Okay so for anyone that has been following my posts my bike seems to be dead stopping when heated, but starts up and runs just fine otherwise. After going through everything from valve clearances to fuel flow, it was suggested that I pull the barrell and look for signs of piston nipping.

Boy was there. Please take a look at the link for the pics. On both sides of the piston you will see marks. Now they are smooth, but probably should not be there. However on the inside of the barrel you will see "2" identical gouges. They coincide with the lower section of the connecting rod where it mates with the piston.

I am pretty sure they are not supposed to be there. I think the piston has flared on one side where it connects to the piston rod. Since the gouges are so deep. Do you recommend me over boring and getting a new oversized piston head??IF so which one? Thanks



https://picasaweb.google.com/115674442219018009194/BsaSecondPics?authuser=0&feat=directlink

email (option): jeff@infinitymedspa.com

Jeff I haven't exactly worked out what has caused those railway lines in your bore? Never the less they shouldn't be there! You will have to take the barrel to an engine machine shop for measuring. Firstly to determine the size of the bore at present, and secondly to estimate how much will need to be removed to clean up the bore. Then you will have to hunt for an appropriate piston before a re bore is undertaken. I suggest you get these measurements and then get back to this site. Ron

email (option): ronpier@talk21.com

strange marks did you see the bore with no marks ie brought the bike assembled if so it might have been there when assembled by someone else

Just a thought, could those grooves be a result of the gudgeon pin, sans circlip, having contacted the bore? The grooves look deeper at the outside edges as might be expected when the flat end of the pin contacts the round bore and the centre of the pin doesn't contact at all.. If so, what happened to the circlip??

Cheers
Pete

email (option): petercomley@web.de

I had this happen with my 1957 Triumph '21'; a gudgeon pin ring had broken and nested two pieces in the piston; that created two DEEP furrows in the barrel, too deep to consider a rebore. Fortunately, the barrels of the 1966 3TA are identical and cheap

email (option): viaconsu # planet dot nl

Hi Jeff...I suspected that piston siezure might be the problem with your engine from the initial postings describing the symptoms. The grooves in the barrel are typical of the damage caused when a circlip comes adrift and either the clip or the end of the pin are forced into the liner. As your pin is still in place this is obviously 'old' damage from the past. Careful measurement of the bore will be required to determine how deep the grooves are but it is often the case that they exceed the limits for any rebore.
If that is the case a new liner will be required.
The piston is scrap and will have to be replaced.
It is not clear from your photos whether the damage on the piston is broadly the same on both sides of the piston..could you clarify that point?
Also is there any damage on the thrust faces of the piston? (front and back?).
If there was an overall bore to piston clearance problem most of the damage would occur there as the piston is a larger diameter across the thrust faces than it is along the axis of the pin.
If the damage is restricted to the sides of the piston there are two possibilities.
First the previous damage was not cleaned up well leaving some raised material on that side of the liner, reducing overall clearance and forcing the piston towards the other side of the bore. With insufficient clearance on both sides siezure is the result.
Second, the conrod might be bent...This 'tilts' the piston, reduces piston to bore clearance, and can exert excessive pressure on the sides of the piston leading to siezure. Sudden locking up of the engine when a gudgeon pin comes out and the extreme forces applied to only one side of the piston have the potential to do this, particularly with the long, flexible conrod of the M20.
So, I would first remove the piston, clean the cylinder gasket face thoroughly and place some suitable parallel bars onto this surface either side of the conrod. Replace the gudgeon pin in the piston and turn the engine over until the pin rests on the bars..you will then be able to determine whether it is contacting squarely on both bars and is lying parallel to the gasket face. If not the rod will require some attention. This must be determined or there is a risk that any repair work you do on the barrel and piston will be ruined when you run the engine again.
Once that is dealt with return to the barrel to sort out the depth of the damage, the current state of wear of the bore and choose between a straight rebore and a new piston or a resleeve job as appropriate...Ian.

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

excellent replky as alwys Ian and everyon else too. Thank you. Just for clarification what are parrallel bars and where do I get them?

email (option): jeff@infinitymedspa.com

Sorry forgot to answer the other question. Yes the scoring marks are definitely almost the same on both lateral sides. No damage to the front and back of the piston. They are scored but still feel smooth...does that matter?

email (option): jeff@infinitymedspa.com

Parallel bars can be made by getting some "ground" finish steel in either bar or round bar form, the idea is to ensure that they are exactly the same measurement when you bring the piston down on them, if you can insert a feeler guage on either side of the skirt of the piston it then indicates that the rod is bent or the small end bush has been reamed at an angle

email (option): davmax@ntlworld.com

I tend to prefer the flat as opposed to round bars. If just the gudgeon pin is fitted to the rod you then have a 'line contact' (or not) as the pin is brought to rest on the tops of the bars...I find this makes it easier to spot any discrepancy.
As Dave says you can get ground stock in various sizes from an Engineering suppliers or you can purchase sets of parallel bars reasonably cheaply from the same sources.
They are handy things to have in the workshop for checking the flatness of any surface, such as carb manifolds, for instance as well as for the use described....Ian

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

does anyone have a pic? Everyone seems to say i need to pull the engine to check?

email (option): jeff@infinitymedspa.com

I just heard back from the machine shop and the barrel will have to be relined. Should I take this chance to fit a m21 piston to the m20 rod and have it overbored for more power? Also what kind of clearance will I need? Is there any other mods to the bike needed if I go this route and I will double check to make sure the rod is straight on the bike..

email (option): jeff@infinitymedspa.com

Hi Jeff...I don't have a picture...but look at it this way. If you put the gudgeon pin through the conrod and were able to turn the engine over until the pin rested on the crankcase upper face it would be lying flat on that face. The only way it could not lie flat is if the conrod was bent..which would tilt the pin off the horizontal.
Of course the pin doesn't go down that far..so putting in two parallel bars, resting on the crankcase face, allows you to effectively raise the crankcase face. Then you can carry out the same test...Ian

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

M21 has same bore as M20- just the stroke is longer for more displacement. Pistons are different .
I would say somebody assembled it wrong in the past without gudgeon pin circlips, took it apart and slapped it together for sale :( Pretty poor seller performance.

email (option): vinver@ns.sympatico.ca

As Vincent said, an M21 piston is no good without the M21 flywheel assembly. Just go for standard...unless you want to do a major engine overhaul. I expect Henk can supply you with a piston if you can't get one there. Ron

email (option): ronpier@talk21.com

I will probably need to get henk email. What s it please?


quote=Ron Pier]As Vincent said, an M21 piston is no good without the M21 flywheel assembly. Just go for standard...unless you want to do a major engine overhaul. I expect Henk can supply you with a piston if you can't get one there. Ron[/quote]

email (option): Jeff@infinitymedspa.com

Henk is on Crete at the moment, but I know he has his laptop with him. ahum@quicknet.nl

Ron

email (option): ronpier@talk21.com