750 - 850 Cases Comparison or 850 Barrels in 750 Cases

I have fairly late model 750 cases which I thought should be as beefy as the 850 cases and what I've heard would be suitable to widen out for fitting 850 barrels. However comparing them side by side to actual 850 cases from a Mark 2a 1974 engine they don't seem as thick in several places. Not shown in the photos is the gussets along the mating surface along the back. The 750 gussets join the main thickness of the case pretty well at right angles whereas the 850 cases have a thick gradual curve where the gusset joins the main thickness of the case.

When I measured the distance between the cylinder cut outs I never torqued the cases together but I did squeeze them together in a vice so the measurements given will provide a fairly accurate idea of the differences. The 850 is .178" wider or .089" extra boring on each side required to fit 850 barrels.

I have 2 sets of 850 cases to compare and they are identical. I only have 1 set of 750 cases that appear to be late model ones as the timing side casting is solid where the distributor plate is located on earlier models. These are replacement cases without original stamped numbers so I can't get the actual date for them. Given the extra thickness of the 850 cases I'd be reluctant to machine out 750 cases to take the 850 barrels. I'd just pick up some 850 cases and go with them. The only negative would be if you wanted to keep a matching numbers bike.

Thank you, that is very interesting. Good photos too.

Mk3 cases were reputedly stronger than the pre Mk3 850 cases, but I don't know for sure where they were stronger?

As I have already said in an earlier post, I had some 750 Combat cases bored out at the crankcase mouth to accept 920cc barrels, and the engine was run as a 920cc. The crankcases were very thin at the 7 o'clock and 9 o'clock position, but the engine survived and survives to this day, despite Roger at RGM saying it WOULD break. Maybe I was just lucky. There was no head work done so although it produced a lot more torque, maybe there wasn't an increase in BHP?

Obviously you are much better using 850 barrels in 850 cases, but it can be done.

Interesting comparison, and food for thought too.

I'd like to see how a set of Maney cases would compare....then again, I'd like to see how a set of Maney cases would look on my bike, too :mrgreen:

I've got my Dreer VR880 left side case that I can take pix of, just for grins...

Reggie said:

As I have already said in an earlier post, I had some 750 Combat cases bored out at the crankcase mouth to accept 920cc barrels, and the engine was run as a 920cc. The crankcases were very thin at the 7 o'clock and 9 o'clock position, but the engine survived and survives to this day, despite Roger at RGM saying it WOULD break. Maybe I was just lucky. There was no head work done so although it produced a lot more torque, maybe there wasn't an increase in BHP?

Obviously you are much better using 850 barrels in 850 cases, but it can be done.

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This is great to hear, thanks for chiming in.

B+Bogus said:

Interesting comparison, and food for thought too.

I'd like to see how a set of Maney cases would compare....then again, I'd like to see how a set of Maney cases would look on my bike, too :mrgreen:

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LCRKEN has addressed this in his "Short Stroke 750 Builld" Thread here:

short-stroke-750-build-t5490.html

Ahh yes, a set of Maney cases. ...wasn't there already a thread started on Norton Dreams?

grandpaul said:

I've got my Dreer VR880 left side case that I can take pix of, just for grins...

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Yes, by all means Paul, I'd love to see how they "measure up"!

These pics are of a set of Kenny Dreer cases I have that are for a large main bearing Falicon crankshaft. Other than the larger bearing bores, they are identical to the other KD cases I have. I'll try to take some more detailed shots later, for comparison with the stock cases. I might have some other shots of the other KD cases, and if I find them, I'll post them too.







Ken

Found some more Kenny Dreer case pictures. When Kenny first started making his own cases, he started the same way Steve Maney did, by just making the drive side half, and mating it with stock timing sides. These are pictures of one of those drive sides fitted to an early 750 timing side half. The difference in case thickness is pretty obvious.







This is a shot of another KD drive side half fitted to a later timing side. It shows the difference in thickness at the front.



Ken

Yep, mine is a drive side VR880 half mated to a timing side OEM 850 half.

Wow! When you consider these are 880 openings you get the idea of how beefy these are. The timing side would get extra strength just from being bolted up to one. Thanks for the photos Ken and Paul.

It's interesting that Reggie has 750 cases opened up for 920 cylinders and that they are holding together.

I remember reading about Ron Wood's Norton-powered flattracker. It was always the drive side case that would fail. Interestingly, the failures would always occur when the throttle was chopped at the end of the straight, just before entering a turn.

If I remember correctly flattrackers don't have front brakes, just down shifting and a rear brake? (It's been a long time since I followed this stuff.)

