This is only my opinion.
This is mostly focussed on the 750 crankshafts, even though it mostly applies to the 650/668 crankshafts as well. The only difference between the 750 crank and the 650/668 crank is the rods fitted, and the left outer web is unique to each motor.
Oil flow to the 750 crank is sufficient. Think of a Bultaco 350 2 stroke, developing more power per big end and spinning faster than the 750S crank. Current fuel/oil mix for the Bultaco is somewhere between 50:1 and 80:1, depending on the type of oil. It’s possible that with too much oil the 750S could create a “bow wave” and cause the big end rollers to skid.
The oil system is both pressured and unpressured. The oil at pressure (typically about 40 psi) is supplied to 7 restriction points, 2 in the cams , 2 spray holes to the underside of the pistons, 1 to clutch centre for cooling, and 2 to the inner crankshaft bearings. Some of these holes are very fine, and would be easily clogged. The inner crankshaft bearings spray oil into the slingers, where it becomes unpressurised. The big ends are supplied by centrifugal force from within the slingers and a 12mm hole in the centre of the big end pin. I cant see how any increase in oil pressure will increase oil flow to the big end. It’ll just overflow out of the slinger
(In the 650/668 there may be a need for the big end oil flow to carry away heat.)
I’ve pushed many of these cranks apart, and without obvious signs of wear, the big end pin is typically smaller in diameter (approx. 35.013) in line with the rod at TDC than it is at angle to the rod. At right angles to the rod I typically see about 35.028 on the inside of the pin (bearing surface only), down to about 35.015 on the outside of the bearing surface.
So we have out of round up to about 15 micron, and then taper of the pin about 15 micron. Most crank bearing manufacturers recommend about 2 micron, some up to 4 micron variance in the pin. It’s the taper that pushes the rod to the outside against the outer web, heats it, and wears it rapidly. I see this more often on the 650/668 engines.
So why does this happen?
My belief is that when the centre pin is pushed into the web, because of the close proximity of the 16mm oil hole in big end pin to the centre hole, the interference pressure, and the metallurgy properties of the integrated web/pin, that the big end pin becomes distorted roughly to the measurements above.
Look at the photo of the back of the inner web. That oil feed hole in the centre of the big end pin is 16mm diameter for 34 mm deep, then 11mm for and additional 23mm. That only leaves a wall thickness of 9.5mm to withstand the pressure of the interference fit of the centre pin. Pressure (typically 10 to 20 tons) is put on the pin and the side of the pin flexes up, pushing the sides of the pin outward at right angles to the rod. This explains why the 35.029 mm across the pin on the inner edge. If the pin goes elliptical, it must also stand that the diameter of the pin must reduce in line with the rod, hence the typical 35.011.
2 new, unused, webs. They measured just on the boundary of oem tolerance specifications on the bearing surface at 5 micron out of round, but within half an hour of assembly of the centre drive pin, they were up to 20 micron out of round with 10micron taper. And when the centre drive pin was taken out, it would shrink back almost to the original shape. Think of the web as being made of hot laver, and the surface hardening as the cool crust. When it is put under pressure (too much interference) it just “flows” to where it wants to be. It’s not rocket science. Just think about it.
New, unused webs, and that’s what they did.
So what is the fix?
Press the crank apart, clean it all up, polish the roller surfaces, and reassemble the centre assembly with new rollers. Then fit the outer webs without fitting the rods, and straighten the crank. Don’t push the outer webs on the last 2mm to give the grinding wheel room to grind the outer edge.
Then the crank is ground, assembled, like a car crankshaft, to get it all straight. Along with out of round and taper, parallelism becomes obvious here. That is, are the holes are drilled straight in the webs and are the pins “in line” with the crank centre line?
And your going to need 2 bloody good tradesmen to do it all!
The grinder has only about 10 to 12 micron “leeway” to get it right. The diameter of the pin needs to stay at about 35.000mm, (from about 35.010 to 35.015 after it was pushed together).
Why 35.000? Add oversize rollers and clearance to the pin, and there is bugger all to be taken out of the rod to clean it up. Grinding the big end on a Sunnen is where you need the 2nd bloody good tradesman. Most of the cranks I’ve pulled apart have been ground with 55 micron clearance, so there is 20 micron “lost” just there.
So, pin at 35.000, clearance at about 30 micron, and 2 rollers at about 5.016mm (the unobtanium oversize rollers) The originals are typically 4.997mm. The big end needs to be ground out to about 45.062mm to reclaim the conrod. About a 15 micron grind, but based on sizing to the final diameter of the pin.
With new conrods, typically 45.047, and 5.016 rollers, the big end pins can be taken down to 34.083mm. Clearance up to 36 micron might give a bit more leeway. The conrod big end is ground to suit the finished big end pin diameter.
Probably better off to target the 35.000mm and rod grind initially. That way, later on, with new rods, you might get a 2nd rebuild out of the crank.
Take it all home and leave it all assembled until you have the conrods and ready to go on. If you press the outer webs off sooner it will all just “flow” back to where it started. It needs the pressure of the interference fit to hold it all in shape.
Throw out the old roller cages and buy Koyo 45*35*20 crankshaft bearings. Take out the 4.997 rollers from the new Koyo and throw them away as well. The 5.016 rollers, at 16,743(?) will fit in nicely. The Koyo bearings have an M type steel cage. Stronger with much better oil distribution properties than the original flat cages.
When you have the rod and bearings ready to go on, press off the outer web, assemble it and press the web back on. Try and keep the time for that less than about an hour, or it may start to grow again…..
Because of the properties the metal it could take a few days to settle into position. So every morning for a few days put it in the sun, then dial guage it and tap it straight again.
Good Luck!
I accept no responsibility or liability for any issues that arise as a result of anyone’s use of this opinion.
