6mm valves

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Hutchison hemi

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Does anyone use 6mm or smaller stem valves and if so do you know of a source for valves and guides without having custom ones made? Is there a reason not to use them? I would think the lighter weight and less flow restriction would be advantageouseven for the street. It is not that expensive to get custom parts as they are only two cylinder four valve engines. Thanks
 
I have not used them, but Kenny Dreer used to sell a 7 mm conversion that he had made by KPMI. I talked to Kenny a few days ago, and he said he was planning to use 6 mm valves on a Commando project bike his is building. He mentioned having them made by KPMI. He plans to use larger diameter hardened wear caps on the stems.

The only reason I can think of for not using them is the increased rate of stem and guide wear compared to the stock size. That would probably not be an issue on a race bike, but might be a concern for a high mileage rider.

FWIW, I have had KPMI make custom valves for other bikes (Yamaha), and the prices were very reasonable.

Ken
 
Hi, Pete Lovel had fitted some 9/32 stem inlet valve to my engine 20 years ago they were 41mm , and worked Ok, but it was a 920 engine with the classic 4s cam .
 
I've run them a lot in Triumphs, I also suspected they may cause earlier valve / guide wear but couldn't say for sure either way.

I am a big believer in the benefits as it allows significantly lower valve spring pressures to be used.

Unless someone works out and proves a formula though, it's a bit of a trial and error process...
 
I am going to look into it further. There does not seem to be any guide wear concerns on other bikes and in the racing industry. In this day and age the 5/16 stems seem to be overkill. They use smaller stems than 6 mm and some are hollow stem with head diameter over 2 inches. I run 5/16 stem 2.300 head diameter valves In my race car. Valve spring pressure 400 lbs. on seat and 1000 lbs. open with .860 lift and 8000 rpm. The question I keep asking is not why to use them but why would you not use them? Thanks all.
 
Hutchison hemi said:
I am going to look into it further. There does not seem to be any guide wear concerns on other bikes and in the racing industry. In this day and age the 5/16 stems seem to be overkill. They use smaller stems than 6 mm and some are hollow stem with head diameter over 2 inches. I run 5/16 stem 2.300 head diameter valves In my race car. Valve spring pressure 400 lbs. on seat and 1000 lbs. open with .860 lift and 8000 rpm. The question I keep asking is not why to use them but why would you not use them? Thanks all.
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I'm behind you 100% sir!

Please keep us all posted on your efforts and findings.
 
Hutchison hemi said:
The question I keep asking is not why to use them but why would you not use them? Thanks all.
Click to expand...

With a 6mm stem the valve adjuster might probably push too much near the edge of the top of the stem where a 9/32" (7mm) stem will just work within the valve adjusters moving limits across the top of the stem.
 
I would be using lash caps with the same o.d. as the stock valves so the geometry should not be affected.
 
In my next batch of heads, I'm doing at least two 850 heads with just a pilot hole for the guides, They will both be for smaller stemmed valves (not sure what size), so they will be using smaller guides for better flow. Just one of the options I offer.
 
I have used a lot of 7mm valves in Norton racers. Both steel and Ti. I was looking for the weight reduction as the flow increase was just not there.

I also built a head for my streetbike using 7mm SS valves. After 1 season of riding [8K?] the guides were seriously worn and noisy. Jim
 
Fullauto said:
In my next batch of heads, I'm doing at least two 850 heads with just a pilot hole for the guides, They will both be for smaller stemmed valves (not sure what size), so they will be using smaller guides for better flow. Just one of the options I offer.
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Makes it easy to re-angle the valves and still end up with 1/2" OD guides.
 
Non Norton but NASCAR and others use or have used 6 mm titanium valves inlet and exhaust with no problems. I have some here that I cut down. The inlets start life with 55mm heads and are CrNi coated.
 
All other things being equal, a smaller diameter valve stem will wear the guide and stem faster than a larger diameter one. For engines with small valve diameters and less lift, like lots of the modern 4-valve multis, that's not much of an issue. Plenty of 4 mm - 5 mm stems out there with no wear issues. For a two valve head with large valve head diameters and high lift, it's of more concern. Probably not significant on race vehicles that get rebuilt regularly, but maybe more so on high mileage street bikes.

FWIW the Harley VROD uses valve head sizes similar to a big valve Norton head (1.575"/1.360", and has 6 mm stems. They also use about the same length of guide. Anyone know if they have a wear problem?

Ken
 
Need to look at the reason why one would want to go with a smaller stem and assess whether it is realistic and worthy.

Need to look at the individual applications when comparing. Is it a cam over bucket, NASCAR with roller tip and large rocker ratio (+1.8:1) or an old British twin with a rocker ratio of around 1.13:1.
 
Dances with Shrapnel said:
Need to look at the reason why one would want to go with a smaller stem and assess whether it is realistic and worthy.

Need to look at the individual applications when comparing. Is it a cam over bucket, NASCAR with roller tip and large rocker ratio (+1.8:1) or an old British twin with a rocker ratio of around 1.13:1.
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The immediate benefit, assuming an engineered conversion that included springs, and retainers would be the big reduction in spring pressures and friction. There are obviously many other potential areas for improvement including improved port performance.
 
Momus said:
The immediate benefit, assuming an engineered conversion that included springs, and retainers would be the big reduction in spring pressures and friction. There are obviously many other potential areas for improvement including improved port performance.
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My points were that I would not try a 5mm valve in a Commando based on citing another motor design with cam over bucket - apples to orangutans. It all really comes down to problem definition as well as opportunities as you outline above but at what cost (ie durability, valve compliance etc.) Smaller stems in a Commando have been used before but someone very knowledgeable on Commando port flowing indicated there's little gain with the thinner stems alone. Now thinner stems and less guide protrusion may be another matter. Though you are correct about less friction regarding lighter springs, my opinion is this is trying to put lipstick on a pig. There's more to be gained by focusing on making more power and increasing reliability to finish a race or a ride.
 
I've no experience of installing 6 mm stem titanium valves in a Norton. I do know that they are run reliably for hundreds of racing miles at high duty ratings in 2 valve per cylinder pushrod and rocker engines. The lateral application of proven engineering is usually how race technology trickles down toward the base of the pyramid. Reducing the valve stem diameter and changing the material to stuff less than 60% the weight means improved port performance and potential cam shaft design. This is well understood and exploited.
 
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