Hello every one i finely got round to taking apart my M20 to have alook why it was smoking more than i do
At first glance the barrel lookt oke bit off wear but the piston

I hoop you guys can figre it out because i am baffeld.

I hoop i can fix this and make normany again this year


Greets Erik

email (option): evandeweerthof@yahoo.com

Is the piston damaged on the top edge in the third picture,if so was it on the side away from the valves? Otherwise it looks to be missing the top piston ring.

That top groove is not a really a ring groove. Ron

email (option): ronpier@talk21.com

The piston, dosen't really look too bad, they always tend to get a little damaged on the area above the top piston ring. You have to remember that the piston is only a carrier for the rings and within reason isn't that critical as long as it isn't that loose that it rattles in the bore, or the ring grooves are that loose that they allow the gasses to get behind the ring. Looking at your piston I think it would go again if the criteria I just mentioned are met. Possibly the only thing I would do would be to fit the next oversize rings and just gap them down to fit after you've honed it a bit to try and remove the ridge at the top

email (option): davmax@ntlworld.com

Yes, the piston doesn't look too bad. Note that there is very little 'blowby' discolouration below the rings. Blowby would normally be present if you had a badly worn bore/piston/rings. It might be worth checking the ring gaps of the existing rings to determine whether they were gapped correctly in the past, particularly the oil scraper ring...
I would carefully clean it up and get the bore to piston clearance checked to make sure it is within limits. If that is OK a light hone and a new set of correctly gapped rings should do it.
One other thing that you could check is the clearance between the inlet valve stem and the guide. If these parts are badly worn you might draw some oil up the guide on the induction stroke...Ian

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

The top groove looked too narrow , but what is it ,deccoration?I was interested in the damage as my old piston looked like it had been peened on one side above the top ring.

Hi Pat..The unused top groove is a 'heat dam' to help protect the top ring from transfered heat. Some modern pistons still have the same feature ...Ian

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

Many thanks for the nice reactions but my question stil stands what is the cause off the damidge?

email (option): evandeweerthof@yahoo.com

Do you have an air filter? Don't forget that if you haven't you have been riding along sucking up dust, grit, little old ladies and suchlike! Some of this will be ejected by the exhaust valves, but some of it will inevitably cause some scoring, when it reaches the heat dam it will probably stay in the groove, but on it's way down there it will cause a little bit of damage. The carbon itself it capable of causing damage if it detatches itself from the top of the piston.

email (option): davmax@ntlworld.com

Yes i a probebly the only m20 with an air filter from KN fore me it is mandetory because i ride a lot of dirt roads as well,could it be that the carbon deposit is glowing so much as to melt the alloi in bear places?
what is the melting temp off this alloi?

email (option): evandeweerthof@yahoo.com

It's difficult to tell with that tarry oil on the crown but the only time that I've seen pistons overheating above the top ring groove was back in my 2-stroke days if I got the ignition timing out.

My old piston had simmilar damage.there was a ridge in the bore that was worse one side and the crankshaft had "issues". These included the wrong oil flinger,knackered drive side ball bearing and the roller bearing race fitted the wrong way round.

I think i found some thing the smal end dos have some un even wear could that be it maybe tilting the piston or even cold welding or hot welding it to the bore? the strange wear is n the valve side off the bore.
The timing hase being inpeckeble so in crosing that off my list Rik but a nice tip .ive nuch wourse pistons in my to stroke days but this baffelde me becouse i could diagnose the problem .

email (option): evandeweerthof@yahoo.com

Hi Erik...If you clean off the top of the crankcase, fit the gudgeon pin to the rod and then put some parallel bars on top of the crankcase, you can turn the engine over until each side of the pin rests on the bars. This is a quick way to see if the conrod is bent, as any tilt in the pin can be clearly seen as it won't contact both bars evenly...Ian

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