750 crank balance factors

Tony said:
Absolutely, couldn't agree more Phil. It was Phil Irvings papers on crank phasing that set me to work on my Twin too.

If the stuck in the mud's at Triumph, BSA and Norton had listened to him in the sixties the whole British motorcycle industry may have been given a fighting chance to survive..... for a while at least!

It's all about conservation of momentum, he actually stated that 76? would be best for absolute power in a parallel twin, as piston A will be traveling at maximum speed on the power stroke when piston B effectively stops at the bottom of the stroke.

A 90? phasing gives the best compromise however between how fast piston A is moving when piston B stops on both the power and exhaust strokes.

Hence that's why my Twin is probably the only vehicle with a willfully eccentric 88? phasing. The original Laverda splines on a twin crank allow you to rotate in 4? increments so we went for a 'close to but less than' 90?.

T.
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Hi

Very old thread but very interesting...

88deg.... That's why Morini with its 1200cc has a 87deg!
 
Only 88 with std SF splined centres - SFCs have larger OD solid centre pins with no splines, so they can be phased to anything you like. Mine went straight to 90 (ie 270, firing intervals 270-450 deg). An old thread indeed! But a goodie. I enjoyed the thread a lot, with Red getting right into twisted twins, rebalancing etc. I have my two triple webs in the shed waiting for me to create my frankencrank and try out something I have a 'gut feeling' about for many years now :D
 
Well, I will not lie.... Since I have to replace the crankshaft bearings..... I only hope an eBay fake 20T press would do the job. and thinking on how to get the pins "not-alligned" properly without a proper machined dime.
Suggestions??  :D
 
20 ton press is way more than you need, the problem with cheap presses is the support of the bed, they can flex a lot. Have you any experience pressing up cranks? If not I would strongly recommend having someone with a proven record on Laverda crankshafts do the job. On Laverda cranks we have made up plates that support around the slingers and other things for both taking it apart and reassembling. It is easy to damage them. Alignment is no more complicated than any crank and as always start by scribing a straight line across the webs. The centre section is as said splined so basic alignment isn't an issue, however throwout can be difficult to rectify there. Unless the centre mains are definitely worn too far or you plan on race type use, it could be just as well to leave the centre alone. The factory only sold the centre as one piece with the centre and both webs iirc.
 
The only longevity issue then could be slingers full. They can be cleaned without dismantling the middle, but it tales care and meticulous cleaning.

The SF cranks were built with a massive press, that basically used the hardened splined centre to cut the splines into the plain mild steel centre holes, so cutting mating splines into it. The bolts then just retain it.
 
Never had much of a problem spinning my 70 750 SF0 above 7500rpm approaching 8 grand - In spite I did melt the pistons once over a lean carb mix and a deteriorating head gasket plus a few other crank case rubber gaskets ;o)

OLD Warrior to say the lest!
 
Piranha Brother 2 said:
The only longevity issue then could be slingers full. They can be cleaned without dismantling the middle, but it tales care and meticulous cleaning.

The SF cranks were built with a massive press, that basically used the hardened splined centre to cut the splines into the plain mild steel centre holes, so cutting mating splines into it. The bolts then just retain it.
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Why bother?

If the slingers are filled with crud, most of it will have passed through the centre mains several times, doesn't take a genius to recognise that these will have suffered.  If the bike hasn't done astronomical miles, the slingers may be filled only to an acceptable level, but it shows it has stood idle for most of its life.  Chances are the bearings may show signs of rust.  After around 50 years, the crank has deserved a complete overhaul.  Nothing is more shitty than having an "overhauled" crank fail after a short time because one has taken a shortcut.

piet
 
sfcpiet said:
Why bother?

If the slingers are filled with crud, most of it will have passed through the centre mains several times, doesn't take a genius to recognise that these will have suffered.  If the bike hasn't done astronomical miles, the slingers may be filled only to an acceptable level, but it shows it has stood idle for most of its life.  Chances are the bearings may show signs of rust.  After around 50 years, the crank has deserved a complete overhaul.  Nothing is more shitty than having an "overhauled" crank fail after a short time because one has taken a shortcut.

piet
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Never bothered to determine exactly what that hard graphity stuff that builds up in slingers is, Piet. Is it just solid from oil impregnated with carbon molecules??? Dunno - Mine were about to head into the crankpin holes when I cleaned them out. Middle mains were fine.
 
The crud is mostly composed of metallic particles, mainly ferrous.  No wonder, with two monster chains flailing around on their sprockets, 5 pairs of gear teeth gnawing away at each other, three selector forks rubbing in their grooves, a camshaft doing its best to chew up the followers, rockers hammering away at the valve tips, valve springs rubbing away on the collars, a clutch push rod trying to press itself through the clutch actuation bar, plain clutch plates being worn away by the bonded plates, etc, etc.  Not to mention the wear at the piston rings and the bores.  Ball and roller bearings wear, 18 of those in a standard 750, couple more in a SFC donk.  Then there are the bronze gearbox and small end bushes that wear away, along with the corresponding shafts and pins.  It all adds up, and whatever is worn from the surfaces does a couple of laps through the engine, grinding off further solids before it is finally caught in the slingers.

