Classic Triumph Bonneville

[Mr. Dazzle]

My new pistons arrived today, ahead of schedule. Spot the difference....



Aside from the obvious crown shape, the new ones have different pins (smaller ID, presumably to make them stronger), chrome plated rings and more 'meat' around the pin holes inside. The new ones also have a vertical groove by the pin hole, not sure what that's about.

They weren't kidding when they said "new old stock", they came in the original packaging complete with the instructions and new clips.



Unfortunately the pins are stuck solid in the pistons. They're supposed to be a easy finger tight fit, but the instructions do say that they might get stuck due to oil coagulation in storage. I'm currently say watching them simmer very gently in hot water....

I now have to gap another set of rings.

[Le_Fromage_Grande]

The vertical groove is for getting the circlips out.

[Mr. Dazzle]

Yeah I just twigged that

Hot water worked a treat, the pins just slid out. The 11:1s are also 'superfinished' on the pin holes (that's a real thing BTW). Dunno if you can see it here, the 11s are on the right. The hole is mirror smooth on them but just simple machining on the standards.



I did another one of those jobs I'm not sure is strictly necessary but since I'm here: Knocked all the very sharp edges off the valve pockets with 4000 grit wet and dry then polished everything with Autosol before a thorough wash in very hot soapy water.

[cheb]

There's something amusingly ridiculous about really high domed pistons. A high quality work around I suppose.

[Le_Fromage_Grande]

Sharp edges promote detonation, so removing them can't be a bad thing

[Mr. Dazzle]

The instructions claim they're "ready for competition use"...I can only assume that what passed for ready in 1973 was different to today. The edges were borderline sharp enough to cut your finger!

[ogri]

The instructions claim they're "ready for competition use"...I can only assume that what passed for ready in 1973 was different to today. The edges were borderline sharp enough to cut your finger!

Probably less problems with detonation in those days unles you used aviation fuel etc?

[Le_Fromage_Grande]

Ready for competition use is marketing, but to be fair to whoever made them not much was known about what caused detonation in the early 70s, nobody uses high domed pistons in modern race engines, that went out in the 1980s, now it's shallow smooth domed combustion chambers and nearly flat pistons.

[Mr. Dazzle]

No one has used a domed combustion chamber in a performance engine for few decades at least, its pent roof now. Goes back to supermofo's point about how his modern bike can run 12 or 13 to 1 when I'm worrying about 11. The chamber shape is a big part of that.

I say "no one".....the yanks still make 2v hemi engines and put them in fast cars, coz of course they do.

[Le_Fromage_Grande]

I'm guessing that what I called a shallow some is actually called a pent.
I have one of the last domed performance motorcycle engines, my 1983 Kawasaki engine has combustion chambers like bath tubs, Suzuki and Honda had gone to 16 valve pent chambers in 1979.

[Mr. Dazzle]

The key thing with a pent roof is that it's a triangular top rather than a hemisphere. To be fair that's as much about getting the 4 valves in as it is anything else. With a 4v head it's hugely easier to get the valves and cams in if the valves are 'square' to each other in a straight line. Imagine the difficulty in actuating two valves which are not on a common axis - possible, but a ball ache.

It also means the spark plug is in the centre rather than hugely offset to one side like it is me auld Trumpet.

As you say, modern pistons tend to be flat topped, but the shape of the chamber roof means you can still squeeze it down to high compression ratios even with flat pistons. My (diesel) car runs 17:1 and has so called 'omega pistons' which are flat topped with a cutout in them...

Things have come a long way in 50 years!

[Rockburner]

The key thing with a pent roof is that it's a triangular top rather than a hemisphere. To be fair that's as much about getting the 4 valves in as it is anything else. With a 4v head it's hugely easier to get the valves and cams in if the valves are 'square' to each other in a straight line. Imagine the difficulty in actuating two valves which are not on a common axis - possible, but a ball ache.

IIRC Rudge had a 'radial' 4-valve head race bike pre war. I think it was pretty fast for the time.

As you say, modern pistons tend to be flat topped, but the shape of the chamber roof means you can still squeeze it down to high compression ratios even with flat pistons. My (diesel) car runs 17:1 and has so called 'omega pistons' which are flat topped with a cutout in them...

Things have come a long way in 50 years!

Isn't that very similar to a 'Heron' head? Where the valves open 'into' the head of the piston?

[Mr. Dazzle]

'Radial' 4v heads are definitely a thing, it's just much much easier to make engines where the valves are in a nice straight line!

I think in my car it's about the way the gases move about and swirl in the chamber. Doubly so when you consider it's a direct injection engine with a complicated multi-stage injection. Modern diesel engines might have 5 or 6 steps to the actual injection event, it's not just sprayed in.

It's called an omega piston 'cause that's the shape of the cross section.

[KungFooBob]

Honda made Radial Four Valve engines, you can spot them because they have RFVC written on the side

[Mr. Dazzle]

Honda made Radial Four Valve engines, you can spot them because they have RFVC written on the side

Indeed...then you end up having overhead cams and rockers! Not so bad on a single, but imagine doing it on a V4 for example - possible, but cost adds up pretty quick.

It's also not too bad fitting everything in on an SOHC engine with a bunch of rockers on a single, but it gets tight on multis.

[Rockburner]

'Radial' 4v heads are definitely a thing, it's just much much easier to make engines where the valves are in a nice straight line!

I think in my car it's about the way the gases move about and swirl in the chamber. Doubly so when you consider it's a direct injection engine with a complicated multi-stage injection. Modern diesel engines might have 5 or 6 steps to the actual injection event, it's not just sprayed in.

Is it injection into the combustion chamber itself? Or into the manifold?

[Mr. Dazzle]

Straight into the chamber, the injector is where the spark plug is in a petrol car. That's why modern diesels have crazy high fuel pressure (~2000 bar), so you spray it in at really really high pressure and get fine atomisation and very fine control.

It's all getting a bit blurred now when you consider the use of multi stage injection, turbulent jet ignition, gasoline direct injection, HCCI etc. etc.

[Le_Fromage_Grande]

It's also not too bad fitting everything in on an SOHC engine with a bunch of rockers on a single, but it gets tight on multis.

Would it ever, the valve gear is wider than the piston on most Honda RFVC engines

[Mr. Dazzle]

How's about a VW VR6 engine?

2 cams, 24 valves, 24 rockers...or is that 12 rockers? I can't remember, I think they might have direct on the valves for half and rockers for the other half.

Plus variable valve timing

[Mr. Dazzle]

Gappin' me rings tonight....



This job has already been done once, but on the old pistons. These new 11:1s come with new - slightly different - rings which need gapping.

You have to adjust the small gap in the circle of the ring. It should be (according to the instructions that came with the pistons) 0.005" for every inch of bore. These pistons are plus 40s, a bit of maths reveals the gap should be 0.35mm. Millimeters cause that's what my feeler gauges read in.

'Tis important that you have the rings square in the bore when you measure. There are fancy tools you can buy to do it, or you can take the more obvious route like I did and use the piston to push it into the bore and get it straight.

After that its just a case of filing the ends down until the gap is correct. You can't put any material back on of course, so its measure three times cut once! I'm glad this is only a two cylinder bike. Check out me fancy chromed ring though!

S'also MUCH better to go too big and not too small. Too big will lose some compression maybe or burn some more oil. Too small will make the engine seize up and destroy it