Views on a big valve cylinder head.

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Reggie

Reggie

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I have a spare cylinder head for my 850 Commando, an RH4, bought partly with a view to doing a big valve conversion, and also because there were several issues with the other head now on my bike such as loose rocker spindles, and I wasn't sure how the repairs to this head would hold up.

Before I send the head off to Mr. Maney or Mick Hemmings, I just wondered if forum members who have had this done would like to comment on if the conversion has been reliable and if there was a noticable difference to the engine characteristics, which I would assume to be more mid and top end (high revs) power. I realise that ideally a big valve conversion usually goes hand in hand with other mods such as more aggressive cams and possibly larger bore carbs and inlet tracts etc.etc and so may be difficult to evaluate as it probably wont have been done in isolation. I would be interested in hearing how other forum members have developed their engines with a big valve heads, and how they feel about the results.

My main motivation for doing this is to gain some mid range and top end power, and hence top speed. Does this mean I am insane? :lol: :lol:
 
Reggie, I think you'll get more than enough with a PW3 or 2S cam. When I blew my 750 at the end of 02 it had a Combat cam which really came in at 4500 rpm and I really miss that surge in 1st and 2nd. My head (Combat) had a lot of work done on it, probably too much but has std valves so it's a bit top endy anyway. Of course if you match that with bigger valves it can only be better but if you want more top speed you'll need quite a bit more HP. If you think that a std 850 has the same cam as a 750 the breathing must be limiting it's potential somewhat .
 
Hi Reggie,
I think Mick H would be the best one to give yopu advice on that, as no doubt he's done a few that haven't had much else done to them. I've got 4 big valve heads, but then all my motors are full race, so I've never tried just doing the head on it's own. I can tell you though, that a badly done head will ruin the motor...I had one go from 70BHP to 44 BHP, just by an "Expert" modifying the head....so avoid CHS like the plague!!
 
I've never modified or had a head modified. If I were to, two people I'd contact: Leo Goff and Mike Libby. Leo of Memphis Motorwerks and Mike of C.R. Axtell tutelage. Leo has extensive drag race experience with a Norton. Mike has flat track and road race experience. I'm certain there are others as capable, it's just that I can vouch for both. I've had stock heads prepared by others which turned out to be a waste of money. Definitely stay away from generic local automotive machinists.
 
Hi John
Is that why my bike is so slow? Or is it just me? Looking forward to next season so much its amazing!
The next engine has a big valve head by Mick Hemmings. I have changed the cam to a PW3 on your advise but will be using the 4s on the big one.
I gave the 7s to my mate for his sprint bike, as you said it was a horrible cam! It seems it isnt any quicker iether! He is fractionally slower than last year with the PW3 & a lot slower than the year before with the 4s. However on strip down its worn quicker than any other cam he has ever had!! Quality control?
ps control yourself with Molly at the do this year.
all the best Chris
 
Often thought about a big valve head, but steered clear. A few things worried me about it, bronze guides, will they still be available in a few years & don't want to blow my motor up.
I know they are pathetic reasons. But with a gas flowed head, PW3, 10-1 pistons, everything lightened & polished etc., I am quite happy at the moment thankyou.
I like the 4S, PW3 comparison, as I think my motor was quicker with the 4S, or am I imagining it.
If I was racing, I wouldn't hesitate to put a big valve head on.
 
Hi Flow
I found the pw3 has loads of drive & maybe a fraction quicker but the 4 feels like your going & has a touch more at the top.
Trouble is its seat of the pants & the clock keeps telling me that my seat doesnt have a clue how fast I am riding!
Chris
 
Thanks for all of the replies.

I am running a 4S at the moment on an otherwise essentially pre MK3 850 standard set up i.e. early airbox with non balanced pipes and with peashooters and amal carbs. I'm not unhappy with the performance as such, but once it gets up to 85 mph the acceleration starts tailing off and it will only just top the ton, on the clock anyway.

I may be expecting too much from this spec. Maybe I should accept it for what it is....... a 34 year old bike that was essentially old technology when it was built, but I like to try different things, and this is one avenue that I haven't tried and am considering.

I know that if I go to Steve Maneys premises which are only about 20 miles from me, that the likelihood of me emptying my wallet are high, and possibly with his expertise with Nortons, his work will be a relatively safe bet to deliver any suggested benefits. But that's why I thought I would court opinion on here first, just to see if anybody was running one and any perceived benefits or pitfalls.
 
