Piston broke .......

[Group Leader]

The old jokes are the best but in this case, the piston really is broke and it's no longer a joke

After covering two or three hundred miles on the little B21 the engine breathing became heavier and it became harder and harder to start when warm.   I also began to noticed a distinct lack of compression particularly when hot.   I checked the oil filters and found small pieces of debris and so, suspecting ring trouble (some of the debris looked decidedly ring like which was a bit of a clue), I removed the head and barrel.

It wasn't a pretty sight with all 3 rings in more pieces than you could shake a stick at but, perhaps more problematic, the land between the rings has been damaged and so clearly it's only use now is as a paperweight so I'll be needing a replacement which, might be tricky due to its age.   The bore escaped relatively unscathed but will need a re-bore just to be on the safe side and I'm just about to try and determine what size it currently is.

A  note from the parts book that it should have a 65-409 piston (and a quick Google for replacements didn't look particularly promising).  However there appears to be no number of that form on the piston but it does have the number 11563, "AE" and a patent number 616820 cast inside the skirt.   I think that might be a Hepolite number although that number appears to relate to a Triumph engine. 

So, my questions are:

1) Is it indeed a Hepolite piston?

2) Is it the equivalent of BSA's 65-409 or is it one that's been used as the correct one was previously unobtainable?

3) The piston has two slots, one either side of the gudgeon pin that separates the crown from skirt fore and aft - is this typical BSA or is it more typical of an alien piston?

4) What are the possible causes of the damage?  One suggestion is fuel wash which would marry up with the tendency for flooding as I believe the wrong carb is fitted which is bigger than it should be.  I had to drop the needle to the lowest slot to get the mixture somewhere closer to correct (although still on the rich side)?

5) And of course the big question, where might a source a suitable replacement?

TIA

Alan

[Y13 Alan G]

Hi Alan. I can't help you with causes of your problem but can only suggest that you try and look at/obtain a piston catalogue to find a piston with a 63 mm bore (Plus 20 thou or whatever you need). Many years ago when I was re-building my G32/12 there were no pistons available (80 mm) but I did find that Ford 105E pistons were the right diameter although 30 thou had to be machined off the head of the piston to get the correct gudgeon pin to crown height. There was ample thickness to do this. Many years later I found an original set of pistons at an auto jumble and fitted them. You may have to search for a while but I am sure there is a suitable piston available if you search. Best of luck, Alan

[JulianS]

My Hepolte catalogue shows 11563 as Triumph Tiger 100, 1939-54. 7.7:1 compresion on the Triumph.

The slots are thermal slots part of piston thermal control.

[Group Leader]

Thanks chaps.   Yes that's what I found from searching various websites it does indeed appear to be  a T100 piston.   That's good because you can at least find those, unlike the correct BSA one which, I suppose, is why it's in there following previous remedial work undertaken by a previous owner.   As it's an AE piston I think that probably dates it to the 70's - 80's?

Now to find out what size I need and someone to re-bore the barrel to suit.

Alan

[JulianS]

There are some significant differences between the pistons to consider.

The Triumph has a greated compression height and a greater overall length.

The Triumph pin is 17.5mm the BSA is 5/8 inch (15.8 mm about)

The pages below from Hepolite 1958 catalogue give the height and length.

[Group Leader]

Thank you Julian that is really useful information.   A friend and I were wondering about the compression height and the skirt length.  Our suspicion being that my piston has had the skirt shortened (as it is flat bottomed) and doesn't have the flywheel clearance that the illustrations I have seen of this piston have.

Time to get the measuring devices out, a job for tomorrow I fancy!

Alan

[Group Leader]

From Julian's information a quick sketch.

It appears that the piston is +0.20" and there is about 4 thou wear on the bore diameter front to back compared to the bottom of the bore (if I've used my mate's bore gauge correctly).    The bore has light scoring all round.    Should I attempt to get a replacement +0.020" piston and hone the bore or go for a re-bore with a +0.040" piston.

Another mystery is what is the origin of the con-rod?   Is it a B21 one with the little end bored out to match the Triumph pin or something else?   

One thing is for certain, no point looking for a correct B21 piston (if such a thing exists now) as the pin won't fit!

Alan

[Group Leader]

Julian do you have the spec for the B21 Super engine anywhere please (or can you point me in the right direction)?

Particularly useful things to know would be: the standard compression ratio, con rod length,  deck height and really pushing my luck, the head's chamber volume.   

I'm trying to work out what the compression ratio was with the T100 piston so that I can see how much higher it was then it should have been.    I've measured the volume of the head's chamber volume as best as I can by measuring how much fluid it will hold (good job the valves don't leak) but I'm sure there's some measurement error included.

