Why are all the aftermarket valve guides counter-bored?

[Dave Erickson]

Two questions about valve guides:

I have a few heads with original Porsche copper valve guides. They are not counter-bored - the inner 10mm diameter runs end to end. All the guides I have purchased are counter-bored about 6mm at the bottom end. What is the reason for the counter-bore?

The original intake valve guides are about 71-72mm long and the original exhausts are about 65mm long. Now, all the guides seem to be the same length. Why the change? Are the 72mm intake guides available from any suppliers?

The shorter bearing surface due to the counterbore makes the guides quite short. Valve guide pilots with collets at the end just barely have enough bore to seat when grinding seats.

[Harlan Halsey]

I usually make my own and they are not counterbored. The only reason I can think of is that the guide exits the aluminum in the passage at an angle. It seems to me that some heads were slightly counter bored to relieve any possible side pressure. Suppliers may be trying to duplicate this effect by counter boring the guide. I wouldn't use such guides.
As for your wanting shorter guides, shortening them yourself is trivial.
The last time I bought guides was about 5 years ago. I bought them from a large 3 letter named valve guide supplier located near LA. It seemed they would do about anything you wanted them to do and they sized them to the tenth.

[Dave Erickson]

All the C/SC heads i have seen are counterbored for the guides on the outlet side, but the new guides have a counterbore as well. The majority of the guides sold in the aftermarket are made by Canyon. I am looking for longer guides, not shorter. Those made by Canyon are about 65mm long.

[Dick Weiss]

Years ago, I checked the early aftermarket silicon bronze guides for concentricity of the OD to the ID
and found several varied between .002" and even as much as .005" which makes it difficult to minimize
cutting the valve seats; I DON'T use a grinder!
Later guides have been machined better, but I still check 'em.

[Dave Erickson]

Dick, good idea. I use a grinder, but off-center guides are a PITA. They might reduce the number of valve jobs a head is good for by half. Harlan's suggestion to make new guides on the lathe is also a good one, although working with bar stock would be a slow operation. Maybe buying oversize guides and mounting them on a tapered arbor would be the way to go. I looked for a source of C90500 bronze tubing in an appropriate size (9/16 or ⅝ o.d. and ⅜ i.d.), did not find it.

[Jacques Lefriant]

Hi
The reason is that the manufacturers want to have less part nos so the relief for carbon build up is for the exhaust only and should be equal to the valve lift. Tehnovance started to include this feature on them and others followed suit.
j

[Martin Benade]

Is that basically ok for intakes too, or is it worth some effort to find non-counterbored guides?

[Jacques Lefriant]

short ans yes,no

[Dave Erickson]

Jacques, don’t you think the guides length is related to how long it lasts? The original intake guides were 71mm long without a counterbore. The current intake guides are 65mm long with a 4-5mm counterbore. So that adds up to 10mm less guide bearing surface, i.e. a reduction of 1/7.

[Jacques Lefriant]

Hi Dave
the more important factors are the material specs and accuracy of the seat machining the current ones usually outlast the originals.
j