Proper filtration would do these engines a world of good, catching particles before they can wreak havoc.

"Fine" is wide definition Quentin.  If you can, you should compare a new C3 bearing to what is fitted to your crank, old ones have a lot more wobble than the new.  This means there is also excessive radial clearance, allowing the crank to literally bend while rotating, if only the outer bearings are renewed.  Doesn't help to lower the vibe level.  An overhauled crank will bend as well, only to a far lesser degree.

piet
 
Tippie said:
20 ton press is way more than you need, the problem with cheap presses is the support of the bed, they can flex a lot. Have you any experience pressing up cranks? If not I would strongly recommend having someone with a proven record on Laverda crankshafts do the job. On Laverda cranks we have made up plates that support around the slingers and other things for both taking it apart and reassembling. It is easy to damage them. Alignment is no more complicated than any crank and as always start by scribing a straight line across the webs. The centre section is as said splined so basic alignment isn't an issue, however throwout can be difficult to rectify there. Unless the centre mains are definitely worn too far or you plan on race type use, it could be just as well to leave the centre alone. The factory only sold the centre as one piece with the centre and both webs iirc.
Click to expand...
Few years ago I took apart few single cranks.
I know the risk of damage is high... But hey, that's the fun!  :D

Also..... Maybe a stupid idea, assuming that I rotate the crank, the camshaft need a new spline machined to the new position.
But, again maybe a stupid idea, if I drill a couple of holes on the flange that is bolted to the timing gear, thread them and use 2 stainless steel grub screws (thick ad the spline width and with flat end) , to hold the camshaft using the old spline?
I hope is clear enough.

About the center bearings. This engine broke the chain and had some damage. Nothing serious but still.... I've found a decent amount of metal dust in the case, and the bearings are also a bit noisy.

Damn...it's 2:00 AM and I can't sleep because I am thinking at the engine and the cases that are going to vapour blasting tomorrow!

  ??? I'm a mess!  :LOL:
 
If you're talking about 750s, the cam is held together by 3 studs (six nuts). There's no spline but a key and keyway. You can eliminate the key joint inside the sprocket and rephase the cam wheel 90 deg for one side by drilling and tapping. The clamping pressure of the flanges against the cam chain wheel hold it all together.

But it sounds like you need to fully rebuild your motor so if you are going to split the crank that's the time to consider rephasing - it's not just a simple twist and voila though. Not something you should undertake unless you know exactly what you're doing. There are about four or five on this forum or beyond that I know of who do this professionally.
 
"rephase the cam wheel 90 deg for one side" 45 degrees if your crank is phased 90 deg. Have to re-mount your second set of points 45 deg as well.
With metal dust doing the rounds in your oil it sounds like your crank should be completely dismantled.
 
Piranha Brother 2 said:
... There's no spline but a key and keyway. You can eliminate the key joint inside the sprocket and rephase the cam wheel 90 deg for one side by drilling and tapping.

... Not something you should undertake unless you know exactly what you're doing...
Click to expand...

Sorry I meant the key, this is my lack in English vocabulary! I thought that the most of the work was done by the key.
As I said before, I have a little experience and I am aware of the risks. But you know...

 
Tippie said:
"rephase the cam wheel 90 deg for one side" 45 degrees if your crank is phased 90 deg. Have to re-mount your second set of points 45 deg as well.
With metal dust doing the rounds in your oil it sounds like your crank should be completely dismantled.
Click to expand...

Yep I know that the cam goes the half of the crank. About the points, well, the stock plate with the ignition points and capacitor is completely missing so I guess I will go for an electronic one.

Thanks
 
"I thought that the most of the work was done by the key." The key is really just used for alignment when it comes to the sprocket itself and is a hassle if you even want to degree the cams in a standard 360 deg motor. When you re-phase the cam it is just in the way and I have cut off the part through the sprocket. I have the key full depth on the flanges, these flanges are the weak point on this multi piece cam setup, they can and do loosen on the cam if not a tight interference fit. The flat faces of the flanges and the sprocket must be in good condition and the tightness of the studs, or 6mm allen screws in my case on my 270 deg motor, hold it together fine, mine have not given a problem after 25 years, most of it racing. The other benefit of using screws is that each side can be individually set to your desired degree if you slot the flanges.
 
Yep I thought about slotting the flanges to have an "adjustable timing" but then I was worried that wouldn't hold enough.
I already have bad experience on the first series of Rotax v60 on the Aprilia rsv. The actual lobes were press fitted and sometimes at high rpm for long period of time (track) they occasionally rotate on the shaft. A common fix was a couple of tig points between the lobes and the shaft.
 
"A common fix was a couple of tig points between the lobes and the shaft" A race mechanic I know uses that method on every at risk point accessible in an engine, has no doubt saved many a dnf.
 
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