750 on 850 Heads

Some interesting design and performance pros and cons between a full hemisphere and a bathtub chamber. I don't know what is best.

Norman White is a proponent of the full hemisphere and a 12.6:1 compression ratio for race. Herb Becker has used the bathtub with excellent results.

What octane are the full hemi and bath tub run on.
What full ignition advance can each tolerate?

From what I've gleaned valve flow shrouding at hi rpm is a down side of bath tubs but then so can too domed a hemi. Then there's the thermo dynamics of surface area vs volume on heat flows and reflections.

Most powerful combustion engines are hemi's


Then again some the most powerful motorcycle are rather flater


Reminds me of an argument I had with my late brother Dale, which would ya least like to get hit with, a .357 or a .45, He won with " Shit Just Surprise Me! "

http://www.secondchancegarage.com/public/585.cfm

Sorry hobot, not so simple that you can reduce it to a few sound bites. It is more complicated than that and getting a bit off topic. If interested in discussing, start a new thread or PM me and I'll do a brain dump.

Hi,I have been looking for an 850 Head for a while. I think I will have to make one out of a 750 head. The early spigot barrel Atlas head has the same Squish area as the 850, so, move the hole centres on the mill, then make special bolts to take oil through the Head Steady bolts. Job done. It must have been done before. The 850 pistons are VW Beetle 1300. There are 18 types in the book. Hope that helps, by the way, there is a slightly dodgy 850 head on eBay at the moment. regards, Paul

Are VW 1300 pistons made by Mahle, or d o I have to look in a Porsche for those ?

Are VW 1300 pistons made by Mahle, or d o I have to look in a Porsche for those ?

acotrel said:

Are VW 1300 pistons made by Mahle, or d o I have to look in a Porsche for those ?

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Mahle would not stay in business if they only supplied pistons to Porsche......

My '64 Volvo had Mahle as standard....and replacement....they were the easiets ones to get for it too....you will find them in quite a few European cars...

Hi, Mahle make pistons for VW, and Harleys. You can use Ford Cortina pistons in an 850 over-bored to 92m. Its surprising what you can do if genuine parts appear to be unavailable. I once found that Bedford lorry pistons fitted a Panther 650.

Back when I was racing in the 60s, if you persisted with a fourstroke there were two t hings you needed - the big front brake, and forged light pistons. Jim Schmidt's pistons and long rods look excellent. A while back, I looked at 73mm Honda Fireblade pistons, but couldn't figure out a cost effective way to compensate for the height difference. I suggest it is extremely important to get the weight on the little ends down, it makes a big difference to performance and reliability. In the old days most pistons were simply cast and machined and many were not even solution heat treated, so a lot of them grew bigger until after they had been used for a long time. The Mahle pistons from Jawa speedway engines were really great in a single cylinder British motor. I don't really know what Norton pistons are like, mine are old but have no cracks and are fairly loose in the bores, so I keep using them. Even though I use methanol fuel, I've never tried to raise the comp. ratio as I don't believe in machining the face of motorcycle cylinder heads. The piston weight is a worry, on every rev the inertia and deceleration and reversal means it has a big effect , and I can imagine the rods stretching and fatigueing.

acotrel said:

Back when I was racing in the 60s, if you persisted with a fourstroke there were two t hings you needed - the big front brake, and forged light pistons. Jim Schmidt's pistons and long rods look excellent. A while back, I looked at 73mm Honda Fireblade pistons, but couldn't figure out a cost effective way to compensate for the height difference. I suggest it is extremely important to get the weight on the little ends down, it makes a big difference to performance and reliability. In the old days most pistons were simply cast and machined and many were not even solution heat treated, so a lot of them grew bigger until after they had been used for a long time. The Mahle pistons from Jawa speedway engines were really great in a single cylinder British motor. I don't really know what Norton pistons are like, mine are old but have no cracks and are fairly loose in the bores, so I keep using them. Even though I use methanol fuel, I've never tried to raise the comp. ratio as I don't believe in machining the face of motorcycle cylinder heads. The piston weight is a worry, on every rev the inertia and deceleration and reversal means it has a big effect , and I can imagine the rods stretching and fatigueing.

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The racers/mechanics paranoia, the number of times I hear a noise and then worry what the ell it is! :shock:
I can well imagine race leading riders worrying about noises, not that I can ever claim to have won a race with enough of a lead to have time to worry.
JohnT

Mick Hemmings has done fully hemisphered re angled big valve heads since the '70s, though I reckon he is not doing many today. I bought an 850 head as a donor...and then ordered a Fullauto from Mick...you know it makes sense....

I had an ex works short stroke head from '75 to '80. It was fully hemisphered and had the valveguide angles machined for large valves from the blank. Pistons were from teh works, Omega domed into the hemisphere, giving around 10.25:1 (still got one somehwere). The works Norton short stroke 750 and 850s were done like this. Modern preference is to retain the squish, but that was not the '70s wisdom for full house motors.....and my bike would outdrag most big twins including an 840cc 180 degree crank Weslake, so it was one of the quicker Nortons around at the time.

Any way, after a couple of seasons we dropped a valve big time....flat in top.....

To recover the head Mick had it welded and reshaped as it originally was, skimmed 40 thou off the face and 60 thou off the barrels and I put in standard flat top Hepolites with valve cut outs and a small 45 degree chamfer cut at the piston edge...

Overall power out put was about the same!

Moral of the story is there is more than one way to skin a cat....

has anybody got any photo's or more info on how to use a 750 head on an 850 barrel ? i am thinking about welding some aluminium dowel into the cylinder head holes and machining flat then re drilling at the 850 centres is this the way it is done? cheers baz

No need to weld as the experts make a thermal interference fit plug then drill to suit.

hobot said:

No need to weld as the experts make a thermal interference fit plug then drill to suit.

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thats interesting,do they reem the existing holes or use them as they are?

They make reasonable larger new holes to get enough meat in the plug size and remaining head metal surrounds then the plugs OD to almost fit with everything @ ambient temp. Plugs and plug holes end up offset from the original head holes of course. Dang a few years ago I would of not known nothing about this so my sense of self worth gear head wise has swollen some. My Peel CHO 750 head now fits on 920 Maney cylinder done as above.