Danno said:

I remember reading about Ron Wood's Norton-powered flattracker. It was always the drive side case that would fail. Interestingly, the failures would always occur when the throttle was chopped at the end of the straight, just before entering a turn.

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Might have had something to do with the crankshaft flopping up and down like a wet noodle. :mrgreen:

Ron built one-piece cranks out of billet, but the cases just couldn't take the 80 or so BHP. Wasn't someone making heavy-duty cases for sidehack dirt racing in the UK?

Many thanks to Rip et al. for empirical, compelling evidence making the argument for using 850 cases where feasible. If I wanted to leave the 850 bottom end undisturbed I could probably use my almost clapped-out spare 750 barrel with .060" or .080" oversize forged pistons to compensate somewhat for the heavier 850 crank in terms of the balance factor. Potential combinations that would make sense for me:

1. 750 crank and Combat cam in original 750 cases wth Combat head (on bike at present)

2. 850 crank and standard cam in 850 cases with RH10 850 head (current spare engine)

3. 850 crank and Combat cam in 850 cases with skimmed-down RH4 850 head (and cam tunnel opened up)

4. 850 or 750 crank and Combat cam in 850 cases with bored-out 750 barrel, heavier pistons and pins and Combat or spare 750 head

All I need is a second Combat cam and oversize pistons to cobble together any of the above configurations over a long weekend. If someone has experience running 750 barrels on 850 cases please let me know. Given that the Combat bottom end has been practically bulletproof since I put in Superblends in '72, the current inventory may just suffice for the rest of my miserable little life. Thanks again to everyone who weighed in on this.


Tim Kraakevik
kraakevik@voyager.net
'72 Combat

kraakevik@voyagernet said:

Potential combinations that would make sense for me:

1. 750 crank and Combat cam in original 750 cases wth Combat head (on bike at present)

2. 850 crank and standard cam in 850 cases with RH10 850 head (current spare engine)

3. 850 crank and Combat cam in 850 cases with skimmed-down RH4 850 head (and cam tunnel opened up)

4. 850 or 750 crank and Combat cam in 850 cases with bored-out 750 barrel, heavier pistons and pins and Combat or spare 750 head

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.......5. Buy a second frame and tranny etc. and have 2 bikes, 1-750 and 1-850

850's are nice for hi-way touring but the lighter crank in the 750 spins up faster and is a lot of fun to ride. I'm thinking along the lines of using my 750 crank in 850 cases with 850 or even 920 barrels and RH4 head mainly because my 750 has the old slotted pistons which I've been hearing all the horror stories about (not to mention gruesome pictures too). Using modern light weight pistons and rods like what Jim Schmidt is selling can keep you on track with your balance factor with the bigger top end. I just have to wait for that lotto ticket 1st.

Hi Tim, about fitting 750 barrels onto 850 cases , no problem , just a question of adding 4 studs where the throughbolts go...
I am using 850 cases where ever I can on my 750, cause they are , as shown previously more strong.........
then after if your 750 is nice , keep it like this .....and yes try to find another frame to put the 850 engine in, that will make two different bikes.....all depends how rich you are!

If someone has experience running 750 barrels on 850 cases please let me know.

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Definitely recommended for peace of mind. I have a set of non matching 73 pre mk3 850 cases (drive side is Gus Kuhn stamped!) with old 750 crank, 850 style 750 bored barrels and 72 Combat head. Works a treat, only downside is I keep getting told I have an 850 so have to explain the barrels etc. Some people are very insistent I don't know what my engine is!! Good luck.

RennieK wrote;

It's interesting that Reggie has 750 cases opened up for 920 cylinders and that they are holding together.

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This bike is now owned by my friend, and he runs two 920s and he has often ridden it very hard, keeping up with German and Jap bikes on biking holidays i.e. significant distances maintaining average speeds (where possible) of 70-90 mph (don't know how he does it without a fairing) and it has held together. The engine hasn't been tuned, and runs a 4s cam, therefore is probably not making any more BHP as the breathing is limited with a standard head, just more torque than standard. I think if it was making lots more power and being run at very high revs constantly as a racer would, who knows?

I wouldn't be averse to putting 850 barrels on 750 cases if I wasn't going to up the power except for what the extra torque the capacity gave me, but obviously the preference would be to use 850 cases or better. The crankcase mouths on this engine were machine out by Pete Lovell who I suspect bored them to the smallest clearance necessary, but as I said the cases are very thin at 7 & 9 o'clock.