Reggie said:
Does this mean I am insane? :lol: :lol:
Click to expand...

No more insane than the rest of us

Reggie said:
I know that if I go to Steve Maneys premises which are only about 20 miles from me, that the likelihood of me emptying my wallet are high,
Click to expand...
Only 20 miles.. have you ever thought of a tunnel? :)
 
As Seeley has said, there's (in my opinion) more to be gained with propper flowing of carbs, manifolds and head than big valving.
 
Hi folks,
For a road bike I think a well flowed head with standard size valves is probably a better all round bike, coupled with a PW3. The PW3 doesn't feel like it has as much power as a 4S because it has more torque from lower down, so you don't get the sudden surge as it comes on cam like a 4S does, but on the track at least, it's definitely quicker. One of the best 750 heads I ever had was flowed by the late Fred Barlow, it had standard size valves and was as quick as any 920. Fred certainly knew how to flow a head, and I'm lucky enough that he also flowed the head on my big valve one thats on the short stroke motor...and that is a very quick commando!

I know a few people who have tried 7S cams....they never lasted more than a few meetings, and I know Gordon could never get their one timed properly, so they binned it!!


Chris, I don't think Molly's coming to the do this year,,,,,shame, I've almost finished corrupting her :-) :shock:
 
Reggie, I think your bike is under-performing for it's current configuration, I believe you should easily get over a ton on it. I would look at and double check timing and carburettor settings, what g/box sprocket are you using by the way?
 
An example of flowed manifold and head. This is a 750 ducati 2 valve, like the old Pantah with the carbs at the rear of the cylinders and the exhaust at the front.
This configuration dictates the manifolds need a curve in them.
The valves were standard 41mm inlet and 35mm exhaust, carbs are 40mm. The cam used was from a 900 injected engine.
The fellow who did the porting and flowing measures the flow before and after he plays with them so he can see the gains in cfm as he goes. He managed huge gains mostly by filling the dips in the manifolds and head inlet ports.

He also stated that he has seen lots of heads ported by experts that were worse than standard heads and were only fit for scrap.

rear inlet manifold with filling

Views on a big valve cylinder head.


rear head port filled

Views on a big valve cylinder head.


heads finished

Views on a big valve cylinder head.
 
Hi all

Mez porting.
What can you say. His son has left the website open for all to look at. Unfortunately Mez passed away & his sons never became involved in the business. (still involved in sprinting?)
Mez tuned everything , classic to BSB. Brilliant reputation & a fair price.

all the best Chris
 
Well I've gone and done it! Despite all the advice, I went to Steve Maneys shop.
I decided to take both heads to see which one may be the best to use for the big valve conversion. I had recently noticed on the cylinder head fitted to the bike a hair line crack that ran from the left hand L carb manifold screw hole that ran towards the head, and that after a few weeks running, had oil on it. I kept telling myself (deny it and it isn't happening) that it was a long way from the valves/guides and must be insignificant :!:
So after removing the head this morning, this is what I found.

Views on a big valve cylinder head.



Views on a big valve cylinder head.



Views on a big valve cylinder head.





I have shown this to Steve, and basically, the head is fairly porous and it will probably be very difficult to impossible to weld the crack :evil:

Therefore, I now have one scrap 850 head :( :(


The good news is that my spare head looks OK. I have talked to Steve who assures me that a stage 2 valve enlargement is the way to go on an 850. Apparently on the torque /bhp delivery, midrange remains the same, but at higher revs the bhp increases :D We shall see. So I now await for the return of my head, and I will be trying it out next year.
 
Yet agin, my computer is not showing the photos. Can they be seen by other forum members?


3 HOURS LATER


Got the pictures showing 3 hours later! Phew
 
Hi Reggie, I'm in work so won't be able to see any pics til I get home (bloody firewalls!!)

Funnily enough, I'm going to be in Steves tomorrow!!!

Cheers

John
 
Considering that it's on the intake side and not likely to pass air, is that really a problem? The danger of enlarging anything on the head or barrel is that your run the risk of there not being enough material in a given area. Obviously using something like FullAuto's new heads will most likely not have these issues.
 
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