Possible cause for seizing

[tinu]

Good evening lads,
I have had a seizing engine early this year, but I didn't find the time to take off the head and the cylinder to have a look at the damage. Now there is some spare time (what a wonder) and the damage was very obvious (look at the pictures). I found a guy close to me who makes the bores fitting new +40 pistons. He thought of insufficient oil feed to the piston(s). As you may see, there are bad scars on the rhs piston, much less on the lhs one.
Is there a possibility, without further disassembly, to check oil feed etc.? What else should I think of...?
The engine was rebuilt by the owner before the previous owner (run 5k since then) and I do now almost nothing about his capability on engines... except he was a motorcycle mechanic. The bike ran smooth and nice, no smoke, no suspect noise (except last run round the block I aborted quickly).

[trevinoz]

Not enough piston to cylinder clearance.

[ironhead]

Hi Tinu.
Seizure on the sides of pistons can be 1 of 3 things.
1- bent rods.
2- bores not bored square to cylinder base flange.
3- crankcase deck not square to crankshaft.
 Most likely the rods are bent (they bend easily especially if gudgeon pins are hammered in    )

[Slymo]

Or as previously mentioned lack of initial clearance or insufficient oil. I personally have issues with other peoples rebuilds. I have had several bikes that had potentially fatal flaws from previous setups. One Royal Enfield had its big end replaced with an expensive roller bearing setup but the old white metal from the previous failure had not been removed from the oilway. Another bsa had an oilway blanking plug left out of the crankshaft. If the bike is sold in a not running or not running well condition I tend to pull them down to component parts.

[Triton Thrasher]

Not enough piston to cylinder clearance.

Yep, the classic four corner seizure.  Can be caused by inducing pinking by labouring the engine, but on a recent rebore with no piston crown damage, it’s almost certainly lack of clearance caused by an ignorant engineer.

Nothing complicated like oil or bent rods involved.

[coater87]

 Flat top pistons, is that a compression plate or are my eyes deceiving me?

 Lee

[tinu]

Thanks a lot for all your answers!
Can a bent rod be checked when still in the crankcase...?
My first idea was the piston clearance, when I read old threads in the forum. The guy at machining shop told me that the clearance is already included in the measurement of the piston. That means the +40-piston mesures only 70 mm plus 28 thou (40 thou minus 6 thou clearance on both sides), ist that correct?
@Lee, you're right, it has a compression plate...

[BSA_54A10]

take the pistons off and then photograph them sitting on a table with good light .
To me looks like too little bore clearance.
Pistons could very well clean up enough to go back in with a hone
Also some photos of the inside of the pistons to verify there was oil up there
And with pistons off thread a length of dowel through the little end eyes to check for truth.
They have to come off anyway so while it is apart costs nothing to check

[Triton Thrasher]

I don’t believe there was 6 thou piston skirt clearance.

[Swarfcut]

Hi tinu, just reading your post again and again to make some sense out of it.

Looks as if the bike has done some 5K  Km with no problems on plus 20 flat top pistons with a  compression plate on the cylinder base. Then you mention plus 40's.  Does this mean you have had it rebored to  plus 40, but need to be sure the oil supply is OK?

 If the plus 20's seized soon after being put in, then the cause is most likely a too tight bore/piston clearance. If it suddenly happened, after some uneventful regular use, then something is wrong.
 If you still have the barrel on plus 20, get the bore honed, clean up the pistons, make sure the rings are free and try again. It may rattle a bit, but you know why, and it is worth doing before spending again. Of course the perfectionists will be in horror, but they are not spending the money.

 Without stripping the motor, the oil pump output can be checked by removing the pressure relief valve....you should get a good  dribble of  oil when you rotate the crankshaft. You can also check for any lift in the big ends, as an indication of any bearing problem due to lack of oil.

 A couple of additional posts were added  while I was typing this.  Bent rods are unlikely if the piston damage is more or less consistent on both pistons. Straight rod across the small end bushes, as suggested. Heating pistons with a hot air gun means the pins will push out with very little force.

