No Crankshaft 1975 Norton 850 Commando MKIII in 750 crankcas
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
But you Can . :twisted:
But you Can . :twisted:
Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
I machined one to fit a while ago. If I remember you need ro remove .030" from both sides of the cheeks where the bearings sit. It is also a good idea to radius both pto shafts to eliminate the sharp corner that can crack and break.
I machined one to fit a while ago. If I remember you need ro remove .030" from both sides of the cheeks where the bearings sit. It is also a good idea to radius both pto shafts to eliminate the sharp corner that can crack and break.
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
Why would one choose to put a 75 crank into a 750? Just curious as to what are the benefits.
Why would one choose to put a 75 crank into a 750? Just curious as to what are the benefits.
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
Why not machine the 750 cases to fit the CRANK ! ? :twisted:
or just use Mk III cases too , trade for the 750 , couldnt be to difficult .
Most of the mk III stuff is beefed up , if your one of these blokes that hasnt grown up .
Why not machine the 750 cases to fit the CRANK ! ? :twisted:
or just use Mk III cases too , trade for the 750 , couldnt be to difficult .
Most of the mk III stuff is beefed up , if your one of these blokes that hasnt grown up .
Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
Mk3 crank is longer on drive side than the earlier cranks in order to fit the starter (or none starter) parts.
The mouth of 850 case is (I believe) larger than 750 cases where the barrels fit.
Apart from the different arrangement for locating the drive side seal there are 4 holes for holding the primary inner case on so you would need to do something about that also.
Mk3 crank is longer on drive side than the earlier cranks in order to fit the starter (or none starter) parts.
The mouth of 850 case is (I believe) larger than 750 cases where the barrels fit.
Apart from the different arrangement for locating the drive side seal there are 4 holes for holding the primary inner case on so you would need to do something about that also.
lcrken
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
To summarize, and repeat some of the already posted info,
750 and 850 (except MKIII) crankshafts are the same except for the flywheel. The 850 flywheel is heavier to counterbalance the heavier 850 pistons.
The MKIII crankshaft is wider from cheek-to-cheek, so won't directly fit earlier cases, has a longer drive side mainshaft to accommodate the electric start mechanism, and has 3/8" fasteners holding it together, instead of the 5/16" fasteners in the previous Commando crankshafts. It also has a slot machined into the timing side cheek to allow the use of a timing plug, but that has no real effect on interchangeability.
It's pretty easy to fit an earlier crankshaft into MKIII cases by putting shims in the cases behind the main bearings, and I've done that a couple of times. It's more difficult to fit the MKIII crank into the early cases, and no real reason to. If you machine the MKIII cheeks to narrow the crankshaft for the earlier cases, you lose the increased bending stiffness of the thicker cheeks, so you might as well just use an earlier crankshaft and ream the holes out to take the 3/8" fasteners. If you try to machine the inside of the early cases to fit the wider crankshaft, you make the already weak cases even weaker. Of course, if you just have the parts lying around and want to use them up, it is possible.
Plenty of differences between MKIII and earlier cases too, but that's another subject.
Ken
B+Bogus said:
To summarize, and repeat some of the already posted info,
750 and 850 (except MKIII) crankshafts are the same except for the flywheel. The 850 flywheel is heavier to counterbalance the heavier 850 pistons.
The MKIII crankshaft is wider from cheek-to-cheek, so won't directly fit earlier cases, has a longer drive side mainshaft to accommodate the electric start mechanism, and has 3/8" fasteners holding it together, instead of the 5/16" fasteners in the previous Commando crankshafts. It also has a slot machined into the timing side cheek to allow the use of a timing plug, but that has no real effect on interchangeability.
It's pretty easy to fit an earlier crankshaft into MKIII cases by putting shims in the cases behind the main bearings, and I've done that a couple of times. It's more difficult to fit the MKIII crank into the early cases, and no real reason to. If you machine the MKIII cheeks to narrow the crankshaft for the earlier cases, you lose the increased bending stiffness of the thicker cheeks, so you might as well just use an earlier crankshaft and ream the holes out to take the 3/8" fasteners. If you try to machine the inside of the early cases to fit the wider crankshaft, you make the already weak cases even weaker. Of course, if you just have the parts lying around and want to use them up, it is possible.
Plenty of differences between MKIII and earlier cases too, but that's another subject.
Ken
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
Ken,
Thanks for the comprehensive reply - very useful information as I've got 850 cheeks and Maney cases - just looking to get a flywheel manufactured if I can get hold of a drawing.
Piero,
The Atlas crank is the same as the Commando as far as I am aware.
As for flywheels, provided the assembly is balanced correctly for the rods & pistons being used, there should be no limitation.
To complete the 'history' the 650SS uses the same cheeks (Introduced for this engine), but the flywheel is different again.
pierodn said:
Ken,
Thanks for the comprehensive reply - very useful information as I've got 850 cheeks and Maney cases - just looking to get a flywheel manufactured if I can get hold of a drawing.
Piero,
The Atlas crank is the same as the Commando as far as I am aware.
As for flywheels, provided the assembly is balanced correctly for the rods & pistons being used, there should be no limitation.
To complete the 'history' the 650SS uses the same cheeks (Introduced for this engine), but the flywheel is different again.
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
Hi Piero, the flywheel of the Atlas is a bit heavier than the commando , but the same about the dimensions....normally Atlas is designed to be balanced at 84% , the commando smaller percentage (52% or 63%, depending if you speak wet or dry!!)
Hi Piero, the flywheel of the Atlas is a bit heavier than the commando , but the same about the dimensions....normally Atlas is designed to be balanced at 84% , the commando smaller percentage (52% or 63%, depending if you speak wet or dry!!)
lcrken
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
One other difference I left out. Early 750 crankshafts (ca. 1971 and earlier, and probably Atlas) had slightly smaller timing side main bearing journals, at 1.1807" - 1.1812", to accommodate the earlier style main bearings. 1972 and later cranks, from engine number 211891, had timing side journals at 1.1812" - 1.1815" to accommodate the "Superblend" roller bearing. The real world impact of the difference is pretty much zero. There have been plenty of cases of upgrading the early crankshafts to the later bearings with no issues. I did the change on my early PR back in the '70s and put a lot of races on that crank with no problems.
Ken
One other difference I left out. Early 750 crankshafts (ca. 1971 and earlier, and probably Atlas) had slightly smaller timing side main bearing journals, at 1.1807" - 1.1812", to accommodate the earlier style main bearings. 1972 and later cranks, from engine number 211891, had timing side journals at 1.1812" - 1.1815" to accommodate the "Superblend" roller bearing. The real world impact of the difference is pretty much zero. There have been plenty of cases of upgrading the early crankshafts to the later bearings with no issues. I did the change on my early PR back in the '70s and put a lot of races on that crank with no problems.
Ken
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Re: No Crankshaft 1975 Norton 850 Commando MKIII in 750 cran
looks like the flywheel per Mk III might not be interchangeable then .
looks like the flywheel per Mk III might not be interchangeable then .
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