750 Head Options

Folks

I have .060"-over pistons and a barrel ready for the next top-end overhaul of my '72 Combat--but a standard 750 head. I'm thinking of taking off .040" to raise the compression ratio--and using 32 X 30mm intake manifolds to use the 932's on the standard head. The cam would still be a Combat. I have 850-style balanced head-pipes and Dunstall mufflers.

Has anyone tried this? The RH10 head had 30mm intake ports to improve torque--but I don't know whether the same would hold true with a standard skimmed 750 head.

The following provides some of my thinking on various engine configurations.


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850


Here's an entry of mine from a while back on crankcase/barrel/piston/head configurations using components available to me at the time. I've since bought 750 and 850 bikes and a nearly complete 750 engine--my engine work since this original message has confirmed the relative merits outlined below:


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

I would be a bit wary of + .060" in a tuned 750 engine without through-bolts. On some cylinders, you're almost hitting daylight at the bottom.

No problem with putting 750 cylinders on 850 cases. It's a straight bolt-on. You just replace the through bolts with stock 750 studs. I've done it several times.

I wouldn't recommend trying .060" overbore on 750 cylinders with a Combat spec engine. Actually, I wouldn't recommend it for anything ridden hard. The stock 750 cylinders are well known for cracking at the front corners of the base when bored larger than .040". On the race bikes we never went larger than .020" over.

I don't think you'll notice a problem from the higher balance factor to get with the 850 crank and 750 pistons. The isolastics tolerate higher balance factors quite well.

Doesn't make much difference whether you use a 30 mm port head or a 32 mm port head. The 30 mm port head will flow as much air as the 32 mm, unless you go to larger valves and major porting mods.

Ken

In Capt. Norton Notes there is a very long discussion on cases and cranks for dragsters, best for the hi rpm was thinner 750 cases but beefed up in places and lightened steel flywheel with super blends. Reasoning and findings were the cases and superblends tolerated more crank flex than the more rigid 850 cases. Much as its rare to here of 8000 rpm Norton nowadays its was all too common in their hey day to make their mark in racing. It does mean less life of course sorta like Achilles, what ya want out your time here, short and glorious or long and dull?

The most potent combo I've experimented with was Combat with a smaller port standard head, flame ring gaskets, no base plate, bigger 34 mm carb. Might be more potent yet with the standard head milled down some to up CR. A stuck throttle ruined this combo in Ms Peel, but was so good I want to try it again in a factory Combat. You do have to fiddle with exhaust system to get the most out of the above, like in 2-1 extractor 1.5" tubes that impressed me.

Its risky to rev old cast iron flywheels into range the 2S cam really wakes up in.
Between 6500-7500 a Combat can lift is front straight ahead or leaned on good traction. Valve float then becomes an issue near factory 7000 redline.

Many thanks to Steve, Kenny and 79X100 for your comments. The barrel is bored and the pistons have been bought, so I'll just have to take my chances whenever the current head and barrel need rebuilding--I've got 53,000 miles on TC's specially ported head and the .020"-over pistons. GrandPaul's photos in the Sell section recently show plenty of metal with .060"-over pistons--but I've heard about broken barrels over the years although I've never had trouble myself.

One nice thing about the standard 750 is popping off the head and barrel together--having a spare barrel and head on hand will keep the downtime to three or four hours. Thanks again, gentlemen


Tim Kraakevik
kraakevik@voyager.net
Two Commandos

Alrighty, got to take some risks to have most fun while still around. You have good set up to tell us what combo's work best, over the years, so get going and start using some of them up like I want too again. I had one cast iron barrel cyrogenic tempered, just in case that gave a bit more toughness. I've also got to figure out a way to help hold head/barrel down by cables to cradle on my boosted special in the works.

I wish you luck with it. The odds aren't that bad. I've seen .060"overbores work in street bikes, but I've also had a 750 barrel with only .020" over crack at the flange on my Production Racer. You pays your money and you takes your chances.

Ken