Any info you can provide will be gratefully received!

Alan

[JulianS]

From the 1938 BSA data book;

The comp ratio  6.2:1.

Conrod length - dont know - but con rod to crank ratio given as 4.17 which seems a bit high.

None of the other details in the book.

Got no more info I am afraid.

[Group Leader]

Thanks Julian.

Yikes - That compression ratio isn't very high!

I'll see if I can get that figure with the calculator I'm using, that will give me a good idea if the value I work out for the T100 piston is correct.

Alan

[Group Leader]

Well I both calculated and measured the compression ratio to be around 10.5:1 which is something over the stock 6.2:1!    No wonder it was sprightly when running but was obviously seriously over-cooked.   Fortunately it now appears the piston and rings bore the brunt of the abuse.     

As the conrod little end is now sized for the the Triumph piston and not the BSA there's no easy way back to stock.  So, I've got a replacement Les Harris Triumph piston at +040 (well, to be correct I've actually got two as they come as a pair for the twin  ), the barrel is away being re-bored and the plan is to insert a 2mm annular copper head gasket between the liner and the head and provide a slightly thicker version of the aluminium base washer/spacer that was fitted when I got the bike.   The intention being to get the CR down to around 8:1 which hopefully will be a lot kinder on the old girl but still retain a little bit of the "High compression" sparkle that I was just getting used to.   I'm just hoping there's enough "capacity" in the tappets to accommodate the increased distance between the followers and the rockers.    Fingers crossed!

Anyway, we'll have to see how it goes.

Alan

[Gtt]

I would think an oversize gudgeon bush could be made to accomodate a standard gudgeon.

[Group Leader]

I would think an oversize gudgeon bush could be made to accomodate a standard gudgeon.

Too late for that ......

[Group Leader]

Well the barrel was returned bored and nicely honed, the crankcase was flushed and blown clear numerous times and the top end rebuilt with the new Triumph piston and my copper compression reducing head gasket.     Measured compression ratio came out ~ 7.5 1 which was a lot closer to what it was when it left the factory!

I'd been taking the Beeza for short Running-In trips around the village, i.e. never very far from home. Nice and flat, low road speed, medium revs being careful, I thought, not to over-rev or load the engine. Having done numerous circuits on different days I thought I'd venture to the next village which did involve going up a gentle hill (but it's Bedfordshire not Devon!) and unfortunately only got halfway up when all power was lost and the engine stopped as soon as I snatched the clutch in. Being but 3/4 mile from home and the first bit was downhill, I returned home with the assistance of gravity and Shanks's pony.

Once the engine was cool I gingerly turned it over but it didn't feel right. A look down the plughole with my little endoscope appeared to show a mark on the rear face of the cylinder wall so there was nothing for it but to whip off the head and barrel and take a look. At least I can do that pretty quickly now!

It was clear that the piston had nipped up on the inlet side with piston material smeared over the rings in their grooves.   The piston was a very nice fit in the bore (maybe just a bit it too nice?) when I assembled it first time so if I dressed the piston back in the effected area, if anything, it would have a smidge more clearance where it nipped up.

Having cleaned the piston up and paid particular attention to the ring grooves I had a look at the rings. You could see that the compression rings had bedded down about 50 % the way across the surface but they had extremely sharp, almost burred, corners. I checked the ring gaps once again and they were inline with current thinking I believe (0.005"/inch of diameter and a minimum of .015" for the oil ring irrespective of diameter). I gave the ring corners the very lightest of stoning, not to put a chamfer on but just to make sure they wouldn't catch and then I made doubly sure that the rings were free in their, well-oiled, piston slots with no tendency to catch or snag when compressed down to their operating diameter.   Fortunately the barrel was undamaged with just a slight polish mark in the affected area so this was lightly manually re-honed.

The oil filters were checked for any debris, all of the oil was discarded and the crankcase and tank thoroughly cleaned once again (not that anything was found). Then it was time to put everything back together so that I could try again!

I've since been doing some slightly larger circuits of about 10 miles, essentially still on the flat, and reasonably close to home at all times.   As the sun was out I did another 3 laps today (the photos are actually from a run a couple of weeks ago) which has put the total mileage up to around 180-200 since the second re-assembly.    Touch wood, it seems to be going alright.   It's fun but slightly challenging to ride as the gearbox has to be nursed quite a lot so no doubt that will require some attention in the near future but I do think its a pretty little thing even if it is bit of a bitsa in some respects   

Alan