Triumph crank and head disassembly

January 18, 2024

The last time I worked on the 1959 Triumph Bonneville T120 I fabricated a triumph pinion puller and removed the crankshaft from the timing side engine cover. The main reason for removing it totally is to get to the infamous “sludge trap” and clean it. When I removed the left connecting rod it was also apparent that the crank would need to be polished/re-grinded as there was scratches in the surface underneath where the connecting rod is mounted.

The DIY pinion puller worked and with some heat applied to the engine case I was able to separate the crankshaft from the casing. I did however have one surprising finding, which was that when I had the crankshaft almost out I rotated it and found that it looked incredibly crooked. Like a milimeter or more play on the primary chain side. I cannot fathom how it would be able to work while this being the case, so I am equal part flabergasted and sceptical of what I saw. But no matter what I need to have it checked and if possible straightened.

Three part crankshaft disassembly

Disassembling the three part crankshaft was not super difficult. There are 6 bolts (if I remember correctly) holding it together. Once these are removed each side of the cranshaft can be removed from the central flywheel.

I was able to use a big nail punch to carefully hit either side of the crankshaft just over the point where the connecting rod is attached and then the two farthest points from here. I used very little force with a hammer and after each little strike I looked to see if it moved. Looking through the holes for the bolts that keep the pieces together it is possible to see even the slightest movement as there is no air between the sides when fully assembled. Using this approach I was able to slowly remove either side of the crank from the central flywheel.

Here are the 3 parts of the crankshaft.

I do not know if I can consider it lucky or not, but what I found when everything was disassembled was that the sludge trap looked absolutely fine. There was nothing at all to clean from it.

But disassembling the engine all the way instead of only focussing on the cylinders and head showed me the wear on the crank, which would have given me problems at some point either way. So better do it properly now.

Removing valves from cylinder head

Once the crankshaft was sorted the next job was to remove the valves from the head to check wear on valves and valve guides.

The valves was a bit reluctant to let go but all of them gave way with a snap and then I could remove the retainers and finally remove the valves and valve springs.

All valves had bad valve guides, but the exhaust valves were especially bad.

I also found that on one of the valves and valve seat some debris had found been stuck and made a significant dent in the valve, while still being present in the valve seat. No need to leak test that valve.

After close inspection of the head a lot of small cracks also became visible. I think there are 3 or 4 places in total.

I am more and more surprised that this engine ran, let alone started on first kick every time. I can understand why it felt a bit sluggish though! The compression will have had any number of escape routes to follow. And if the crank really is/was as bent as it seems then there must have been major resistance just in spinning it.

Now that everything is disassembled I can start working towards being able to re-assemble the engine. First major task is figuring out whether I need a new crank or not. After that I need to figure out whether I can have the head welded to fix the cracks or whether a new head is cheaper.

If changing the head, then I might as well revisit my thought of putting on a Morgo 750 kit. I abandoned that thought as it would require a new 9 bolt head and did not seem to add that much extra performance. But if I have to look for a new head anyway, then it might be back on the table with a Morgo 750 kit.