Valve guides, guide fits, and why it matters: A general overview

[T. Cotten]

I'm talking about roughing out a window weight Folks!

Of course it is finished out between centers on a pilot. That's a given.

My father called it "pig iron", and it is as hard as the Hubs o' Hell.

(Credits to VirtualIndian posts from a Kiwi or a fellow from Oz, cutting them for racing 741s if I remember right. Probably not. But if there may be a superior material cheaply available, I'm going to investigate, at very least.)

....Cotten
PS: BoschZEV!

What happens if a finish is too smooth?

[BigLakeBob]

Cast Bar.JPG

[BoschZEV]

What happens if a finish is too smooth?

Too smooth is too bad. Some amount of roughness is needed to retain lubricant. It happens that I'm rebuilding the damper on a bike right now on which I plan to replace the present O-ring with a T-seal, so only a few days ago I consulted the Parker manual for what surface finish the shaft should have. In principle, at least, with 500-grit stones on my Sunnen external hone I could achieve 1–2 microinches. However, the Parker manual says "Surface roughness values less than 5 micro-inches are not recommended for dynamic seals, however, as an extending rod will be wiped completely dry and will not be lubricated when it retracts. ... The most desirable surface roughness value is from 10 to 20 micro-inches."

The same consideration is at work with valve guides, but specific values depend on the material of the guide and that of the valve (and if any coating is on the valve stem). That said, while the ideal roughness value for window-weight guides may not be in the literature, I suspect any value of roughness whatever will result in the same lifetime for the valve if you use that material.

[T. Cotten]

Too smooth is too bad. Some amount of roughness is needed to retain lubricant. It happens that I'm rebuilding the damper on a bike right now on which I plan to replace the present O-ring with a T-seal, so only a few days ago I consulted the Parker manual for what surface finish the shaft should have. In principle, at least, with 500-grit stones on my Sunnen external hone I could achieve 1–2 microinches. However, the Parker manual says "Surface roughness values less than 5 micro-inches are not recommended for dynamic seals, however, as an extending rod will be wiped completely dry and will not be lubricated when it retracts. ... The most desirable surface roughness value is from 10 to 20 micro-inches."

The same consideration is at work with valve guides, but specific values depend on the material of the guide and that of the valve (and if any coating is on the valve stem). That said, while the ideal roughness value for window-weight guides may not be in the literature, I suspect any value of roughness whatever will result in the same lifetime for the valve if you use that material.

Whoa, BoschZEV,..

Weren't we talking about making guides?

The bores are properly honed with a cross-hatch after installation, and even reamed have tool marks that hold oil,... if you want it.

Can the outside be too smooth?

...Cotten

[Peter Cooke]

I installed dozens and dozens of guide liners, Gene! They are a great last resort to get something running just to sell it. But it better be water-cooled.

....Cotten

Could you expand on this please Tom. Is it a heat transfer issue?

[T. Cotten]

Its always about heat, Peter!

A solid piece of metal conducts best. (I think I remember Chevy heads machined with integral guides, but I wasn't focused,... Coulda been Fords maybe.)

Any joint to another piece is a barrier, and traditionally this is practically alleviated by a firm press-fit. Holding the guide in place is easy, because there really isn't much on the valve to grab and make it move. Its all about an interference fit that mushes the metals together for better heat transfer.

Guide liners are yet another heat barrier, and the interference can vary with the wear upon the original guide. Even if they were followed by by something like a ball-burnisher, gaps happen.

And any change in the bore of the guide affects run-out on the seat. And its rarely in a good way, especially when it gets as fat as a liner.
Do you want to sink the seat to China just to save a guide?

....Cotten
PS: The whole "Fluid-Weld" thing is that it fills gaps for better conductivity. Too bad its so expensive, but its shelf life seems pretty good.

[chuckthebeatertruck]

This one is getting interesting.

I enjoyed reading the additions and seeing different preferences.

I think we need more stuff like this.

[chuckthebeatertruck]

I originally didn't comment on the chuck verses collet comment but I would like to share my 40 years of almost daily machining knowledge here. I am in no means disgracing anything, just adding my 2 cents. I agree that a collet is better, but I believe most motorcycle/machinist have a lathe with just a chuck.
.

