricant before it runs all over the crankcase - easier to prevent than to clean up.

Cut a piece of thin metal to put on top of the rags below the bore bottom so you don't get the hone caught in them!

You will likely find that the drill will still turn fast enough that you need to be right snappy about stroking up & down to establish the crosshatch. Keep it moving when you stop, and press the stones together to get it out of the bore - DO NOT pull it straight out after it stops turning - it puts nice straight scratches to cause oil consumption!

Clean as mentioned.
Clean as mentioned.
Clean as mentioned.
Clean as mentioned.
Clean as mentioned.

Think it's clean enough?

Clean as mentioned.
Clean as mentioned.
Clean as mentioned.

OK, if you can wipe the clean oiled towel through the bore without any trace of black - if not, well you know!

FR Millmore

Flex hone is a wonderful product for this procedure. Rather than using 3 long stones, it uses dozens of small "balls". You get it rotating, then slide it in while it is spinning, do the honing, and remove it while it is still spinning. Takes about 20 seconds and no holding of stones to avoid vertical scratches.


Pretty simple.
C Holm

Thanks again for all the advice, one last question before I start.

I can vary the speed of my drill from 10rpm upwards! Any advice on a sensible rpm to start off with?

Peter S

I know this will make some giggle but one of the best lubricants for honing is lard. When done, clean up with dish deturgent and water. One of the best hones is made by Sunnun. It has four stones in a holder that is adjustable for diameter. The stones are held vertical to the cylinder walls, no springs are used.
For softening the carbon, soak it in carb cleaner.
John H

Im just still curious about the whole water misting. Anyone else even heard of it ? Let alone done it ?

What did I say above?
FR Millmore

"Im just still curious about the whole water misting. Anyone else even heard of it ? Let alone done it ?"

It works well, and also works as a pretreatment for using seafoam directly into the intake via vacuum hose.
C Holm

I used to work in a shop back in teh ealry 90s and i have several times used water to clean the carbon, it does work, but do not go hog wild, too much water and you will bend a rod, water doesnt compress!
Darren Brandon

Thanks Darren thats what I wanted to hear!
What do you use a squirt bottle or something ?

Also FRM I think we all duly noted your love for kerosene

we used an old windex bottle set so spray not mist, but i suppose you could use a water pistol if thats all you have. just keep the revs up while doing it, it will steam and shimmy and shake then smooth right out.

This thread was discussed between 24/05/2007 and 07/06/2007

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