valve stem sizes

[jerry atric]

further on my quest for air tightness, I wonder if anyone can help with the correct valve stem diameter of the S engine inlet valve please? Nearest I can find is Greg Bender's 1940 Alce manual. this says 9.93mm. The diameter of mine is  9.43 but there is no wear ridge and  it's apparently  fairly new. it's too loose in the guide so could I get away with just replacing the guide?  I dont want all the hassle of buying a new valve only to find the diameter of repos valves is now 9.43mm. I contacted guzzi retro but no reply after a  week😟

[RICHARD YAMANE]

My thinking on this is valve stem clearances affect oil control more than air control. Loose stems are better than tight within reason. Especially with modern fuels. Rotax had a problem with their later, late 90's, 605 engines where the valves would stick and prevent compression. Fuel chemistry and clearancing has probably been improved since but the lack of lead and "dry" alchohol fuels were the suspected villan.

Rick Yamane
Importer of Vape/Powerdynamo ignition systems and Polisher of bikes at Motion Pro

On 04/23/2026 3:46 AM PDT jerry atric <baconsl...@gmail.com> wrote:

 

 

further on my quest for air tightness, I wonder if anyone can help with the correct valve stem diameter of the S engine inlet valve please? Nearest I can find is Greg Bender's 1940 Alce manual. this says 9.93mm. The diameter of mine is  9.43 but there is no wear ridge and  it's apparently  fairly new. it's too loose in the guide so could I get away with just replacing the guide?  I dont want all the hassle of buying a new valve only to find the diameter of repos valves is now 9.43mm. I contacted guzzi retro but no reply after a  week😟

 

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[jerry atric]

thanks rick,

its an external valve, hence sucking in air. interestingly, the quoted clearance is much less than the later Gtv. which had huge clearance  but maybe ran hotter? 

[Alan Comfort]

I would replace the guide, or fit some K Line valve guide liners.

On Thu, Apr 23, 2026 at 3:46 AM jerry atric <baconsl...@gmail.com> wrote:

further on my quest for air tightness, I wonder if anyone can help with the correct valve stem diameter of the S engine inlet valve please? Nearest I can find is Greg Bender's 1940 Alce manual. this says 9.93mm. The diameter of mine is  9.43 but there is no wear ridge and  it's apparently  fairly new. it's too loose in the guide so could I get away with just replacing the guide?  I dont want all the hassle of buying a new valve only to find the diameter of repos valves is now 9.43mm. I contacted guzzi retro but no reply after a  week😟

--

[Alan Comfort]

In my humble opinion, if you can wiggle the valve in the guide, it is too loose. Nonetheless, the engine will still run OK but may smoke a bit more than you want as oil mist is sucked past the valve stems.

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[Alan Comfort]

Oil getting past the valve guides is not an issue with an open valve system.

[Andrew Nahum]

Out of interest, my Sport 15 with external exhaust valve seemed to run fantastically hot.  Are those motors all like that?  It never seized. I’m just judging by the heat radiated from the engine and the speed it cooked any oil that got onto it.  Andrew

On 23 Apr 2026, at 17:02, jerry atric <baconsl...@gmail.com> wrote:

thanks rick,

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[Alan Comfort]

How does your spark plug look? If the ground electrode and centre electrode have a white tinge, you are running hot. Brownish tinge is what you want.

If white, try a larger main jet in the carburetor and a little less advance with the ignition. Modern fuel can cause all kinds of issues with these old engines.

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[John O Regan]

HI All,

I have seen several vintage engines fail after being fitted with k liners or similar products

Open valve air cooled iron engines survive best with iron valve guides .

9.43mm valve in a 9.5 bore guide is a shade under 3 thou clearance, 

A week or two ago I was repairing an IOE inlet housing from a Rudge Multi that was made without a guide originally

I was measuring up some valve stems and 9.43mm cropped up being the stem size of what I think are John Deere valves? I wondered if that was a clearance size for a 0.375 guide bore?

Careful guide measurement with a telescopic  gauge, bore micrometer or gauge pins will reveal if the guide is worn hourglass shaped.

Luckily with Rudge the original factory drawings are available so I could get the valve length rocker angle and fitted spring length correct. I made a jig to hold the inlet housing so that when bolted in place the wobbly guide bore was central. then using a boring head trued up the oval guide bore and continued to 7/16in. An iron guide from the local engine shop was 0.5in od and 0.313 bore. I mounted the guide on a mandrel to keep it concentric and turned a length to be a light press fit in the housing and left a shoulder on top for security.

a shortened BSA Gold star inlet valve needed to be reduced near the head so it would sit against the seat.

Stem to guide clearance is about 3 thou, I'm happy with that.

I made collets to fit a turned down portion of the stem and taper to match the Rudge spring cap. 

