Triumph Bonneville Engine Rebuild

Triumph Bonneville Engine Rebuild – 1972 T120 R

Overview of the 650 engine

This engine was out of a 1972 Bonneville, at first sight, it did not look promising.  It had been in a bike that looked like it had been parked outside decades ago. Now a ripe Triumph Bonneville engine rebuild project.

It is a ubiquitous 4 speed 650cc parallel unit construction twin. Unit construction means that the engine and gearbox are built together in one set of crankcases, unlike earlier pre-1963 Triumph 650s where the engine and gearbox are two completely different assemblies.

In fact, the main elements of this engine followed a very similar design to the earlier Speed Twin engine from the ’30s. And the same basic engine expanded to 750cc, and with a 5-speed cluster in the gearbox continued for Triumph until the end of the Meriden type bikes in the mid-eighties.  Seems very odd but reassuring in this age where everything seems to be changed every 5 minutes. 🙂 

Triumph Bonneville Engine Rebuild subject.

There was silt in the inlets, and the carbs had almost vapourised.  The pistons were so stuck that it required “lethal force” to move them.  But at least all of the parts seemed to be there, so there was a good basis to start from.

Silt in the inlets on the Bonneville engine.

You can read more about the initial strip and removal from the frame in my Bonneville T120R Restoration log.

The Strip Down Process

Before I can enjoy doing the Triumph Bonneville engine rebuild by putting it all back together, I first had to strip the whole thing down. By the time I got the engine on the bench in an engine stand, I had already removed all of the primary side and cylinder head, the details are in the other post.  But now I wanted to get the engine completely apart to see what havoc the years of neglect had caused.

With any strip down, it really pays to make plenty of notes, and take plenty of pictures.  We are lucky to live in the digital age where we can take as many photos as we want. Sometimes the biggest problem is finding the one I want among the many hundreds I’ve taken.  Sometimes the only way to remind myself how something went together originally is to use an image taken weeks before.  

The parts books are not always right, sometimes the draftsman in the office has drawn things in the wrong order, and sometimes the originally intended part didn’t make it to production, and the part in your hand bears no resemblance to the drawing.  Examples are the Thackery Spring Washers in the rockers, and the OIF head steady if you want to check this out.

Front Sprocket Removal

As the clutch and alternator had already been removed the next step before the gearbox could be stripped the next step was to remove the front drive sprocket.  This lives behind a cover plate held by six screws.  

Triumph Bonneville front sprocket removal.

There is no big drama removing this, especially if the engine is still in the bike so you can put your foot on the brake.  If the engine is out it isn’t too hard to work out some way to secure the sprocket while you undo the nut, maybe using a chain wrench. To do the job you will need a  big box spanner or socket for the gearbox sprocket nut and a puller is needed for the sprocket itself.  

First, you remove the tab washer and undo the nut on the end of the main shaft.  This needs a 1 ⅝” box spanner. 

Box spanner being used to remove the main shaft nut.

Then you need to use a two-legged puller to pull the sprocket off.

Removing the sprocket with a puller on the T120.

 If you are changing the chain and sprockets on one of these bikes it is a pain that you have to strip out so much to get at the front sprocket, so they frequently get left.  

I was quite pleased that for this bike the sprocket was still in good condition when I got it out so I could reuse it. 

Front sprocket removed from the case.

Once you have the sprocket off that is it for the primary side of the engine, now you can then move around to the other side to give the gearbox your attention.

Gearbox Outer Cover Removal

Obviously the gearbox oil needs draining before stripping anything.  The drain plug is easy to find underneath. You could get confused by the smaller screw in it used to check the oil level. 

Removing the gearbox cover from the Bonneville.

Before the cover will come off both the gear lever and kickstart lever need to be removed.  Then it is a simple job to remove the screws that hold the cover on. This is one of those covers where there are about 5 different sizes and types of fasteners used to hold it on.  It is worth making good notes because it can be confusing when you come to put it back on.

The outer cover has the gear change and the kickstart spring in it.  The spring will release when you pull it off, but shouldn’t really cause a problem.  Besides the kickstart spring, there is also the clutch actuator and the gear change mechanism.  All of which will need removing to clean and check.

Triumph Bonneville kickstart spring.
Bonneville clutch and gear change mechanism removed.

Kickstart Removal

The ratchet for the kickstart is attached to the end of the gearbox main shaft.  There is a tab washer to fold back and a nut on the end. You will have to find some way of stopping the shaft from rotating while you undo the nut.

I use a vice wrench on the cog, it is hardened metal so you would have to be pretty brutal to cause damage.

Mole grips to stop the shaft from rotating.

Usually when these are undone the parts come apart quite easily, in this case, they didn’t but the use of a three-legged puller soon had it off.  There are quite a few parts so a shot of how they were fitted will help later.

Three-legged puller removing the kick start.
Bonneville kick start parts removed and laid out.

Gearbox Inner Cover Removal

Once you have the kickstart ratchet off it is quite easy to remove the inner cover, which is not the same story when you put it back 🙂

The gearbox inner cover on the Triumph Bonneville.
Gearbox inner cover being removed.

