SF2 crankshaft nut

[natm]

Just because you did it doesn’t make it right, use a rattle gun for ffs

Just because you did it doesn’t make it right, use a rattle gun for ffs

That engine puts out 82Nm so another 13 in top gear will be no problem, unlike a rattle gun that could strip the nut or damage the crank (ffs). Yes, you are right, just because I did it, doesn't make it right, but I'm a retired engineer who has rebuilt gearboxes on many different makes of motorcycles and did all my own spannering when I raced motocross. I have used this method several times and the bike runs perfectly. I should add that when I bought my SF3 I had to heli-coil about a dozen threads on the engine because of ham fisted previous owners/ mechanics. I use a rattle gun to remove the crank nut.

 

[Lyndoneil]

Tightened the bolt with my neighbours Sealey rechargeable rattle gun, I couldn't get my air impact driver to work, stripping despite stripping it down, cleaning and re-oiling 3 times.
The Sealey gun states it tightened up to 370 ft/lbs, didn't over do it, put some loctite on the bolt and being a fairly fine thread on the bolt it tightened quickly, eased it off once, just to cheak that it actually felt tight and them re-tightened and gave it a couple of short rattles, as I was slightly concerned about damaging the crankshaft thread.
Bent the new tab washer into place then reassembled everything.
I've no idea what torque the bolt is tightened too, just have to hope it's enough, but in truth half the time self maintenance work is done on these old machines it's a case of hope that's right, tight enough, seals, not too tight etc.
Will find out eventually, just have to listen for any strange noises for a while.

 

[CLEMTOG]

" I wouldn't like to put that sort of force through the oil pump drive."

you are not putting any strain on the oil pump drive, it is the gear that is splined onto the crankshaft (same spline as the engine triplex sprocket) and it's teeth drive another gear which is the oil pump, so the forces are not harrmful, to the pump, the factory tool works very well but they are rare (mine was lost long ago to a "borrower") but new ones that are not particaularly nicely crafted are available from an ebay seller based in Lisbon (Ireland) they work very well but are not elegant, a bit spiteful in your hands, and very heavy, but certainly have acurately cut teeth to engage the crankshaft oil pump gear and lock the crank. Most people wouldnt notice or care about the "feel" of the spanner, but if you can tell the difference between a Snap On 13mm spanner in your hands (eyes closed) and a Chinese mud metal equivalent, or even an Elora type, you will know what I mean.

CLEM

 

[Dellortoman]

Something in another thread about locking the crankshaft during a leakdown test gave me an idea. It may turn out to be impractical, but for the sake of innovative discussion, here it is anyway ...

Using air pressure in the cylinder may be an alternative method of tightening the primary drive crankshaft nut up to the correct torque if you don't have a torque wrench that'll go that high, or a suitable way of locking the crankshaft. There are a whole lot of unsuitable ways to lock the crankshaft but lets not go there. They've been discussed previously in this thread and this method dispenses with them anyway.

You'll need an air compressor with an adjustable pressure regulator and accurate gauge, and an adaptor to feed the compressed air into the spark plug hole. You'll also need a suitable socket for the nut and a breaker bar that you can lock against a footrest or something.

The consensus among Laverdisti is 120 ft.lb as a suitable torque spec for that nut on Breganze twins and triples. If you position the crankpin on the cylinder nearest to the primary drive side at 90° ATDC on the power stroke (both valves closed) you can work out the cylinder pressure required to generate that torque. It's a pretty simple calculation.

On a 750 twin or 1200 triple (80mm bore) it works out to 127 psi. That doesn't take into account any friction between piston and cylinder bore messing up the calculation, so maybe add 10% to account for that. That'll make it around 140 psi. You might have to tweak the pressure regulator on your compressor up a wee bit. It may not be super accurate because of the friction business, but I'd suggest it's more accurate than using a rattle gun or jumping up and down on the end of a breaker bar.

On a 1000 triple (75mm bore) the piston area is a bit smaller so it'll need a higher pressure. The theoretical cylinder pressure to produce a torque of 120 ft.lb is 144 psi before accounting for friction, so you may need to go to 160 psi. That's probably about the limit of a typical compressor from a tool shop anyway.

I reckon you could mitigate the friction error by introducing a bit of vibration into the system. It's a similar concept to the way that a concrete vibrator allows the wet concrete to flow and air bubbles rise out. That could be done by holding the cylinder at the specified pressure while tapping gently on the breaker bar with a small hammer.

You'll just need to mark the 90° crank position with a felt tip pen or something similar, and keep re-setting it back there until the crankshaft stops rotating in the thread of the nut.

Using cylinder pressure is a reasonably gentle way of tightening the nut. It doesn't require any cowboy methods of locking the crankshaft, and you're not going to overload anything with a home workshop air compressor. Typical cylinder pressure for a petrol engine during the combustion stroke is 400 - 900 psi, so 160(ish) psi isn't going to break anything.

Don't be tempted to hook up a hydraulic pump that can go to thousands of psi!

Now that I've written it all down, it sounds like a lot if faffing about, so perhaps not such a great idea. But I don't mind making a fool of myself so I may as well hit the post button rather than delete it. Who knows, it may trigger someone else to have a better idea..

 

[Tippie]

Or use a rattle gun

 

[The Sock]

Something in another thread about locking the crankshaft during a leakdown test gave me an idea. It may turn out to be impractical, but for the sake of innovative discussion, here it is anyway ...

Using air pressure in the cylinder may be an alternative method of tightening the primary drive crankshaft nut up to the correct torque if you don't have a torque wrench that'll go that high, or a suitable way of locking the crankshaft. There are a whole lot of unsuitable ways to lock the crankshaft but lets not go there. They've been discussed previously in this thread and this method dispenses with them anyway.

I would be very concerned about the loadings on the conrods, if using the above. It's way outside the use the rods were engineered for, to say nothing about the bearings.
A thrown rod is an engine killer!

 

[Paul Marx]

The art of making simple things complicated.
Paul

 

[sfcpiet]

I would be very concerned about the loadings on the conrods, if using the above. It's way outside the use the rods were engineered for, to say nothing about the bearings.
A thrown rod is an engine killer!

Conrods need to cope with several tons of force in alternating directions thousands of times per minute, a couple of psi air pressure won't even impress them or their bearings.

piet