1971 Triumph Bonneville Restoration: Gudgeons, Johnson Rods & Cylinders out! (oh my)

I’ve got a degree in English, but working on the old Bonneville is stressing out even my vocabulary:

That’d be your gudgeon pin right there.

Gudgeon: one who will bite at any bait or swallow anything, credulous or gullible person Mer. V. I. i. 102. (that might apply to this project)

Definition of gudgeon

1: PIVOT sense 1, JOURNAL (we’re getting closer)

2: a socket for a rudder pintle (a rudder pintle?)

Definition of gudgeon pin: WRIST PIN

Definition of wrist pin: a stud or pin that forms a journal (as in a crosshead) for a connecting rod (that’s it!)

They can make up anything! Nobody knows! Well, you need a new Johnson Rod in here!

A few people know, George. Consumer ignorance is expensive, but who has time

to understand the machines they depend on?

Gudgeon pins? After the massive fight that was removing the seized cylinder head, I was bracing for misery, but I was able to pop the circlips (!) out and tap the gudgeon pins (!) through with no sweating and swearing. Pink Floyd was playing on the computer and I think music from its era calms the old Bonneville down when I’m working on it.

The circlips popped out easily and a 3/8 inch socket extension was the perfect size to tap out the ‘wrist’ pins. I thought the one on the seized side might cause more problems because it had been stuck in place with moisture but it didn’t and both came out easily leaving a motor bereft of its pistons.

British Cycle Supply Co. delivered my 750cc kit quickly and without any headaches (and also for about $200 less than comparable kits out of the USA), so I got the shiny new bits out and had a look:

You’d think a 750cc head would look significantly different to the stock 650cc unit, but you have to look closely to see a difference in size.

I had a close look at the connecting rods and they appear to be in good shape and the bottom end of the motor moves freely. I think next steps are to remove the rest of the engine from the frame, give the oil-in-frame a deep clean and then recoat it with some quality paint. With the frame cleaned up and sorted I’ll strip the rest of the motor and clean everything out to ensure nothing’s whacky before I begin the great rebuild.

I could just chuck it all back together now and hope for the best, but if it ends up having other niggles rushing things at this point is just false economy. I’m not worried about making the bike look mint, but I do want it to be dependable and that wouldn’t be the case if I haven’t checked it over completely after such a long (30 year?) layoff. An engine out deep restoration was always the intention here and I’m not in any kind of rush since the other two bikes are both five by five and felt fantastic on their first rides of the year this past weekend.

The gudgeon pin itself looks to be in good shape, which is good because the head kit didn’t come with new ones. The circlips came out nicely too but the kit did come with those. It amazes me that even specialized material like that in these cylinders can withstand the extreme forces they operate in. This Bonnie had done at least twenty thousand miles on these cylinders and they’re still in remarkable shape considering they were face to face with more than 17 controlled explosions every second (a spark plug typically fires about 17 times per second at 2000rpm!). You’d think all that heat and violence would cause more wear than I’m seeing in the beating heart of this motor.

This is the 3/8 inch socket extension I used to tap the gudgeon pins out. Nothing grabbed or slowed the process down and I barely had to apply any pressure. I might have even been able to push them out had I been so inclined. I caught the pin as it came loose and the cylinder lifted off the connecting rods easily. Again, considering the extreme conditions the heart of every engine operates under, I’m impressed by the engineering and metallurgy that makes all this possible.

The new cylinders don’t have the carbon build up from all those combustion cycles and look pretty spectacular in their pristine newness:

Here’s the whole kit from British Cycle Supply. It includes the cylinder head, cylinders, piston rings, circlips for the gudgeon pins and a head gasket. I swiped the last one they had in stock for $688.75CAD ($535USD – told you I got a good deal). Unfortunately I might be the last person on the planet to get a 750cc kit that cheap. Prices will be going up on the new stock and probably even higher than that as we’re in an inflationary spiral thanks to Putin overreaching and years of pandemic fueled broken supply chains; we live in interesting times.

Another tool came in that’s interesting. I was reading Practical Sportsbikes last week (highly recommended if you love bikes and getting your hands dirty on them), and editor Chris mentioned an endoscope smartphone camera he was using on a project bike to inspect the internal parts of an engine. I found one on Amazon for under $40CAD and it came in on the weekend. It lets you see parts of the engine that haven’t seen light before. It’s an exciting thing that offers you a look into the secret life of engines. I’m looking forward to using it as i continue to work on the Bonnie.