advice please!

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sharon piquet

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Good Morning everyone....after a computer crash....finally back to business....hubby has yet another question for all you experts....

Will a RH10 head from a 1974 850 work on a 1973 850 that has a RH8 head.

as you can probably guess I have no idea what this means...but I will forward any and all info to Jim who will know.

thanks and have an amazing day!!!
 
Yes, in fact the RH10 is perhaps the most desieable head for an 850. Some of the other castings had problems with cracking or porosity (weeping oil).
 
RH10 is the best head out there, when you can find them. Next is the RH4.
 
sharon piquet said:
Good Morning everyone....after a computer crash....finally back to business....hubby has yet another question for all you experts....

Will a RH10 head from a 1974 850 work on a 1973 850 that has a RH8 head.

as you can probably guess I have no idea what this means...but I will forward any and all info to Jim who will know.

thanks and have an amazing day!!!
Click to expand...

If your hubby really has an RH8 head, tell him to guard it closely. RH8 is 1973 750 short stroke race kit.
 
illf8ed said:
sharon piquet said:
Good Morning everyone....after a computer crash....finally back to business....hubby has yet another question for all you experts....

Will a RH10 head from a 1974 850 work on a 1973 850 that has a RH8 head.

as you can probably guess I have no idea what this means...but I will forward any and all info to Jim who will know.

thanks and have an amazing day!!!
Click to expand...

If your hubby really has an RH8 head, tell him to guard it closely. RH8 is 1973 750 short stroke race kit.
Click to expand...

Then what the heck is it doing on an 850?!!!
 
Then what the heck is it doing on an 850?!!!

750 short stoke uses the 850 bore and probably the barrels shortened so has the through bolts.
 
Yes of course, the short stroke would need a bigger bore. I guess I should have figured that out, but then what is the difference in the head?
 
Bigger inlet valves (approx. 41 mm vs 38 mm std) with thin stems (approx 7 mm vs. 8 mm std). An RH8 head will fit any 850 but the pistons will probably need cutouts for the valves.
 
kommando said:
750 short stoke uses the 850 bore and probably the barrels shortened so has the through bolts.
Click to expand...


I think the short stroke 750 barrels were the same height as standard 850 barrels, as I believe it was the short stroke pistons and conrods which compensated for the difference in stroke.
 
Bigger inlet valves (approx. 41 mm vs 38 mm std) with thin stems (approx 7 mm vs. 8 mm std). An RH8 head will fit any 850 but the pistons will probably need cutouts for the valves

Gday John Tickle, thanks for the above info, wondering if the RH8 inlet port was increases aswell to compliment the 41 mm valves? Ive purchased the RGM 42 mm inlets,then turned down to 41 to avoid the tangles and ported the inlet out to 33.5 mm on my RH10 head. Like mine, Im also assuming the RH8 had No exspensive reangling valve angle job done? Any info is much appreciated!

Foxy
 
RH8 head had different inlet valve angle (26 1/2 degrees vs 28 degrees std). Fitting a bigger inlet valve without re-angling will cause valve clash with most camshafts. It can be avoided by cutting the exhaust seats very deep and shortening the exhaust valve stems accordingly.
 
thanks for all the info everyone....truly appreciated!!! but....I posted incorrectly....so sorry read his note wrong and he left to work a double so wasn't around to correct matters....should have said... will an RH10 head work on a 1973 850 that has a RH4 head. like I posted this may as well be Greek to me....just have to be more careful of my typos!!
 
sharon piquet said:
....I posted incorrectly...

..should have said... will an RH10 head work on a 1973 850 that has a RH4 head.
Click to expand...


No problem, and the same info applies to the RH4-RH10 head swap.
 
Typo? Oh, well...one of the joys here is the stuff you learn by accident.
 
C'mon guys, we all know that a Fullauto Technologies head is at the top of the tree. By far.

I know this guy.................
 
the same info applies to the RH4-RH10 head swap.
Click to expand...

Although...RH10 heads have 30mm inlet ports-instead of 32mm (but carbs used were still 32mm).
 
It is my understanding that the RH10 heads fitted 32 mm manifolds but tapered to 30 mm. Something to do with velocity. Now why didn't they address that on the exhaust side?
 
Good Morning everyone....after a computer crash....finally back to business....hubby has yet another question for all you experts....
Click to expand...

Fascinating review of various heads but sure reads fishy as a spoofing set up to me.

IIRC port narrows-venturi should be ~80% of valve size to keep flow speed up.
Also must make sure this '73 pistons do not stick up .05" proud of barrel to extend into the head chamber some. Standard 750 heads need rim milled out for 850 size jugs. Head bolts end up with less meat around them.

Everyone should know the 8 valve head is best but alas never in production so far.
 
["RH8 head had different inlet valve angle (26 1/2 degrees vs 28 degrees std). Fitting a bigger inlet valve without re-angling will cause valve clash with most camshafts. It can be avoided by cutting the exhaust seats very deep and shortening the exhaust valve stems accordingly."] Quote

Thanks for that John and appologies to Sharon for high jacking this thread!
On the subject of sinking the exhaust deeper to avoid the tangles, whats the ratio of depth to shortening valve stem?
Eg; 20 thou deeper exhaust, therefore 20 thou shorter valve stem?
A DR-Hillier pm'd me some time back saying he has fitted 42 mm valves to his commandos with standard cams and valve angles, and claimed he hasnt had any problems with the dreaded valve clash?? Thoughts :?:
Foxy
 
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