Big valves in a Fullauto head

swooshdave said:

You'd probably need a flat where the valve contacts but would it work otherwise?

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One thing to keep in mind here is heat transfer vs. sealing force, that is two very important drivers for the actual valve seat width. You want it as small as possible and as big as necessary. Radiused seats are relatively bad in terms of heat transfer.


Tim

Tintin said:

One thing to keep in mind here is heat transfer vs. sealing force, that is two very important drivers for the actual valve seat width. You want it as small as possible and as big as necessary. Radiused seats are relatively bad in terms of heat transfer.


Tim

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Seat width is mainly important on the exhaust valve. The intake valve leads a pretty easy life so I keep them very narrow.

The exhaust seat seems to work best on a Norton at around .060 to .080 wide. When they get wider than that they tend to collect carbon on the sealing surface and the heat transfer goes away and soon you end up with a burned valve. Jim

Tintin said:

swooshdave said:

You'd probably need a flat where the valve contacts but would it work otherwise?

Click to expand...

One thing to keep in mind here is heat transfer vs. sealing force, that is two very important drivers for the actual valve seat width. You want it as small as possible and as big as necessary. Radiused seats are relatively bad in terms of heat transfer.


Tim

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Which is why I said to keep it flat where the valve hits the seat.

Jim, are the re-angled intake valves at 26 degrees? And hows the rocker geometry in relation to the re-angle and do you have to modify the rockers and or pushrods?

Mick

ML said:

Jim, are the re-angled intake valves at 26 degrees? And hows the rocker geometry in relation to the re-angle and do you have to modify the rockers and or pushrods?

Mick

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The jig I use when I cut the seats and guides rotates with the rocker arm spindle as center. So the head is centered on the stock guide and seat first to bore the stock seat and guide out. Then the head is rotated 1.5 degrees on the spindle axis. That way the rocker arm to valve stem contact point stays the same and a slightly shorter pushrod is needed.

I don't remember the exact angle off the top of my head. The jig I built years ago uses holes with pins that are drilled at the right angles for stock intake and exhaust. Another set of holes is for re-angled intake and exhaust at 1.5 degrees from standard and 2.5 degrees from standard [which I have only used a few times for really big valves]. Jim

pierodn said:

For a layman as myself this is a wonderful scienze fiction topic! And Jim is a UFO.

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Nice.

So how many of these did you do up Jim? ...or do we have to do our own ; )


Nice work Jim! It's amazing how much goes into these modifications. Thanks for another interesting and informative post!

RennieK said:

So how many of these did you do up Jim? ...or do we have to do our own ; )


Nice work Jim! It's amazing how much goes into these modifications. Thanks for another interesting and informative post!

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Thanks,
So far I have only done 4 big valve Fullauto heads. I will be glad to do one for you if you want one.

I would hate to guess how many big valve conversions I have done on OEM heads over the last 20 years. Jim

I consider this type of work to be a bit of a bargain, expensive though it may be. The customer pays for the labour and gets decades of R&D thrown in.

Glen

+1 on the decades of R&D in the value. Another reason I love this forum....I had no idea that all this stuff for Nortons was out there until I landed here on access, I was just trying to source parts for my '69 basket case and presto now I know......Now all I need is some extra cash Cj

One point I missed Jim.....

You say the head was bored for 850 valve guides, but I didn't understand what guides were in there as it came from Fullauto?

Ok, so did the head start life as an 850 head?

Does Fullauto use the smaller valve guide size in the 850 heads?

What would you use as the base of a big valve/re-angled valve head for an 850 or 750 short stroke build?

about 850 guide : I had a discussion about my 750 head with a loose guide , more than an oversize repair guide can cure. I proposed to fit 850 guide as they are beefier and tend to wear less .I was told it was not a good idea because there will be less metal left in the head . The option was to have an adhoc siize guide made .
What do you think ? knowing that a Fullauto head will be the definitive anwser

Nice work Jim. Exelent!!

So far I have only done 4 big valve Fullauto heads. [/quote]

I hope it's soon will be 5 big valve Fullauto head . One for my 750 short stroke.

SteveA said:

One point I missed Jim.....

You say the head was bored for 850 valve guides, but I didn't understand what guides were in there as it came from Fullauto?

Ok, so did the head start life as an 850 head?

Does Fullauto use the smaller valve guide size in the 850 heads?

What would you use as the base of a big valve/re-angled valve head for an 850 or 750 short stroke build?

Click to expand...

All of the Fullauto heads use the 750 style guide at .501 OD. That makes it easy to re-angle the hole and fit an 850 guide. I re-cut the taper on the end of the 850 guide so it is smaller and does not block the port so much.

