750 on 850 Heads

Can any one here say they have seen or done the following placed a 750 head on a 850 barrels , I am looking for a head for my 850 Mk3 need a RH4 which is correct ( RH9 at 10.5:1 not for road bike ) RH10 only 30 mm intake ports can be made to work . But can a 750 head like a RH5 from a 750 with some slight mods to the head bolt holes work successfully re being able to seal up ok , like some feed back before hunting for one thanking all Dave .

I'm not sure ,however I blew up an 810 and replaced it with 750 barrels ,an 850 head and the original valvetrain.
It worked OK but did not seem to make quite as much power.

Pm sent

Hi , yes it's possible .....Steve Maney is doing such modifications, and Ken Canaga had done it ,he showed that somewhere in the forum, and following his job a friend of mine with tooling facilities had modified a RH5 head to fit a 850 bolt pattern ( as some Maney 750 alloy jugs got the 850 pattern, id more spaced ........).You just need a milling machine!!!!

Left Coast Racing coverted Peel's 750 CHO head to fit 920 Maney, by opening the head piston bores to 920 size, leaving signicificantly larger squish bands and moving bolt holes apart by putting in Al plugs that are re-drilled. 10.5 CR should work a treat on a street bike if paired with a 2S or other slightly-more agressive low rpm CR bleeding over lapped cam and decent octane or skilled throttle hand and shifting not to lug into detnotion, so might not be able to nail WOT in 2nd going 15 mph w/o a ping or two before getting into the non detonationing rushing range. I did a survey here and over lists to find Commando's are very detonation resistant and only got a few reports from racers over doing it or loaded down touring in dry air Mt area with crappy gas so they had to back off hi throttle till cooling down and better gasoline. By Far the only routinely expected detonation trouble occurred by those de-tuning the wonderful Combat by putting a standard cam in but not lowering the CR. Might be able to google 750 head on 850 or 920 Commando to find Ken's photo's of how he does it. Push rod length will be the main tough detail to diddle for good valve/rocker geometry not the basic mods to fit what ever head to what ever engine size.

I'd be interested to know what barrels fit the commando engine. Is the cylinder base gasket common to the 650ss, Atlas, 750 commando, 850 commando ? My experience has always been more with 650 & 750 cc Triumphs where everything fits.
Which heads fit the 850 commando without machining ?

Which models was the RH9 10.5 to one head used on and what size are the inlet ports ?

As has been said, it can be done. Pete Lovell who is based in UK altered one for me in the 1980s.

acotrel said:

Which models was the RH9 10.5 to one head used on and what size are the inlet ports ?

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This link was posted by L.A.B I think some time back......

http://www.nortonownersclub.org/support ... tification

Also more in here than I even want to know about Norton heave weight twin heads

http://atlanticgreen.com/nhth.htm

Here's Peel's 750 CHO head, holes plugged and drilled wilder and rim bored wider so the 10.5 piston can slip into. Wider pistons than 750 in 750 case need the cases opened up.

acotrel said:

I'd be interested to know what barrels fit the commando engine. Is the cylinder base gasket common to the 650ss, Atlas, 750 commando, 850 commando ? My experience has always been more with 650 & 750 cc Triumphs where everything fits.
Which heads fit the 850 commando without machining ?

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The only heads that fits the 850 without machining are the 850 heads.

750 and 850 cylinders will fit either 750 or 850 cases. You do need to screw in the 750 base studs on the sides to fit 750 cylinders, and remove them to fit the 850 cylinders. It's also a really good idea when fitting 850 cylinders to 750 cases to helicoil or use inserts in the holes on the sides where the through bolts screw in.

I don't know what it takes to fit 650 or Atlas cylinders to Commando cases.

Ken

. . . not to mention that the 750 cases will need machining to accept the bigger 850 barrels where the "liners" protrude below the crankcase openings.

Ken is absolutely right about helicoiling the crankcase bolt holes where the four 850 through-bolts clamp the barrel to the cases--850 or 750. Mechanics of all marques have been known to throw wrenches around when head bolts strip out crankcase threads




Tim Kraakevik
kraakevik@voyager.net

kraakevik said:

. . . not to mention that the 750 cases will need machining to accept the bigger 850 barrels where the "liners" protrude below the crankcase openings.

Ken is absolutely right about helicoiling the crankcase bolt holes where the four 850 through-bolts clamp the barrel to the cases--850 or 750. Mechanics of all marques have been known to throw wrenches around when head bolts strip out crankcase threads

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You're probably right about that. The last time I put 850 cylinders on 750 cases the cylinders were sleeved out to 920, and I had to bore the cases out to clear the larger sleeves anyhow. I've done a couple other 850 cylinders on 750 cases, but that was some years ago, and my memory of the details is pretty poor now.