Thanks for adding this. Sometime in the future someone is going to ask how it could be done, and you gave a few options.

[T. Cotten]

So who has a collet that accepts a window weight, Folks?

Thanks in advance....

.....Cotten

[Peter Cooke]

Its always about heat, Peter!

A solid piece of metal conducts best. (I think I remember Chevy heads machined with integral guides, but I wasn't focused,... Coulda been Fords maybe.)

Any joint to another piece is a barrier, and traditionally this is practically alleviated by a firm press-fit. Holding the guide in place is easy, because there really isn't much on the valve to grab and make it move. Its all about an interference fit that mushes the metals together for better heat transfer.

Guide liners are yet another heat barrier, and the interference can vary with the wear upon the original guide. Even if they were followed by by something like a ball-burnisher, gaps happen.

And any change in the bore of the guide affects run-out on the seat. And its rarely in a good way, especially when it gets as fat as a liner.
Do you want to sink the seat to China just to save a guide?

....Cotten
PS: The whole "Fluid-Weld" thing is that it fills gaps for better conductivity. Too bad its so expensive, but its shelf life seems pretty good.

Thanks Tom. It appears I have some thinking to do. The shop that did my cylinders used them but I've since discovered they're not OEM style guides anyway. Either from something else or home made. So perhaps the solution is simple. New OEM style guides.

[T. Cotten]

You won't need a solution, Peter,...

Until they fail;
Might as well get your money's worth if you can, particularly since any replacement will sink the seats deeper.

....Cotten
PS: Unless you're doing The Cannonball.

[gharper]

Cotten,

I used bronze liners for a reason: The X is a pocket valve engine so the intake valve is held in a cast iron cage which drops down into a big hole in the top of the cylinder and held in place by the rocker tower and a big nut. The guide, cage and seat is all one casting. No room to replace the guide and barely enough room for a liner. The rocker arm geometry is horrible, with no lubrication. Consequently the guides are always shot. Also, they used a 15 * valve seat! I chucked the cage up in a 4 jaw, dialed it in and did everything in 1 operation except the final hone and gentle lap of the valve seat to get a good show of contact. I used a Jeep Cherokee intake valve, cut down the shaft, and the seat is 46*. First operation was to carefully bore the sloppy 5/16" guide out with a carbide boring bar to true it up. Next was using the K line reamer to remove the last one or two thou. Then drove in the liner, seated it, reamed it to within about .001 of finish size and then cut the seat. Once that was done, a final hone with a .003" clearance. End result is a decent valve and guide setup, much better than the original. Picture attached, note the valve guide is way off center in the casting. That wasn't me, that was the poor QC in the X factory! Most of the guides were horribly off center, at least the meat of the casting was. Yes, it's thin there, some even cracked a tiny bit as the wall is so thin. Those are spares now but the ones I'm running are much better.

intake cage X.jpg

The exhaust was another story. These are screw in guides, a horrible design. Not enough meat in the cylinder to locate the center and bore the cylinder for a press in guide, it would never hold, so stuck with the screw in guide. I made the mistake of removing them and installing liners and reinstalling. These are STEEL guides by the way, not cast iron. Again, shot to hell but hard too, tore up the first reamer. I also made new guides but regardless of what type, once screwed in the valve center and stem angle are never the same, so lots of seat material needs cut to establish a seat. I have since decided to not remove the guides in the future and instead mounting the cylinder on a steel plate, attach that to the mill, locate center as best as possible and drill, ream and line the guide in place. Again, a big mistake to remove the screw in guides! The exhaust I fitted at .005".

Time will tell how I did.

[T. Cotten]

Rocker geometry is what ate the Chubble liners, Gene....

Since the intake cage is small and manageable, you might consider Wilson Plank's approach with oven silver-soldering: Sweat in a rod to completely fill the bore to the liner diameter, and then re-bore to the stem diameter.
This would eliminate at least one insulating gap, and provide a more durable bearing surface. Maybe.

The exhausts are hard for me to comprehend! (Threads reduce the interference contact, and multiply the gap.)

.....Cotten