 Piston manufactures state the nominal bore, plus the oversize, and this is the size the finished, honed bore should be. The piston is manufactured to a size which takes into account this running clearance. Some machinists discard this information, measure the piston and add on a clearance from their experience, usually with water cooled engines which run to a tighter bore than air cooled motors. Some are not too happy to be told they got it wrong, so folks don't complain and just go somewhere else, so the guy continues to bugger pistons and bores for years, unaware of the misery he has caused.
 Clearance is the diameter of the piston measured at the pin height, at 90 degrees to the pin, compared to the bore diameter. Pistons are slightly oval, being narrower each side, compared to thrust faces, so the measurement is the thrust faces, front to back, not the pin end sides. The piston crown above the top ring is narrower than the skirt, which is why you can sometimes see the top ring when you look down the bore and foolishly convince yourself it's worn out and needs a rebore! The piston running clearance is in the BSA published data for your model year.

 Swarfy.

[Greybeard]

The piston crown above the top ring is narrower than the skirt, which is why you can sometimes see the top ring when you look down the bore and foolishly convince yourself it's worn out and needs a rebore!

That is great advice 

[tinu]

Hi Swarfy
As you read right I did 5k km with no problem (smooth riding, no racing) with +20 pistons and compression plate. Sometimes there was little pinging on steep hills. The seizing happened on top of a mountain pass (Susten, 2'260 m above sea level) on a straight street running aprox. 80 km/h.
I had a close look at the bore with the machining guy. The scars where too deep, he said they will still be there after just a hone. So I bought a set of new hepolite flat top +40 pistons...
There was an equal amount of oil inside both of the pistons, but when I removed the gudgeon pin, the hole was damaged. I'll have a quick look at the shop to make some more pictures from the piston and the bores. And I'll talk again to the guy about clearances. 
I'll keep you updated...

[Triton Thrasher]

Generally, if it can do 5000 km it should really run forever.

Never let it pink though. Change to a lower gear.

[Swarfcut]

 tinu, Just had a look at the Susten Pass, Switzerland. Thanks to Google. Looks a truly spectacular road to ride, it is listed in the category "World's most Dangerous Roads"

 I think it's a case of the motor working hard, with  perhaps a bit too much advance for the fuel being used, despite the compression plate, and the mixture on the weak side for the altitude. Overheated, seized and nipped up before too much damage done. TT is right, never let it pink, back off, change down, even add a bit of choke to richen up and aid cooling. Looked to be a reasonable and reliable build until challenged with something out of the ordinary for most of us, maybe everday riding for you.

 New stock Hepolite Pistons are not the same as the ones we trust from UK maker AE (now no longer in business as such after numerous takeovers). The new style packaging features simulated greasy finger prints as if they can't be bothered to use clean hands to pack them, and being distributed by W*SS**l is your guarantee of hassle. There are numerous threads on the forum about the quality of the various commonly available rings and pistons. Go by their suggested running clearance, as the modern materials used are unlikely to match the expansion rates of original pistons, and makes published data from way back redundant.

 Your new pins should just slide into the small end bushes, make sure the bushes are OK and the oil hole matches the one in the rod, as the bushes sometimes creep on the small end eye. Was the damaged hole on the pin boss of the piston, or the rod? Have a good look at the base of the clutch (Drive) side con rod. There should be a small bleed hole that points towards the central flywheel.

 Swarfy.

[muskrat]

G'day tinu.
As mentioned by others most likely not enough piston clearance, hard work going up the mountain and pinging. If the ignition timing or carburation is a little biased to one side that side will heat up more than the other.
To check little end bush's I have a shaft the same size of the pin about 10 inches long. It should pass through both rods.
My last nip up on the A7 plunger had similar damage. (purists look away  ) I just cleaned up the piston, give the bore a light hone and put the bad piston in the good bore and the good piston in the bad bore with new rings. Done over 3000 trouble free miles since.
With those flat tops and a compression plate you could run the bike on kero 
Cheers