This is mostly focussed on the 750 crankshafts, even though it mostly applies to the 650/668 crankshafts as well. The only difference between the 750 crank and the 650/668 crank is the rods fitted, and the left outer web is unique to each motor.
Oil flow to the 750 crank is sufficient. Think of a Bultaco 350 2 stroke, developing more power per big end and spinning faster than the 750S crank. Current fuel/oil mix for the Bultaco is somewhere between 50:1 and 80:1, depending on the type of oil. It’s possible that with too much oil the 750S could create a “bow wave” and cause the big end rollers to skid.
The oil system is both pressured and unpressured. The oil at pressure (typically about 40 psi) is supplied to 7 restriction points, 2 in the cams , 2 spray holes to the underside of the pistons, 1 to clutch centre for cooling, and 2 to the inner crankshaft bearings. Some of these holes are very fine, and would be easily clogged. The inner crankshaft bearings spray oil into the slingers, where it becomes unpressurised. The big ends are supplied by centrifugal force from within the slingers and a 12mm hole in the centre of the big end pin. I cant see how any increase in oil pressure will increase oil flow to the big end. It’ll just overflow out of the slinger
(In the 650/668 there may be a need for the big end oil flow to carry away heat.)
I’ve pushed many of these cranks apart, and without obvious signs of wear, the big end pin is typically smaller in diameter (approx. 35.013) in line with the rod at TDC than it is at angle to the rod. At right angles to the rod I typically see about 35.028 on the inside of the pin (bearing surface only), down to about 35.015 on the outside of the bearing surface.
So we have out of round up to about 15 micron, and then taper of the pin about 15 micron. Most crank bearing manufacturers recommend about 2 micron, some up to 4 micron variance in the pin. It’s the taper that pushes the rod to the outside against the outer web, heats it, and wears it rapidly. I see this more often on the 650/668 engines.
So why does this happen?
My belief is that when the centre pin is pushed into the web, because of the close proximity of the 16mm oil hole in big end pin to the centre hole, the interference pressure, and the metallurgy properties of the integrated web/pin, that the big end pin becomes distorted roughly to the measurements above.
Look at the photo of the back of the inner web. That oil feed hole in the centre of the big end pin is 16mm diameter for 34 mm deep, then 11mm for and additional 23mm. That only leaves a wall thickness of 9.5mm to withstand the pressure of the interference fit of the centre pin. Pressure (typically 10 to 20 tons) is put on the pin and the side of the pin flexes up, pushing the sides of the pin outward at right angles to the rod. This explains why the 35.029 mm across the pin on the inner edge. If the pin goes elliptical, it must also stand that the diameter of the pin must reduce in line with the rod, hence the typical 35.011.
2 new, unused, webs. They measured just on the boundary of oem tolerance specifications on the bearing surface at 5 micron out of round, but within half an hour of assembly of the centre drive pin, they were up to 20 micron out of round with 10micron taper. And when the centre drive pin was taken out, it would shrink back almost to the original shape. Think of the web as being made of hot laver, and the surface hardening as the cool crust. When it is put under pressure (too much interference) it just “flows” to where it wants to be. It’s not rocket science. Just think about it.
New, unused webs, and that’s what they did.
So what is the fix?
Press the crank apart, clean it all up, polish the roller surfaces, and reassemble the centre assembly with new rollers. Then fit the outer webs without fitting the rods, and straighten the crank. Don’t push the outer webs on the last 2mm to give the grinding wheel room to grind the outer edge.
Then the crank is ground, assembled, like a car crankshaft, to get it all straight. Along with out of round and taper, parallelism becomes obvious here. That is, are the holes are drilled straight in the webs and are the pins “in line” with the crank centre line?
And your going to need 2 bloody good tradesmen to do it all!
The grinder has only about 10 to 12 micron “leeway” to get it right. The diameter of the pin needs to stay at about 35.000mm, (from about 35.010 to 35.015 after it was pushed together).
Why 35.000? Add oversize rollers and clearance to the pin, and there is bugger all to be taken out of the rod to clean it up. Grinding the big end on a Sunnen is where you need the 2nd bloody good tradesman. Most of the cranks I’ve pulled apart have been ground with 55 micron clearance, so there is 20 micron “lost” just there.
So, pin at 35.000, clearance at about 30 micron, and 2 rollers at about 5.016mm (the unobtanium oversize rollers) The originals are typically 4.997mm. The big end needs to be ground out to about 45.062mm to reclaim the conrod. About a 15 micron grind, but based on sizing to the final diameter of the pin.
With new conrods, typically 45.047, and 5.016 rollers, the big end pins can be taken down to 34.083mm. Clearance up to 36 micron might give a bit more leeway. The conrod big end is ground to suit the finished big end pin diameter.
Probably better off to target the 35.000mm and rod grind initially. That way, later on, with new rods, you might get a 2nd rebuild out of the crank.
Take it all home and leave it all assembled until you have the conrods and ready to go on. If you press the outer webs off sooner it will all just “flow” back to where it started. It needs the pressure of the interference fit to hold it all in shape.
Throw out the old roller cages and buy Koyo 45*35*20 crankshaft bearings. Take out the 4.997 rollers from the new Koyo and throw them away as well. The 5.016 rollers, at 16,743(?) will fit in nicely. The Koyo bearings have an M type steel cage. Stronger with much better oil distribution properties than the original flat cages.
When you have the rod and bearings ready to go on, press off the outer web, assemble it and press the web back on. Try and keep the time for that less than about an hour, or it may start to grow again…..
Because of the properties the metal it could take a few days to settle into position. So every morning for a few days put it in the sun, then dial guage it and tap it straight again.
Good Luck!
I accept no responsibility or liability for any issues that arise as a result of anyone’s use of this opinion.