Originally the Rudge had a slotted stem and the "flat pin" has tapered ends , which is somewhat unusual??

John

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[jerry atric]

thanks guys  for  some good info. I'll go for a new guide, keeping the same  valve,I suspect it is hour glassed. I'm not to confident in fitting it myself. I had one  replaced professionally on my old GTV and they cracked the head... glad I didn't have to pay the repair cost!

[RICHARD YAMANE]

I have a GTV. Not sure why I keep forgetting this. 

Rick Yamane
Importer of Vape/Powerdynamo ignition systems and Polisher of bikes at Motion Pro

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[John O Regan]

Hi Jerry

" I had one  replaced professionally on my old GTV and they cracked the head."

Some "professional" ????

Iron guides in iron heads do not require much interference, as the expansion rates are the same, plus the guide is always hotter than the head. 

If the engine shop is not familiar with old Mòtorcycles walk away.

John

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[jerry atric]

I was using one of the premier classic/vintage engine  rebuilding companies in my area. it was their third attempt at fitting a bronze exhaust guide, the previous 2 had seized on the  valve and pulled out. It was a nightmare. the repair was huge necessitating a new valve seat, which they did.  A week later, the new seat fell out! I was told later that they had  a rogue engineer... could have explained it

[John O Regan]

HI Jerry,

Broken cardinal rule. Iron head = Iron guides on aircooled engines !!!

I had a wrecked BSA A10 iron head to fix some time ago, it had been fitted with bronze guides!!!

My reasoning is that the bronze expands more than the iron head and with nowhere to go it will squeeze onto the valve stem. then likely start moving in the head if the valve does not stick open.

John 

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[jerry atric]

interesting, John.

metallurgy is beyond  me, here's what I gleaned during the frustrating months it went on.

Guzzi Retro: We always fit bronze bushes as all new valves are stainless so incompatible with iron guides

Company doing job: Stainless picks up on iron but modern silicon bronze is better at self lubricating anyway.

original GTV specs: I  can't  remember the clearances now but they were much  greater than expected, presumably because of the  very problems I was experiencing.

Jerry

[Paul Compton]

On Sun, 26 Apr 2026 at 08:53, jerry atric <baconsl...@gmail.com> wrote:

> Guzzi Retro: We always fit bronze bushes as all new valves are stainless so incompatible with iron guides
> Company doing job: Stainless picks up on iron but modern silicon bronze is better at self lubricating anyway.
> original GTV specs: I can't remember the clearances now but they were much greater than expected, presumably because of the very problems I was experiencing.
> Jerry

Stainless Steel valves are available with Chromed or coated stems to
be compatible with cast Iron guides.

I've done a few guides on Morinis and the occasional racing Triumph
(Daytona 500). On those engines I used Colsibro and fitted the guides
with undersize bores, brush honing to fit. Race fit was for an oiled
valve (3in1) to just fall under its own weight (less than 1 thou).
Seat is then cut to be concentric with the guide. Valves then seal
with no need to lap. The oil retaining finish in a honed guide will
tolerate a much tighter clearance than a reamed one.


--
Paul Compton
www.paulcompton.co.uk (YouTube channel)

[John O Regan]

Hi Jerry,

There's stainless and theres "stainless"

A very broad range of materials!

Way back c1950 when valve materials were being developed for high performance motorcycles, one of BSA's subsiduary companies "Jessops" made special steel's

One such metal developed for valves is code named "G2" which is an "Austenitic steel", it is non magnetic, but does that make it stainless steel?

I have been guided by "Tuning for speed" regarding material selection for old engines, it has not let me down in all these years🙂

John

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[jerry atric]

Oof! I'm more  confused than ever now. Either way, I'm constrained by what people like Guzzi Retro sell. As long as as it  starts and runs I'll be happy😂

[pouma1954]

A decent engineer should find car valves that can be adapted.

There's still some of them about.

Payl

Envoyé depuis mon appareil Galaxy

-------- Message d'origine --------

De : jerry atric <baconsl...@gmail.com>

Date : 26/04/2026 19:03 (GMT+01:00)

À : Guzzi Singles <guzzi-...@googlegroups.com>

Objet : Re: [guzzi-singles] valve stem sizes

To view this discussion visit https://groups.google.com/d/msgid/guzzi-singles/455a5c4e-56dc-45ea-ab3b-7a890b4e4c21n%40googlegroups.com.

[jerry atric]

I agree, Paul but budget dictates the less exotic path.

[Andrew Nahum]

There should certainly be an automotive valve that suits. Most car ones are too short, Some years ago the head came off the (original) exhaust valve on my 1919  Triumph model H. As I recall I found a truck valve with the right stem diameter.  I think we had to turn down the head.  You don't use modern stem/guide clearances in an antique motor which was made to much more open fits.

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