Top end tear down 790-865cc Triumph engine

Detailed top end tear down 900cc Triumph engine

This is the beginning of a series of posts in which I will disassemble and reassemble a 865cc 270 degree Triumph Bonneville motorcycle engine.The engines are marketed as 900cc (actually 865cc) and 800cc (actually 790cc) to clear up any confusion.

270 degree engines would be :

  • Speedmaster
  • America
  • Scrambler

360 degree engines would be :

  • Bonneville
  • Bonneville T100
  • Thruxton

The 270 and 360 degree engines share much of the same construction, so other than timing marks most of what is up here will apply to both engines, as well as the 790cc versions. It’s nice when you have a good design and things don’t change too much. The engine I’m working with is out of the bike. Top end engine work can easily be done with the engine in the motorcycle chassis.

tear down 900 Triumph Bonneville engine
Remove 4 bolts holding down valve cover.

Quite simply there are just 4 bolts holding the valve cover down. Remove them and under the cover is the valve train. You will see the cam chain which drives the “drive gear” for the cams and then the cams, intake and exhaust which turn by meshing with the drive gear. The cams are held in place with the cam block caps and there is an oil line which feeds oil to the assemblies.

valve train components of Triumph 865 engine
valve train components of Triumph 865 engine

It is best to remove the cams when the timing marks are aligned. You still have to be real careful, but this way there is no pressure from the valves and springs pushing upward as you take the cam caps loose. Be sure to mark everything as you do not want to mix the cam caps up!

alternate view valve train 865 engine
valve train 865 engine alternate view

To pull the cams out you will need to remove the oil feed line. Take note that there is only crush washers on top, between the bolt and oil line. There aren’t any between the line and cam cap. Remove the 2 bolts and wiggle the line out of where it sits in the head.

oil feed line removal of bonneville engine
oil feed line removal.

Next you need to line up the cam timing marks to remove the cams.
If you are not real sure about lining up the timing marks on the camshafts, you might want to remove the alternator side cover from the engine and align the timing marks on the alternator rotor with the line where the crank cases split.  This will put the engine at Top Dead Center on the left hand cylinder.

timing marks 865 triumph bonneville engine
timing marks for lining up camshafts

The cams are in the correct position when the camshaft timing marks are aligned with the timing marks on the drive gear, and “TOP” will be facing up on the drive gear.  You can simply put the bike in gear and use the rear wheel to spin the motor around to line things up. Be sure you know what you are doing. If you are not sure then remove the alternator cover and check the rotor for alignment marks. Always better safe than sorry.

the "top" marking on camshaft
TOP marking on right hand side of engine.
difference in timing marks on camshaft
Exhaust marks are the lines and Intake marks are the dots.

One last VERY Important thing before removing cams…. You must insert a special tool which locks the 2 cam gears together. If you look closely at the cams you will notice that the gears are 2 piece. They are a spring loaded gear most likely to keep the valve train quiet and accurate valve timing.

special locking pin tool to hold spring loaded cam gears together for removal camshafts

The cam gear locking pins install easily and with the pins in place it’s now time to take the cam block caps loose and remove the camshafts.

special locking pins hold camshafts for removal.
Notice the 2 piece gear and locking pin installed. Now the camshafts are ready for removal.

Be sure to number the cam blocks so the caps go back where they cam from.

Do Not Mix them UP !

I prefer to engrave them with a dremel tool so I don’t have to worry about washing the numbers off when cleaning up parts. Using a number stamp on something this small could distort the cap unless you are careful.  I would just avoid it, engraving is faster and easier too.

number cam blocks. do not mix up.
cam blocks numbered so they are not mixed up.

Break all the cam caps loose first, then go around and remove the bolts and caps. If your timing marks are aligned, the valves shouldn’t be pushing the cams up as you take the caps off. Be very careful if for some reason a valve is pushing the cams up. Always handle the cams with great care, they are pretty fragile. Same goes for the bearing surfaces they ride in.

sequence for loosening and tightening cam caps
tightening / loosen sequence for cam caps

After removing cams you have access to the valve caps and shims. If you just had to shim the valves as part of regular service, this is where you would stop taking things apart. I’m covering the whole top end so I’ll keep on going, as the cylinder head and cylinder block will be removed.


valve caps and shims
valve caps with shims on top… aka shim over bucket

Next is to remove the valve caps. This is where the old cup cake pan comes in handy so you don’t mix the valve caps and shims up. The valve caps run in a bore and should not really be mixed up if possible and keeping the shims in their respective place will speed reassembly of the job. It is best to measure the valve clearance before removing cams, once again to speed reassembly of the engine. This way if some of the valves were out of specification you could replace the shims and put it together and just check that you got it right.

cup cake pan for keeping valve shims in order
valve caps or buckets removed from engine and organized to go back where they came from.

