With the engine in pieces that is - not with snake oil fuel additives!
I have an old Triumph Bonneville with pistons that look like this;
The head is in a similar state, but that's going off to get rebuilt anyway. What's the best way to clean these pistons up? Do I need to take 'em out - I'd rather avoid the hassle TBH. The bores appear to be in good nick, but I need to have a proper look later.
They're 360 degrees apart, so one fires while the other is on the intake. That's "the norm" for parallel twins AFAIK - so you're basic CB250, CB500 etc. will be the same I think. Twins that are 180/540 degrees apart rock side to side loads. Modern stuff with 270 degree cranks etc. tend to have balance shafts.
I think the original "big bang" engines were Brit twins re-cammed to fire two at the same time, but don't quote me. This bike sparks both cylinders every time and has completely separate carbs and exhaust for each cylinder, so it'd be a pretty easy mod. Literally just change the cam I think.
Mr. Dazzle wrote: ↑Fri Aug 14, 2020 8:26 pm
With the engine in pieces that is - not with snake oil fuel additives!
I have an old Triumph Bonneville with pistons that look like this;
The head is in a similar state, but that's going off to get rebuilt anyway. What's the best way to clean these pistons up? Do I need to take 'em out - I'd rather avoid the hassle TBH. The bores appear to be in good nick, but I need to have a proper look later.
Being an old twat but not that decrepit I can remember doing this.
Drop the pistons down the bore a few millimeters, put a fillet of grease around the circumference of the bore/piston. Wire brush it clean.
Drop the pistons and this will leave the ring of grease with the crap stuck to it at the top. Clean it out, jobs a good 'un.
I'd somehow just assumed that they'd fire 180 degrees apart.
I guess it makes it easier to set the timing and dial the cam in if they're both TDC at the same time and the points/timing doesn't need to be too complicated.
It's also cheaper to make a 360 degree crank than a 180 one!
Plus like I said a 180 crank makes a weird rocking motion, so you still get vibes just of a different sort. AFAIK parallel twins are pretty much always 360 degree unless they're a bit special.
180 cranks also have an uneven firing order, power strokes are seperated by 180 then 540 degrees. This old girl doesn't idle well at the best of times before you throw in an uneven firing
Compression was a bit low before I started taking it apart....about 100psi on one side and nowt on the other. That "nowt" is why I started taking it apart!
If you want good compression and neglible oil consumption, hone the bores with stones no finer than 180 grit, wash the bores with soap and hot water until the bore surface does not blacken a paper tissue and then rub the bores with just enough thin oil to prevent rust.
Make sure the top ring gap is no smaller than spec (0.008”?), file the second ring gap to 30% bigger than the top ring gap, put one drop of oil on the piston skirts, front and back, no oil on rings.
Start the engine and instantly take off up the road with at least half throttle. Make the engine pull hard in bursts for a couple of miles.
Then you’ll have bedded in the rings and you should find it good and stiff to kick over.
You've got that far - I'd whip the block off and remove the pistons. Chances are you have a broken or at least badly worn ring. Cleaning carbon off in situ almost always ends up with bits dropping where you don't want them.
Unless you're a better mechanic than me you'll want 2 ring compressors to refit the pistons.
Ian.
1952 Norton ES2
1955 Francis Barnett Cruiser 75.
I suppose if I take the jugs off I can also take them to work and measure them to see if they're overbored, that way I'll be able to get the correct rings....
You will of course know to stuff the crankcase openings with rags as you lift the barrels just in case there are stray bits of ring trying to make their escape...
Ian.
1952 Norton ES2
1955 Francis Barnett Cruiser 75.
There's a whole series of videos from Low Brow customs on YouTube detailing Triumph 650cc strip/rebuild. It's about 10 hours total run time with all the tricks detailed.
JackyJoll wrote: ↑Fri Aug 14, 2020 11:46 pm
Make sure the top ring gap is no smaller than spec (0.008”?)
Seems there’s a not brilliant rule against editing mistakes.
Minimum ring gap in the manual is 0.010”, now that I’ve checked.
The recommended gap was changed to 0.015”-0.018” in a late 1960s service note. I think that was to reflect the actual gaps being seen on new Hepolite parts, rather than the previous minimum being unsafe.
An end gap that's a bit big is not a problem. Too small on the other hand....
And I would be cautious about believing anything I saw on youtube especially from a "custom" shop. I think the TOMCC have videos by Hughie Hancox and others which would be far more use.
Ian.
1952 Norton ES2
1955 Francis Barnett Cruiser 75.
Well the aforementioned videos certainly have the air of expertise IMO, he gives off the vibe of someone who knows what he's on about...but YMMV.
I will take the cylinders and pistons to work and measure 'em properly to see what size they actually are. The recommended clearances I've seen from various sources are 4-5 thou, but as high as 6 on racing engines. I shall see what I've actually got.