Ken

acotrel said:

Which models was the RH9 10.5 to one head used on and what size are the inlet ports ?

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The RH9 Norton part number 065213 10.5:1 32 MM PORTS remarks 1974 high performance never seen one so can only assume it was a special for racers . :?

As an aside, given enough notice, your Fullauto Technologies 750 cylinder head can be ordered with 850 head bolt spacing or just about any variation in machining, or, indeed as a raw casting. I have given Jim Comstock an 850 head with very small ports so that he can perform his magic act on it, just to see what he can do. I have another one the same should anyone want to indulge his fantasies with a porting tool.

Thanks for the info and the link - much appreciated. The head on my 850 has 32mm ports, which I've tapered over about 25mm from 34mm at the manifolds , however I never knew what the numbers meant. I presumed the comp. ratio was about 9 to 1, and I'm using methanol fuel. 10.25 to 1 would be a bit better, but the alcohol works even at low comps, despite what some might believe. I've got two motors, I'll have a look at the numbers on each of them. Does the Norton factory get the higher comp. by machining the face of the head, or by machining a raw casting to have a smaller hemisphere ? I believe Jim Schmidt's pistons and long rods give a higher comp. so the pistons must not come right up to the top of the bores in a standard motor ? One thing I really like about Commando is that there is no lump of piston poking up into the combustion chamber. I don't know why you guys fully hemishere the heads and remove the squish band. I would have thought it would be one of the motor's design strengths ? My 850 motor has relatively little done to it , yet it is much faster than my mate's very fast 650 Triumph that I rode years ago. I know it is quick enough to win in our period 4 historic racing against 1100cc CB750s.

Here's an entry of mine from awhile 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, '74 RH10 850

I only really know factory Combat and they all stick their pistons up .050" above the cylinder surface, ie: up through the gasket and into the head somewhat. Combats got most their 10 CR by milling .040" off head and a bit more can be gotten by removing the base gasket/plate and use of Flamering instead of copper standard gasket.

All Norton Cyclinders tops are cut on a .003" slant, to the rear IIRC. Indexing that true bumps CR up a tiny bit too. I had this done on Ms Peel when discovered then later found reference its innate factory issue. Ken Augistine of World's Straightest Commando is one of them.

There have been lots of hemi hi powered race engines w/o any squish bands left so don't seem to be that big a denotation limiting power factor, likely because the dome space left at TDC is still swirling enough compared to the flater 4 valve chamber and they use hi enough octane. Ken Canaga did it the best way I know by trimming piston to almost same radius as the head leaving wide angled squish bands. Ms Peel head above gives like 3/8" wide bands in sections that pistons should come close to .040" of for extra detonation tolerance into the 17:1 zone -116 octane with inner inner cooling. Peel has enough squish area to likely get some flame jets in-out with a Singh Groove or two.

There's two schools of thought on the size to TDC volume, most aim for small as possible, which in Peels 750 CHO head on 920 jugs gives ~43 ml, though I've read of some Norton twins a few less ml's than that, w/o bath tub non hemi mod of course.

JMS piston kits gets various CR by the pin placement and tallness of crown.
Peel has his 920 10.5 set giving ~7.5 effective with late closing cam.

I am paying attention to your methanol habit Alan both for its power potential but more so for its refrigeration while making full power, then might be able to inject NOS as the air resistance builds near top rpm and over boosting impeller.

acotrel said:

I don't know why you guys fully hemishere the heads and remove the squish band. I would have thought it would be one of the motor's design strengths ?

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Actually, I don't think many of "us guys" convert standard heads to full hemi. The RH2 heads that Norton homologated as a Combat 750 for AMA racing, the RH7 heads for the customer short stroke 750 engines, and the RH8 heads from the factory short stroke 750 race kit all came from the factory with a full hemi chamber with no squish bands (as well as re-angled inlet guides for larger valves). Those are the only full hemi Commando heads I have seen. I'm sure someone, somewhere has converted a standard squish head to full hemi, but I haven't heard of anyone doing so. It certainly isn't a common mod. Quite a few people have offered big valve mods with re-angled guides over the years, but most of them keep the conventional squish combustion chamber design. At least two tuners have also offered a conversion to a bathtub shaped combustion chamber, but it still had squish areas.

Ken

i rebuilt an 850mk2 motor with a de tuned 750 combat top end ie (1mm plate under the barrel) and this engine runs superbly