750 case reinforcement

acadian · Aug 13, 2017

Found a few mentions of reinforcement mods for early 750 cases, mostly involving beads of weld around the main bearing flange on the primary side, as well as around the rear case/cradle bolt holes. One idea from a local machine shop was to weld an aluminum support plate around the main bearing flange inside the case, my concern would be shrinking the ID of the bearing recess.. any thoughts?

madass140 · Aug 13, 2017

I have no experience with beefing up the early cases but if you were to weld a stiffener as you suggested then I would advise not doing so
with the outer bearing race installed.

texasSlick · Aug 13, 2017

I have heard/read that beefing up the drive side bearing area with such a plate on the outside of the case merely moves the failure area further away radially from the bearing hub. I see no reason why inside the case would be any different.

It would seem that such a plate, outside or inside the case, would afford some additional strength. Whether the increase is worth the trouble is another matter. If the engine is stock, or only mildly warmed up, the stock case is good enough, .... if the engine has been built into a monster, the plate is not good enough.

Again, I have no direct experience ..... I am only rehashing what I read somewhere.

Slick

CanukNortonNut · Aug 13, 2017

acadian,
What is your game plan for this early case? :? Racing??....you have better options. + you need more money.

Keep it stock for road use and It will serve it's intended purpose. 8)
IMHO
Cheers,
Thomas

o0norton0o · Aug 13, 2017

A few things...

firstly, the weakest point should always fail first, so is the crank itself weaker than the cases? I always wonder about that...

Secondly, Is there any interaction by changing one and not the other?? I was told once by a norton "guy" that the early cases were more flexible because they were a thinner casting, so they flexed rather than broke... I don't know if it's true. It seems counter intuitive to me, but it was suggested to me, and I thought it was an interesting idea to repeat here to hear opinions.

Thirdly, is there a different route to change the amount of force being applied to the cases and crank by changing the mass of rotating crank and pistons.... like maybe superlight pistons ala jim schmitt... This way the cases don't need to be as strong...

Certainly a race bike tries to do it all. Stronger cases, stronger crank, and lighter pistons. If I had to chose only one, I'd chose the lighter pistons,... but I'm only guessing whether less flexing due to lighter mass would outlast stock pistons and a strengthened crankcase...

I'm sure some of the real mechanical experts could tell you which is best... I just wonder if they would all agree 8)

CanukNortonNut · Aug 13, 2017

o0norton0o said:
like maybe superlight pistons ala jim schmitt... This way the cases don't need to be as strong...

Certainly a race bike tries to do it all. Stronger cases, stronger crank, and lighter pistons. If I had to chose only one, I'd chose the lighter pistons,... but I'm only guessing whether less flexing due to lighter mass would outlast stock pistons and a strengthened crankcase...
e 8)

I did this to my P11 Atlas era engine.
That is where I would go. but it still takes lots of money. :shock:
Cheers,
Thomas

acadian · Aug 13, 2017

Already have the new steel rods, standard pistons though but with a hotter cam, will be dynamically balanced once collected. Not racing this one, and no, not in a position to splurge on Maney cases or an aftermarket crank. This was just a proposal from the machine shop guys I thought was interesting, but if no real benefit for my purposes I'd likely forgo it altogether given the problems that may emerge (need to re-bore the bearing recess etc...)

acotrel · Aug 14, 2017

I repaired a set of 850 cases that had cracked horizontally through the drive side main bearing. I welded a piece of 8mm aluminium plate to it. The guy who gave the cases etc to me, had used the commando to race against the Kawasakis etc. on the Great Ocean Road in Victoria on Sundays and had regularly revved it to 8000 RPM. My motor is tuned to pull much harder at lower revs - the limit is 7000 RPM. And it only just touches that on most gear changes as the load comes off.

grandpaul · Aug 14, 2017

acadian said:
One idea from a local machine shop was to weld an aluminum support plate around the main bearing flange inside the case, my concern would be shrinking the ID of the bearing recess.. any thoughts?

As there is barely enough room there to insert a spacing shim for the main bearing, I don't believe you can add any material INSIDE the case, else you won't be able to nip up the cases with the crank in place.

mdt-son · Aug 14, 2017

acadian said:
Already have the new steel rods, standard pistons though but with a hotter cam, will be dynamically balanced once collected. Not racing this one, and no, not in a position to splurge on Maney cases or an aftermarket crank. This was just a proposal from the machine shop guys I thought was interesting, but if no real benefit for my purposes I'd likely forgo it altogether given the problems that may emerge (need to re-bore the bearing recess etc...)

acadian: Weldig on aluminum c/cases will have two undesired effects - warping, caused by release of internal stresses (from the casting process), and build-up of residual stresses in vicinity of the weld zone, which in due course will produce fatigue stresses when superimposed with mechanical stresses. These residual stresses have a magnitude of the yield stress of aluminum. The following article demonstrates this:
http://www-llb.cea.fr/fr-en/activ03-04/p074-075.pdf

Residual stresses may be avoided by post-weld heat treatment. However, this and the release of internal stresses will not "unwarp" your casting!
Thus, be prepared for turning / milling work after PW heat treatment - especially at the crankcase joint between the halves. Maintaining dimensions inside the c/case and the "mouths" for barrel spigots will be rather difficult, and a taylored sheet metal "gasket" / spacer is proably neded. It`s a bodge. For the most part, the timing side crankacse halve needs trueing as well, as they deform with the number of heat cycles.

If I were in your shoes and you need more durable c/cases, I`d consider a pair of new (strengthened) crancases by Steve Maney. He used to provide a drive side halve matched to customer´s OEM timing side halve - these days I think he wants to supply complete crankcases only.
http://www.stevemaney.com/products.html

Other expensive alternatives are available .....

-Knut

acadian · Aug 14, 2017

mdt-son said:
acadian said:

Already have the new steel rods, standard pistons though but with a hotter cam, will be dynamically balanced once collected. Not racing this one, and no, not in a position to splurge on Maney cases or an aftermarket crank. This was just a proposal from the machine shop guys I thought was interesting, but if no real benefit for my purposes I'd likely forgo it altogether given the problems that may emerge (need to re-bore the bearing recess etc...)

Click to expand...

acadian: Weldig on aluminum c/cases will have two undesired effects - warping, caused by release of internal stresses (from the casting process), and build-up of residual stresses in vicinity of the weld zone, which in due course will produce fatigue stresses when superimposed with mechanical stresses. These residual stresses have a magnitude of the yield stress of aluminum. The following article demonstrates this:
http://www-llb.cea.fr/fr-en/activ03-04/p074-075.pdf

Residual stresses may be avoided by post-weld heat treatment. However, this and the release of internal stresses will not "unwarp" your casting!
Thus, be prepared for turning / milling work after PW heat treatment - especially at the crankcase joint between the halves. Maintaining dimensions inside the c/case and the "mouths" for barrel spigots will be rather difficult, and a taylored sheet metal "gasket" / spacer is proably neded. It`s a bodge. For the most part, the timing side crankacse halve needs trueing as well, as they deform with the number of heat cycles.

If I were in your shoes and you need more durable c/cases, I`d consider a pair of new (strengthened) crancases by Steve Maney. He used to provide a drive side halve matched to customer´s OEM timing side halve - these days I think he wants to supply complete crankcases only.
http://www.stevemaney.com/products.html

Other expensive alternatives are available .....

-Knut

good info, here and throughout responses, answers my question... will leave as is, and save the road racing for the T140

750 case reinforcement

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