Has a piston seized ?

[Russell]

I’ve been for a run tonight on our Brum and when coming to a junction, after pulling in the clutch, the engine cut out, after coasting to a halt I tried to start it again but the kick start was solid. I pushed it to a nearby pub to check it over and everthing looked okay but the kick start would not turn the engine, it would kick through with the clutch in, and the primary drive chain would tension when pushing on the kick start, so I came to the conclusion the engine had seized and pushed it home. On further inspection one of the beaks of the pilgrim pump was dry so I can only presume one cylinder has been starved of oil.

Are my presumptions correct, and if so whats the best prceedure to free the engine, and what damage has likely to have been done ?

Sorry for my ignorance in these matters, but all help welcome.

Cheers
Russell

[BRIAN MARSHALL]

Hi, Not good news ! When the oil supply fails various problems then follow, not necessarily in the same order. The first point of call for incoming oil is the crankcase sealing glands, which in a standard engine is a metal-to-metal seal, so the interface will score and ‘pick up’. The next place for the oil is the main bearings, which soon get damaged if run dry of oil. After the main bearings the oil would get onto the rear face of the cranks, and by centrifugal force it gets thrown outwards into the groove on the edge of the cranks, and from there into the big-end bearings, so they also suffer if the oil supply fails. The cylinder walls get their lubrication by means of the petrol vapour passing through the crankcase picking up some of the oil that has been lost out of the big-ends, which collects in the ‘wells’ at the bottom of each crank chamber. Once that has been used up, the piston rings and bores will run dry of oil, then overheat, causing the pistons to expand more than usual, to the point of seizure. Usually the piston metal will melt in small areas, and be ‘wiped’ onto the bores.
Perhaps your best initial action will be to take out the spark plugs and squirt some penetrating oil into the bores. You could also take off the transfer port covers and see if this reveals any damage. After standing for about 24 hours, put the bike in top gear, then rock the bike backwards and forwards to see if it frees off. Even if it does free off you shouldn’t run the engine again until both it, AND THE PILGRIM PUMP, have been stripped for inspection and repairs.

[BRIAN MARSHALL]

Hi again, It may also be sensible to remove the alternator and its ‘door’ from the L/H side of the engine, and the distributor/oil pump set-up from the R/H side. You will then be able to see which half of the engine has been starved of oil, and often this is confirmed by the big-end eye of the conrod and adjacent metal being ‘blued’ from the excessive heat that has been generated. Unless you are a fairly capable mechanic, it will then probably be best to hand the job over to a Scott specialist, because if a piston is seized badly it can be one hell of a job to get the cylinder barrel off the crankcase ! Applying hydraulic pressure into the affected cylinder, via the spark plug hole, may get the piston moving, but this can only work if the both the inlet and transfer ports are not open to the bore, ie. the stuck piston has to be well up the bore when it seized. If it is a really bad seizure it may be necessary to cut through the conrod with a plasma cutter or die grinder, to enable the barrel to be removed from the crankcase. Hammering on lengths of wooden dowel down the spark plug hole might just shift a seized piston, but is fraught with the risk of doing damage, and it really needs some past experience of how big a hammer, and how hard to wallop !

GOOD LUCK !!
Brian

[Russell]

Thanks Brian, I have applied some penetrating oil to the bores, and will let you know how I get on. I’m pretty sure its the left hand cylinder that has been starved of oil, its the left hand side of the pilgrim pump that is dry, and on removing the spark plugs the left hand one is relativly clean compared with the right hand which is more sooted/oiled. If I can free the cylinder, will it require a total engine strip down or just a top end strip down to check for cylinder damage ? – Cheers Russell

[BRIAN MARSHALL]

With the alternator removed, a very simple job, you should be able to see if the big end bearings and conrod eye have blued, and if there are any metallic fragments in the ‘well’. (Use a magnet, one of those telescopic pocket ones). If the big ends are OK the main bearings might be OK too, and the piston seizure would be the main worry. IF you can get it moving freely you will be able to give it a partial inspection via the transfer ports. If piston metal has ‘wiped’ over the piston rings I’m afraid a strip-down will be necessary. If it looks OK it may be that just the skirt of the piston seized, and it all depends on the amount of damage that occurred. At least if you are able to turn the engine, it becomes a straightforward job to get the barrel off for a proper inspection and repairs. Personally I would not try and restart an engine that has suffered a seizure, as there may be deposits of aluminium in the bore, and damage to the piston is even more likely, both of which will need some attention. If you find metal particles in the crankcase well then obviously some skilled help is vital. And of course your Pilgrim pump needs an overhaul to rectify whatever caused it to fail on one side, assuming of course that it wasn’t a case of very bad ‘adjustment’ that made it stop pumping on one side. The oil tank and feed pipes should also be thoroughly flushed out. When I rebuilt a 1950 Flyer, over thirty years ago, I found a good handful of sand, grit, and small pebbles in the tank. A nasty bit of sabotage ! No wonder the engine had died !

[Kevin N Bayliss]

After many years of cutting corners and hoping that will do instead of doing the job properly on my old RD 2 strokes, I have even changed a broken piston at the side of the road and got home, but it only takes a small fragment of metal or a broken piston ring to cause catastrophic damage which could result in a scrap engine which happened far too frequently, good job there where plenty of spare engines around. Nowadays I hope I have a bit more sense.

