Hi chaps

an issue is raising it's head (potentially literally)on my M20. The piston crown is about 2mm above the top of the barrel at TDC so it needs to have a pretty thick head gasket to get any clearance at all. I have it all apart again at the moment and the piston seems to have just touched the head though I couldn't hear anything when I kicked it over (and I did a lot of kicking). There also seemed to be a fair amount of compression. Another thing is that there were a load of base gaskets too (about 5mm including a steel one) which I didn't refit at first but I couldn't get the valves to have any clearance so I had to refit, but I now realise the piston would have been far too high. Now the valves are able to be adjusted ok but there's still insufficient room for the valve lifter cam to fit. The previous valve lifter was ground off to suit this.

At first I thought that there might be other valve lengths available from, say, a later model M20 or M21, but this seems to be pointing towards an issue either with a rogue piston, or more likely (or maybe not) to the barrel being too short. Possibly from something else?

I wonder if I have a hybrid of nearly fitting parts? I wonder if someone has or can point me to some workshop data so I can check some of these components dimensions.

I can get dimensions of the plot as existing if anyone can help

Rick

email (option): rick.howell@talktalk.net

Hi Rick,it seems you may have a M21 crank fitted.the M20 has a stroke of 94mm
the M21 has a stroke of 112 mm, so a difference of 18mm halved would be 9mm.
You need to check the stroke,a rule would show the length reasonably accurately
without any further dismantling.If that's the case,you need the correct piston
which has a different pin to crown dimension.
hope this helps.
Cheers Rick

email (option): richardholt@rocketmail.com

Thanks Rick

I checked the stroke and it's definitely 94mm. Checked the height of the crown of the piston above the liner/barrel top face at TDC and it's 1.7mm.
That would suggest a tight fit even with a solid copper gasket though if that's how it should be then I'll go with that. It doesn't seem right to me especially as the base has been packed out by at least 4mm.

I have wondered if the piston is incorrect as it is domed at the edges by about 4mm then flat topped. The only stamping on it is a letter A which I assume is the tolerance mark. It clearly hasn't been machined recently but in view of the time available maybe I'll machine a few mm off the crown if there's sufficient thickness to do so.

If I can get it to run sufficiently well (might not be perfect) then that will do for me.

the rickster (in Exeter)

email (option): rick.howell@talktalk.net

It seems you have an incorrect piston fitted, the M20 piston has a completely flat top...

Taking a measurement from the center of the gudgeon pin to the top of the piston would confirm this...

The 'static' clearance between the piston and the cylinder head will reduce once the engine is running due to flexing of the crank assembly..

So, even though you appear to have sufficient clearance that may not be the case with the engine running when it may still be possible for the piston to contact the head...

I would recommend a 1mm (.040") clearance (measured statically) between the piston and the head with the gasket fitted....This will ensure there is no contact with the engine running...

If you machine the piston crown take care not to make it too thin...I don't generally go below .150" when machining OHV pistons and a little more might be advisable for the sidevalves which run at higher temperatures...

What is the bore size currently?....Ian

email (option): ian@wright52.plus.com

Hi Rick,

The original or pattern piston defiantly should not be domed.

Another point worth to conceder:

If you want to keep using this piston, which can be fine,
It is best to keep exactly the same height / thickness you had under the barrel,
As if you take 1mm of it, just as example, now the top ring would go 1mm further/higher in the barrel,
To a place slightly narrower than the rest of the stroke, as it did not get worn by the top ring when it worked before.
This tiny “step” or / “stair” makes the ring contract very very very slightly
Every time it is on the TDC, and expend/open again when starting to move down the barrel,
And that usually breaks the top ring, sooner or later.

Same goes for the bottom ring,
If using a much thicker gasket under the barrel.

Best regards,
Noam.

email (option): noam10@gmail.com

Noam makes a good point..You must hone out any wear ridge or carbon build up if you are altering the position of the barrel...

Worth noting as well that 'pop up' pistons, which protrude above the barrel were used by Harley Davidson in the K Models to improve gas flow in the head without reducing compression ratio..

Therefore, plan B could be to machine the head to clear the piston, rather than the other way round...
So, you would start off with a standard 1mm gasket and then machine out the combustion chamber squish area to give the correct clearance over the piston..

The only problem then would be where to source a replacement piston when you need one eventually!

I run a domed piston with a machined head in my own bike.
Though that is a larger capacity engine the same principle apply....Ian

email (option): ian@wright52.plus.com

Thanks Noam and Ian - all valid points and helping me to come to the right decision. I'll take the barrel off tomorrow and measure up the piston and bore and see if a skim is on the cards.

I feel that the piston is expendable (and I have a lathe) but a head isn't (and I don't have a miller) so I'll try to get by with what I've got (or another gash piston if someone has one) and then look to getting the barrel rebored/honed with a correct piston at a later date.

0.150" Ian? Say 4mm as an absolute minimum piston crown thickness?

BTW how much crown/head clearance does a standard piston normally have?

the rickster

email (option): rick.howell@talktalk.net

4mm will do it...

Head clearance with the standard set up is approx. the thickness of a compressed composit gasket but is also dependent on the thickness of the base gasket used....

I experimented with closing down the clearances to improve the squish.

I found a point was reached where there was static clearance but the piston contacted the head when running..

Aim for a .030"-.040" (.75mm-1.00mm) clearance as noted in a previous post...Note that .040" (1mm) clearance is considered the maximum desirable to obtain correct function of the squish area...Ian

email (option): ian@wright52.plus.com

I would dump the domed piston and fit flat top...on my engines I machine deck so there is 15thou clearance and fit 0.6mm solid copper gasket...if you want more compression you can fill some of combustion chamber with weld

Just to be clear...

Do you mean a .015" clearance between the piston and head without a gasket and then an additional .0235" (.6mm) head gasket, making a total clearance of .0385"?....Ian

email (option): ian@wright52.plus.com

No Ian, 15thou from top of piston to face of block then with a 0.6mm solid gasket gives about 35thou approx.

Well, an update. I saw this when I rebuilt and thought nothing of it, but now....

The piston is marked "Indian 85734" and "Lo-Ex Lynite", "Reg.T.ms. A.c. of A. Patent 1799837"

It has an O/d approx. of 82.00mm - the unworn cylinder bore is approx. 82.20mm - and the height from centreline of gudgeon to crown of the piston is 45.98mm. There appears to be 6.4mm of metal thickness in the crown as a minimum (and there's a rib under the centreline for strength) so perhaps a couple of mm off the top won't hurt.

Crikey, if Bert Munro can do it......

Rickster

email (option): rick.howell@talktalk.net

all done, runs a treat.

Thanks for the advice, chaps, we're on a roll.

Rickster

email (option): rick.howell@talktalk.net