Bagheera is unwell

Oh dear. Things had been going so well.

So well in fact, that we had decided it was time for the wife to have a go in the back for the first time, and do to a ride out to the local branch of the Triumph Owners’ Motorcycle Club annual BBQ to show her off (the bike, not the wife).

Up the big hill out of Henley on a hot day she started to struggle, then cut out completely. Pulling into a turning it was clear by the blue smoke coming off the engine that things were badly wrong. The engine had got way, way too hot and was burning off the traces old oil that had leaked onto the cylinder head. 

It had got so hot in fact that the fuel in the carburettor had boiled, which was probably the reason it cut out. No amount of roadside fiddling was going to sort this out, so it was time to call Mr AA and kick back for the customary three hour wait.

The next day it still showed zero interest in starting, so it was time for some fault finding. An engine will only run if it has three things; fuel, spark and compression. It makes sense to check these things in order.

So first I removed the carburettor, which now cool showed no problems or signs of damage. Then I checked out the electrics and found good battery voltage, healthy spark and the spark timing correct as far as I can tell without running the engine. The oil system is working properly too.

So the problem must be compression. I connected my compression gauge, held the throttle full open and kicked the engine over vigorously. A healthy cylinder will indicate at least 100psi. Mine only had 60psi on the left and just 25psi on the right. Again; oh dear.

A loss of compression can be caused by a damaged or leaky valve, a leaky head gasket, damaged pistons or piston rings or a combination of all of these. It was time for some serious dismantlement.

Then I took off the fuel tank, oil lines, exhausts and head steady bracket. That took about half an hour ans is all you need to remove the cylinder head on these classics - on a modern bike that would be half a day’s work. Mind you, modern bikes almost never need to have the head removed because they are so reliable.

Once the rocker boxes were off (the rear one is a right PTIA due to the lack of clearance to the frame) the push rods simply lift out and the nine cylinder head bolts can be loosened and removed and the head lifted off, when all was revealed…

Mmm. So. Not great but could easily have been worse. The heat has damaged the piston crowns but luckily did not burn through. That would have sent debris throughout the engine leading to a full rebuild. The cylinder head has not cracked, nor are the valves obviously damaged although the heat had burned off all traces of carbon. 

The head gasket, which on this bike is solid copper, shows traces of a leak between the cylinders probably due to the head warping in the heat, although it could have been like that for a long time.

With the piston damage I need to dig further and remove the engine barrels. This requires a special thin-wall ring spanner to access the cylinder base nuts, which I do not have. But a bit of time spent carefully grinding down a normal spanner solves that problem.

The other gotcha on these engines are the cam followers - small metal pieces that ride on the camshafts in the bottom end of the engine and transmit their motion to the push rods, which in turn run up the front and rear of the engine to the rockers which in turn operate the valves. If you are not careful they simply fall out when the cylinders are lifted clear and fall into the engine. A simple elastic band holds them in place.

Things are not terribly pretty in here either, the right piston has suffered a partial seizure either due to the heat or debris from the piston crown. The cylinder shows a corresponding mark and will need attention too.

Finally, I strip down the cylinder head by removing the valves and springs. For this you need to compress the spring and remove a tiny metal collar called a collet that connects the spring to the valve stem. Again a special tool is required, but luckily my neighbour has one. 

The intake valves are OK, but the exhaust valves show signs of overheating. The valve guides that they run in also have a bit too much slop in them. This is a common problem with these engines over time but it needs to be fixed.

So in summary I will need:

• New pistons and piston rings (in a fractionally larger size)
• The cylinders to be professionally rebored to fit the new pistons
• The cylinder head to be skimmed flat
• Valve guides to be replaced or repaired
• New valves and the valve seats cut to match

In other words, a complete top-end rebuild. The pistons, rings, valves and gasket set will be around £200 while the engineering work will be around £300.

But on positive side, I will then have an effectively as-new engine that should give many years of good performance.

Of course I still need to solve the original cause of the overheating. I have some suspicions, but I can’t check until the engine is running once again. Which may be some time.

TOTALS TO DATE: Hours: 148. Cost: £6310. Miles ridden: 180. Breakdowns: 3 (one serious)