I would have to concur with Brian and state I would not restart the engine without a complete strip down. It doesn’t take long to strip and rebuild and that way you will know for sure that all oil ways are clear, no debris, piston rings are free in the groves, bearings have not been too hot and started to fail. You might have got away with it but is it worth taking that chance. Good luck. Kev

[Russell]

Thanks for all the advice, we have managed to free the engine using the method Brian suggested, it freed quite easily. We checked through the transfer ports and no visible damage, also checked through the alternator door and no obvious overheating or traces of metallic fragments (just a very small amount of “filings” in the crankcase well). We’ve now stripped off the head and barrels and all looks okay, there is a small amount of “smearing” on the right hand piston (not the one that was seized), so it looks like we’ve got off without any damage. Just the pilgrim pump to check now.

We now need a gasket/seal set to rebuild and also a barrel coolant drain tap (as that got damaged while stripping it down), have you any recommendations of where to obtain these, also anything to watch out for while putting it back together?

[Dripfeed]

Hello mate try the ebay site that deals with old stationery engines check your tap as a good number of Scotts have had oversize taps fitted my Scott drain tap is 1/8 b s p .A tip that I would sugest is to turn the tap lever to face the cylinder block this is then less likely to get knocked open. Regards D F.

[BRIAN MARSHALL]

If the original 1/8″ BSP thread for the drain tap has corroded badly, and is not recoverable, you will need to go oversize, and as 1/8″ drain taps are easy to find, the best way to go is to install an adaptor sleeve into the oversize hole, with a 1/8″ BSP hole to take a replacement tap. This keeps things ‘standard’. Be VERY careful when drilling and tapping the oversize hole, as water jackets are surprisingly thin, and always corroded internally. Some castings have the drain hole lower than others, and these are usually OK, but some have it, for no good reason, about 3/4″ higher up, where the metal is thinner and more fragile. Why this was done is arguable, but it means that after draining down, a fair amount of coolant is left festering in the water jacket, causing corrosion when left standing.
Brian

[Russell]

The thread in the barrel is fine so I just need a replacement tap, the outside diameter of the thread is roughly 10mm so I presume this is 1/8″ BSP as you have said, I’ve managed to find this – https://www.completeautomobilist.com/products/338-straight-drain-tap-1, does this look correct ? or can you suggest an alternate source ?

Also any suggestions of where to source a set of gaskets/seals ?

Thanks again – Russell

[BRIAN MARSHALL]

Hi, That tap looks fine. The Club Spares Scheme has all the gaskets, but don’t forget to specify long-stroke Brum engine when you order.
Brian

[Russell]

I received the new tap today, but unfortunately it is a slightly different size to the one I have, the new tap is physically slightly smaller, and its thread outside diameter slightly larger at 9.7mm (old is 9.55mm) and the thread seems slightly finer on the new one, so I guess mine is none standard. The tap on mine fits into a small adapter (which the new tap won’t screw in to) which in turn fits into a slightly larger hole on the cylinder block, the diameter of the thread of the adapter that fits into the block is 10.85mm but I have no idea what type of thread it is, or that of the original tap, any ideas ?

[efr215]

Take a scrap of wood, trim it square to a size slightly larger than the core of the threaded hole, taper the end slightly to provide a lead and screw it into the hole as far as it will gp. There is nothing very fussy about all this and if one dosent work then a scrap of wood is cheap enough!

With any luck when the wood is unscrewed there will be a clear impression of the thread in the wood which can be measured to find the pitch. Once the pitch is known that will narrow the search for the right thread.

For reference 1/8″ BSP has an O.D. of 0.383″ — core 0.337″ — pitch 0.0357″ — T.P.I. = 28

Near threads are:-

3/8″ BSF — O.D. = 0.375″ — core = 0.311″ — pitch = 0,050″ — T.P.I. = 20
3/8″ UNF — O.D. = 0.375″ — core = 0.399″ — pitch = 0.0417″ — T.P.I. = 24
Metric, 10mm is a bit small, 12mm a bit big, there are 11mm (0.433″ O.D.) threads and also a 1mm (0.040″) pitch series.

These are the common ones but there are more obscure ones too, if you could post the pitch I can dig deeper.

[Russell]

Thanks for the advice, I can measure the pitch of the thread in the cylinder from the “adapter”. The best I can measure it, is for the old tap – O.D. = 0.375″ core ~ 0.33″ pitch ~ 0.039″ and for the “adaptor” that screws into the cylinder O.D. = 0.427″ core ~ 0.36″ pitch ~ 0.053″. Does that help ?

[efr215]

The old tap is almost certainly 1/8” BSP, the dimensions, allowing for a wear and small measurement errors is too close to be anything else.

The adaptor is more of a problem, 0.427” is a “nothing” size but it is not far off 7/16” (0.437”). The trouble is that it is also almost exactly 11mm but that is also not a “preferred” size.

After a good deal of head scratching I suspect that what we have here is a hole opened up for a 3/8” BSP Helicoil insert, (the 0.036” pitch is almost exactly 28 tpi which is correct for 1/8” BSP), it would seem that the Helicoil insert has for one reason or another failed and has then had the insert made.

Does that make sense to you?

[Author]

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