Next up before pulling the head is to remove the oil feed line, the automatic cam chain tensioner, cam chain guides and the cam drive gear. Easy enough. First thing is the oil line. Be careful with the sending unit, they are fragile. The picture shows the last time this one was alive. I just barely bumped it with the ratchet and knocked it’s head off!

removing oil line feed to cylinder head.
Remove oil feed line to head.

After the oil line, remove the cam chain tensioner. Remove the big nut and then the 2 bolts. Sometimes they are a bit stuck and need a tap to come lose from the case.

removing cam chain tensioner
Remove automatic cam chain tensioner.

IMPORTANT! Do not ever “half remove” the chain tensioner. It is automatic and it will adjust itself and if you just put it back the cam chain will end up too tight and things will break and the fun will come to an end. Always remove it completely and put it back properly.

cam chian tensioner assembly
Cam chain tensioner removed.

With the cam chain tensioner removed the drive gear can come out next. It’s a shaft with a bolt holding it in place. The cam drive gears are all basically the same, there seems to be different shafts and spacers.

removing the drive gear for cams
removing cam drive gear

Whats important here is that you Do Not drop the spacer(s) down into the engine when you are removing the drive gear. Mine had 1 spacer, some have 2, one on each side of the gear. Be sure you know how they go back.

cam driver gear spacer
the spacer you don’t want to lose sits next to the bearing inside the gear.
shaft side of cam drive gear
shaft side of cam drive gear

Now the chain guides can be removed. There is one half hidden bolt in between the intake ports for the rear chain guide, and there are 2 torx bolts in the head for the front chain guide. The guides simply lift out.

cam chain guide bolts front guide
front cam chain guide bolt
front cam chian guide removed
front cam chain guide removed
hidden rear cam chain bolt
location of rear cam chain guide bolt
rear cam chain guide removed
rear cam chain guide removed

Now finally it is time to remove the head bolts, which are actually nuts, not bolts. If you are working with the engine in the motorcycle, hand tools are best. Mine being out on the bench I used a air impact. It is fine to use an impact to take head bolts loose.
–> Never, ever, ever install head bolts with an impact! If you are assembling an engine on the bench, you need a stand bolted to the bench to properly hold the engine to torque the head bolts.

head bolt tightening sequence
head bolt loosening sequence. Tightening sequence is opposite !
detail of head bolts
socket head cap nuts with shoulder for cylinder head studs.

A quick knock with plastic or rubber dead blow hammer and the cylinder head pops off. The oil drain tube seals are tight and what holds it on a bit.

removing head
a quick tap in the right spot.
underside of cylinder head
underside of head

A quick look at how one piston is at top dead center and the other isn’t also tells you this is a 270 degree engine.

short block 270 degree engine
short block

Wiggle the cylinder block a bit and lift it up some and put rags under the cylinders so nothing falls down into the crankcase. I’m pulling the whole motor apart, so I didn’t bother.

removing cylinders
removing cylinders

Be very careful with the pistons and cylinder studs when the cylinder is removed. You do not want to bang the pistons or rings up and you don’t want to scratch the cylinder hold down studs.

cylinders removed
cylinders removed from engine

That’s it. The top end is now removed.

Engine Identification

There are 2 different engines as shown above with the different models. The alternator rotor is one way to tell as there is a left and right cylinder timing mark on the 270 degree engine, and a single timing mark on the 360 degree engine. The other way to tell is by looking at the cam lobes from the right hand side of the motor, when the timing marks of the engine are aligned correctly.

cam position of 270 degree engine
Position of cam lobes of 270 degree engine when timing marks are aligned, as seen from right hand side. Intake is at 3 o’clock and Exhaust at 1 o’clock.
cam position of 360 degree engine
Position of cam lobes of 360 degree engine when timing marks are aligned, as seen from right hand side. Intake is at 1 o’clock and Exhaust at 11 o’clock.

I’ll quickly discuss here what is meant by 270 or 360 degree motors.
The crank is what makes these motors either a 270 or 360 degree engine. On a 360 engine, the traditional Triumph style of crankshaft, both pistons go up and down in the cylinder at the same time. Because it’s a 4 stroke engine, one cylinder fires and the other is on the exhaust stroke. This design is inherent to have vibration because both pistons come to a stop and change direction at the same time, at the top of the stroke and at the bottom of the stroke. The flywheel is there to counter act this, but doesn’t cancel out all of the vibration. Without balancing the flywheel assembly properly the crankshaft bearings, both mains and rods, would take an enormous beating and the engine would self destruct in short order.

The 360 degree crankshaft.
The 360 degree crankshaft.

The 270 degree crank helps cancel out vibration because the pistons don’t change direction at the same time. These engines have a sound more like a 90 degree v twin because of the firing pattern. In addition to flywheel counter weights there are also balance shafts on both engines to smooth things out.

Worth mentioning is that Phil Irving, an Australian engineer, first designed the 284 degree crank (76 degrees apart in firing order) and Yamaha is credited with first pioneering the 270 degree 2 cylinder inline engine some time around 1996, with the TRX850.

The 270 degree crankshaft.
The 270 degree crankshaft.