Classic Triumph Bonneville

[JackyJoll]

You’ve already had my info on ring gaps. Works for me: did 500 miles over the weekend.

[Mr. Dazzle]

Pistons back on. Its a long old stroke motor is this...

Before sticking them back in I had to check the piston/bore clearances. On paper I shouldn't need to, but I've learned with this old bike that the paper values don't count for much!

Unfortunately it's not as simple as it once would have been. In my old job we were fully vertically integrated, meaning that we did everything from initial design to customer delivery. The upshot of that was a massive metrology cupboard chock full of all the micrometers you can shake a calibrated stick at.

In my new job we do all the engineering ourselves and we do all the assembly ourselves, but most of the actual metal bashing is done out of house due to higher volumes. This mean far less measurey stuff knocking about. I was able to blag a micrometer big enough to measure the pistons accuratey, but we don't have a bore mic that goes up to the necessary 2.8 something inches.

So I came home today with some accurate piston sizes and I had to assume the bores were cock on 40 thou over stock size. Making that assumption, the piston/bore clearance is a tiny smidge over 0.005" - which is about perfect. I also realised on the way home I have more than one set of feeler gauges, so I could do a second test. If I put the piston in the bore and insert a 4 thou gauge on one side I can get a 1 thou gauge between the piston and bore on the other side, but a 2 thou is a bit tight. So that means the piston/bore clearance is over 5 thou but less than 6

TL:DR - the pistons are the right size

[KungFooBob]

Have I asked this before (I've had a drink). Is it big bang?

[Mr. Dazzle]

No, it's not. One fires while the other is on the intake.

You can make it big bang really easily; All you do it take one cylinder's push rods out and weld the rockers together. It's wasted spark ignition so it'll just run like that no problem.

BTW: Hindsight tells me that I should have cleaned all that old gasket off the casing before starting reassembly. At least I had the foresight to check the studs are all good, hence the lone nut.

[Mr. Dazzle]

Cylinders back on tonight.

I first had to go to B&Q to get the official Triumph ring compressors and piston alignment tools.



After that it was just a case of making sure the gaskets were in place, the cams had a nice dose of assembly lube and away you go. I was expecting to need 2 or 3 stabs to get everything together but it just slid on like butter first go!

I was actually a bit suspicious of how easy it was, but I figure if the rings or pistons are on the piss it would be HARDER to get it together.



Putting it 4th gear and spinning the rear wheel shows that everything moves nice and smoothly. There's gentle 'sssshhhhhhpppp' sound as the pistons come up and then a properly steam train 'chuff' as they go back down. The two pistons move together so the crankcase gets pressurised as they go down.

[Lutin]

The two pistons move together so the crankcase gets pressurised as they go down.

Isn't there some kind of (timed?) breather to help with this?

[Le_Fromage_Grande]

The two pistons move together so the crankcase gets pressurised as they go down.

Isn't there some kind of (timed?) breather to help with this?

Nope, it's like a single cylinder bike in that respect, it's worth having a really big, free flowing breather on singles, I imagine these are the same.

[Mr. Dazzle]

There is a breather, its just sized for running at 6000rpm. When just turning over slowly at low speed I imagine you don't hear much from it. The sound I could hear was coming from around the tappets, which are normally hidden away inside the push rod tubes and also covered in oil.

On this particular bike the breather has been rerouted a bit and exists just above the rear wheel. Genius.

[JackyJoll]

The two pistons move together so the crankcase gets pressurised as they go down.

Isn't there some kind of (timed?) breather to help with this?

Pre- mid 1969 ones had a timed breather, but it wasn’t very good. This bike appears to have the later big hose plumbed to the primary case.

[Mr. Dazzle]

I found some old pic from before the disassembly. The breather is the browny-gold pipe coming backwards out of the casing at almost the middle of the pic.



Is then makes it's way backwards along the bottom of the seat. On a stock bike it follows the rear mudguard and point at the floor aft of the rear wheel. That set up would have been changed for this bike's racing career cause it wasn't allowed. The current setup is clearly a bit more modern cause its held on with cable ties.

[Le_Fromage_Grande]

The breather looks small for the engine, on racing singles we used to run a 1" pipe to the catch tank, any smaller and you lost power, we tried to get negative pressure in the catch tank, but I don't think we ever achieved enough to make a difference .

[Mr. Dazzle]

That is what came with the engine....in fact AFAIK this bike's is bigger than stock. That's not to say Triumph got it right, the often didn't from what I can see

Don't forget that pressure in the crank case helps you push the pistons back up again. In a perfect world it would be neutral, anything you lose in pressure would be regained by pushing the pistons back up thus helping compression and exhaust...a perfect spring. It is never perfect of course.

[JackyJoll]

That breather and the passage to it through the main bearing are big enough to discourage oil leaks better than the previous tiny bore timed breather. The timed breather was useless on the 650 at high rpm.

There’s no reed valve or anything in Dazzle’s to get crankcase pressure below atmospheric.

[JackyJoll]

Don't forget that pressure in the crank case helps you push the pistons back up again.

I’ll happily forget that! Do any of your work’s engine designs try to keep pressure in the crankcase?

A less dense atmosphere in the crankcase gives less windage. Greater pressure differential above and below a piston ring gives better ring sealing.

[Mr. Dazzle]

No, but its not the disaster everyone assume it is. Everyone forgets it works both ways. Similarly any negative pressure has to be overcome during the 'up' parts of the piston's movement.

Ideally you'd make it so it never changes.

As for sealing....the pressure above the pistons is an order of magnitude higher than that below it, so while crank case pressure makes a difference to sealing its not that big a difference.

The main aim these days is just to keep oil inside the engine.

[Rockburner]

No, but its not the disaster everyone assume it is. Everyone forgets it works both ways. Similarly any negative pressure has to be overcome during the 'up' parts of the piston's movement.

Ideally you'd make it so it never changes.

Maybe some sort of bellows arrangement could be fashioned?

[Mr. Dazzle]

This engine is probably as bad as it gets...or maybe a single. IL4s have nominally constant crankcase volume for example, although the shape of that volume still changes. Normally its not that big a deal.

[Mr. Dazzle]

I tried to start rebuilding the rocker boxes today. Everything is clean now and I've got new seals etc.

I'd heard the task described as "like putting a cat in a toilet"...

Cat number 1 successfully inserted into toilet!



Cat number 2 is more reluctant, so I'll have another go tomorrow. First one only took 10 mins once I'd figured it out, but clearly I've not completely figured it out.

I used the official B&Q piston alignment tool as a rocker alignment tool aswell. The rocker spindles are 0.5", aka 12.7mm, so a bit of 11mm dowel is just a bit skinnier. Sanded to a point at the end it makes the perfect assembly aid. It'll also come in handy if I'm ever attacked by a tiny vampire.

[JackyJoll]

You can make or buy a rocker cover gasket with individual pushrod holes, to align the pushrods with the rockers, as you fit the cover.

[JackyJoll]

the pressure above the pistons is an order of magnitude higher than that below it

A damn site more than an order of magnitude during combustion.