The preferred head for any big valve conversion is going to be the Fullauto head as there is extra metal around the ports to work with.
Next in line is going to be the small port 750 or 850 head. A small port 850 head [rh10] is going to be more work as a sleeve must be installed in the guide hole. They will not flow as well as the Fullauto head without some filler in the port. You are also limited in how much you can widen the bowl and port area without breaking through. Jim

JRD said:

about 850 guide : I had a discussion about my 750 head with a loose guide , more than an oversize repair guide can cure. I proposed to fit 850 guide as they are beefier and tend to wear less .I was told it was not a good idea because there will be less metal left in the head . The option was to have an adhoc siize guide made .
What do you think ? knowing that a Fullauto head will be the definitive anwser

Click to expand...

Repair guides are available up to .015 oversize from Kibblewhite. If I need to go larger than that I cut down an 850 guide and make a custom size.
As far as I am concerned Norton should not have gone to the larger 850 style guide on the later heads. It was done because cast iron 750 guides were fragile and often broke off and went through the engine.
They should have gone to a bronze alloy guide-they don't break- but they were a lot more expensive and required a plated valve stem for long life.
A large 850 style guide is a problem in any of the 32mm port heads. It leaves a thin section that is bound to crack like a lot of RH4 heads did. Jim

Kvinnhering said:

Nice work Jim. Exelent!!

So far I have only done 4 big valve Fullauto heads.

Click to expand...

I hope it's soon will be 5 big valve Fullauto head . One for my 750 short stroke.[/quote]

Hi Kvinnherring, how is that project going?

Steve

comnoz said:

SteveA said:

One point I missed Jim.....

You say the head was bored for 850 valve guides, but I didn't understand what guides were in there as it came from Fullauto?

Ok, so did the head start life as an 850 head?

Does Fullauto use the smaller valve guide size in the 850 heads?

What would you use as the base of a big valve/re-angled valve head for an 850 or 750 short stroke build?

Click to expand...

All of the Fullauto heads use the 750 style guide at .501 OD. That makes it easy to re-angle the hole and fit an 850 guide. I re-cut the taper on the end of the 850 guide so it is smaller and does not block the port so much.

The preferred head for any big valve conversion is going to be the Fullauto head as there is extra metal around the ports to work with.
Next in line is going to be the small port 750 or 850 head. A small port 850 head [rh10] is going to be more work as a sleeve must be installed in the guide hole. They will not flow as well as the Fullauto head without some filler in the port. You are also limited in how much you can widen the bowl and port area without breaking through. Jim

Click to expand...

Thanks Jim, very useful info.

Steve

Thanks a lot, JIm.

This is very interesting. Doing heads you're always chasing something. I've been putting different bigger valves in A65 heads. And have gone back to the 44.5mm valve for the one I'm doing. Not so good on small bores but good on the 80mm. Thankfully it doesn't need re-angling. I had it with a 38mm manifold which is ok but with way too much work lifting the guide area and going through between the fins to do it properly like the first one and it's nearly 900 and not 744. So try as I might I could not get the same flow from it. So I sort of shelved it, not enough flow to justify the port size. Then I did the little 34mm 42mm valve version which works so nice on a stock bike or std bore long stroke 750.

On about the 6th one of those heads I found I can get flow up to a point but it's sort of stuck at that level and they are all about the same, with maybe 5cfm variation between the good ones. Measuring through the 34mm carb.

So I shoved a 34 manifold and carb on the old one with bigger port with 44.5valve. And low and behold it's 20 cfm or so better than the great little one. Hardly much less than through a 38mm carb, and looks like a very fun thing because of how much smaller the port can be. It's about 70% better flow than a stock head and should end up about 15% by volume bigger than stock. It's just a matter of filling it where it doesn't need empty space and trying to keep the flow or improve it a bit. That chasing thing that has you straining over 5cfm, then I get 20 so easily. And now it's reverse engineering.

So many of the BSA guys do not get it. That hp is dependent on flow and that high speed in the port gives so much return by way of response. Trying to book in for the dyno today and he didn't even understand, thinking if you reduce flow by 5cfm you will lose top end, but I think not when it's got plenty and pushed in so much harder by its speed, hopefully all the time.



I also want to reduce the exhaust to 35.8mm or 37mm because these exhausts are a bit big from what I have read.

With anything such as this, you never know what the benefits are until you get the modified part onto the bike. If you succeed in getting more torque, you often do not know it until you change the gearing. If the power band moves up, you can reach the rev limit before you experience the benefits. Dynos and flow benches probably only give an indication of what might happen in service. The rest is supposition until you try the changes in the situations which represent normal use. When you do that, the motor needs to be set up with everything optimised. It is not as simple as an additive process. You can buy all the good parts and think you will build the perfect motor, but it is usually the way in which the parts are assembled which is more important. If you change the flow in the combustion chamber, you might do better if you also incorporate a different cam or exhaust system. One of the main strengths of the Commando engine is it's smooth strong power delivery. In road racing and on the dirt, a bike which is peaky is usually slower.