Removing the Gearbox internals

Removing the internal parts from the gearbox is quite easy, a little bit of jiggling here and there and they are soon out.  When you first look at these parts on the bench, it always seems a miracle that they work at all.

They are actually quite simple, so it is worth your while taking the time to work out how it works, this understanding will help when you are putting it back together.  

Inner workings of the Triumph Bonneville gearbox.
Cogs removed from inside the Bonneville's gearbox.

Oil Pump Removal

There are only two nuts holding the oil pump on, and a slider that slips over a stud on the inlet cam nut.  For such an important job this must be one of the most simple mechanisms ever. As long as there is no wear, and it does run in oil, and there are no blockages, it will just keep going.

Any wear is usually caused by grit in the oil not being filtered out, a good filter, and regular oil changes will see the oil pump last forever.

Triumph Bonneville oil pump in situe.
Bonneville oil pump parts removed and laid out.

Camshaft pinion Removal

The crankcases can be split without removing the camshaft pinions. If you intend to replace the cams or feel that there will be wear you need to check it is probably best to remove these first.  I wanted to check everything on this engine so I definitely wanted to remove them.

The things to pay attention to are the fact that they are left-hand threads, so you need to go the opposite way to what you are used to when undoing the nuts.  The pinions will not come off without the correct pullers, and they are also used to replace them so you do need to get them.   

Removing the camshaft pinions.
Cam shaft pinion reverse thread.

Crankshaft Pinion Removal

The crankshaft pinion will not come off without the correct puller.  The specialist puller has three legs that slide into a recess at the back of the pinion.  It is nearly always the case that the legs will need a little grinding off to get them to go in.

Even with a factory puller, this may be needed, so don’t be surprised when you are taking your new £40 puller to the grinding wheel to modify.  With the puller correctly fitted the pinion comes off quite easily.  

Modifications to the factory puller for removing the Crankshaft pinion.
The crankshaft pinion being removed from the Bonneville.

The pinion can be put in the wrong way when being reassembled, you want to avoid this at all costs.  If it is put on the wrong way the recess for the puller is in the wrong place, and this is a big problem.

It is still possible to get the pinion off, but it will mean you end up making the pinion unusable by grinding a slot into it.  Not something you want to do. 

Crankshaft pinion removed from the Triumph Bonneville.

Piston Removal

Before splitting the crankcase it is best to remove the pistons.  They are held to the conrods by gudgeon pins secured by wire retaining clips.  Once the clips are removed it is likely that the gudgeon pin will not come out.

I have a handy little press that pushes them out.  Don’t just take a hammer and drift to the or my could damage the conrods.

Removing the piston gudgeon pins with a special press.

Splitting The Crankcase

I always find it a little exciting opening up the crankcase of an engine.  I should get out more 🙂 If you didn’t already remove the sump plug and let any oil out, this will make it less messy.  There are a number of through bolts that need removing and a few machine screws on the primary side.  

Primary side screws removed.

The crankcase is unlikely to just fall apart, it will be held by the sealant between the cases, and the fit of the main bearings.  The crankshaft is likely to stay in the bearing on one side, so you need to take this into account when splitting it.

Do not put anything between the joint faces to try to pry them apart.  A copper hammer, or using a piece of wood as a drift tap various parts of the casing to get it moving. With a little patience, the cases will soon start to move.

Once you have them apart the crankshaft will still be stuck in the timing side bearing.  Usually, all you need to do is use gravity to pull it out. Use some pieces of wood to support it.  If you feel the need to hit the end of the crank, use something to protect it.

If you just hit it with a hammer you could cause the end of the crank to mushroom slightly which will need specialist work at a machine shop to fix properly.

Triumph Bonneville crank case split.

Removing The Bearings

The timing side main bearing comes out easily using a bearing drift.  Don’t forget to use heat to expand the case a little first. I use a plumbers soldering torch, or even my oven.  Make sure you keep the drift square so that you don’t cause any damage to the case.

The outer race of the primary side bearing will still be stuck in the case.  At first sight, this looks impossible to remove. The preferred method is to heat the entire case (in an oven, don’t tell the wife) and drop it onto a wooden surface and usually, the bearing race will just drop out.

Another method is to weld a bar across the inside of the race, sometimes the welding will cause the race to shrink and come out, or you can use a drift on the bar to move it. If doing it this way be careful. The case is alloy and has a lower melting point than the bearing race, you need to really be careful to avoid damaging the case.

Using an oven to heat the engine case to remove bearings.
Bearings removed fromt he Triumph Bonneville.

Just a couple more things to do before the strip is complete.  I will add these shortly before I get on with the rebuild

3 thoughts on “Triumph Bonneville Engine Rebuild – 1972 T120 R”

  1. Really great website. Inspirational. I’m now tempted to consider starting the restoration of my sons Norton ES2…..1956…..500 single. It’s been in crates for a few years now although much has been done to the frame and body parts. I’ve dry stored it and wrapped the engine. No experience with this type of work. We’ll see!

    1. Thanks for your comments, Mike! Hopefully your Easy 2 will be back on the road again soon. It’s like doing a big puzzle, the trick is not to rush it, slow persistent progress is best. Joining the owners club, or at least their forum is a good idea. Once the engine is in the frame, and it is stood on its own wheels, it does start to get exciting.

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