What is causing that knock?

Tippie

Tippie

Hero member
Location
Dr?bak, Norway
SF broken crank.jpgSF crank big end.jpgSF crank outer web.jpg

Before leaving on the 400km ride home, the bike developed a fairly serious knock, which quietened down with a few revs. So it was decided to chance it and ride the old SF carefully back home, keeping the speed way down to just 110 kmh.
Upon return we checked it for the usual culprits like loose cam flanges, primary sprocket etc. Compression was good and nothing really seemed amiss, so there was nothing to it but dig out the big tools and :devilish: SPLIT THE CASES:devilish:.
How many motors do you know that would have made it home with a minor inconvenience like this?
 
Lee,

Seems not that uncommon, especially with the stepped pin GT(!) crank in your pics. Never seen one myself, but I've heard many stories about these issues.

I once managed to limp home with a broken crank pin in my old Matchless single, more than 80km before I decided to call it quits. The groaning noises coming from the primary drive were straight out of a horror movie! The pin had snapped at an angle, with a portion jutting into and dragging the timing side web with it. I was more than amazed when I tore it down!

piet
 
It was at the right side of the engine. As far as I know there is just the generator and the starter gear. Maybe the engine would have stopped running when the battery was empty 😀
 
What is a stepped pin crank doing in an SF? I thought they'd been phased out when the SF came out. They have the reputation of breaking when the bike is used hard.
Paul
 
andre g said:
It was at the right side of the engine. As far as I know there is just the generator and the starter gear. Maybe the engine would have stopped running when the battery was empty 😀
Click to expand...
The starter functioned so I imagine that the dynamo did as well.
Paul
 
sfcpiet said:
Lee,

Seems not that uncommon, especially with the stepped pin GT(!) crank in your pics. Never seen one myself, but I've heard many stories about these issues.

I once managed to limp home with a broken crank pin in my old Matchless single, more than 80km before I decided to call it quits. The groaning noises coming from the primary drive were straight out of a horror movie! The pin had snapped at an angle, with a portion jutting into and dragging the timing side web with it. I was more than amazed when I tore it down!

piet
Click to expand...
GT and S.
Paul
 
It's a mate of Lee's Rob. How they managed to start it with that is beyond me - you'd think the starter would just grind the two broken faces together. The owner has many Laverda twins and mixes and matches bits and pieces as needs arise!

The stepped crankpin explains a lot.
 
sfcpiet said:
Lee,

Seems not that uncommon, especially with the stepped pin GT(!) crank in your pics. Never seen one myself, but I've heard many stories about these issues.

I once managed to limp home with a broken crank pin in my old Matchless single, more than 80km before I decided to call it quits. The groaning noises coming from the primary drive were straight out of a horror movie! The pin had snapped at an angle, with a portion jutting into and dragging the timing side web with it. I was more than amazed when I tore it down!

piet
Click to expand...
AMC singles seem prone to it. Recently shown pics of a 500 which ran nicely but no drive. Pin had broken clean on drive side. No other damage at all.
I'm going to assume after getting home on that twin a lotto ticket or three were purchased....
 
GregT said:
AMC singles seem prone to it. Recently shown pics of a 500 which ran nicely but no drive. Pin had broken clean on drive side. No other damage at all.
I'm going to assume after getting home on that twin a lotto ticket or three were purchased....
Click to expand...
Especially the early type with the 2-piece pins, pretty weedy... add about 50 years of wear and tear, you can wait for it to snap!

I'm actually running stepped pins in my long-stroke GTL, bearing races ground 3mm eccentric to the pressed portions. Has held up well since '94, but I did take care to use the best material I could find and include suitable radii.

piet
 
My S is running its original crank with stepped pins.
Crank rebuilt by Phil Todd around 2000 as a matter of course during restoration
I doubt that the crank has 100 000 km on it.
Paul
 
sfcpiet said:
Especially the early type with the 2-piece pins, pretty weedy... add about 50 years of wear and tear, you can wait for it to snap!

I'm actually running stepped pins in my long-stroke GTL, bearing races ground 3mm eccentric to the pressed portions. Has held up well since '94, but I did take care to use the best material I could find and include suitable radii.

piet
Click to expand...

Blimey, that's interesting Piet. But I'm not sure if I understood it correctly. Does the 3 mm refer to the radius? Which leads to the next question, have you achieved 77 (774 cc) or 80mm (801 cc) stroke with it?
:unsure:
 
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Jo said:
Blimey, that's interesting Piet. But I'm not sure if I understood it correctly. Does the 3 mm refer to the radius? Which leads to the next question, have you achieved 77 (774 cc) or 80mm (801 cc) stroke with it?
:unsure:
Click to expand...
80mm stroke, Jo. 3mm eccentric = +6mm stroke. The pin is 30/36mm, so that works out perfectly! My calculations came up with 805cm³. Nice and punchy, from way low down. Who needs revs when you have punch? ;)

The radii referred to the step between the narrow and wide bit. Get these wrong and the whole caboodle goes pear-shaped very quickly. I had the crank apart a couple of years ago for balancing, everything looked in top shape, not even the signs of bending that are so common on all cooking 750 cranks.

The whole project materialised when I had one of these early cranks (#1109 actually) on the bench with a stray 1200 piston laying around... but it's not as straightforward as it may sound.

piet
 
Thanks Piet! Which leads me to the next question: What kind of conrods did you choose for it?

53536249801_1556f1b96a_b.jpg


You probably explained it here. If only I had listened better instead of snapping shots ...
:cool:


Sorry for hijacking your post Lee!
 
Carillos were fitted when I first built it. For balancing, I reverted to carefully prepared OE rods because they are considerably lighter and suited my goals better. The Carillos have been set aside for my next project.

Don't think Lee will mind at all... he loves this sort of stuff! ;)

piet20190522_195925.jpg
 
Hi Piet,
I'm still lost. What I do not understand is the point that an OE rod would be too long if the crankpin moves 3mm upwards (in tdc). Sure you can shed some light on this ...
;)

Cheers,
Jo
 
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