Valve guides

[BK911]

A buddy of mine works in a machine shop that builds American V-8’s for the track, with the occasional street engine thrown in. He said on some of the street engines they don’t replace the valve guides if they are close to being within tolerance. They thread the inside of the guide, which raises some of the material, and then they ream it out to size.

I am considering this for a bone stock 2.4T rebuild. The guides are at the lower limit of the tolerance window. I know I can just replace the guides, but if this other method works on 300hp V-8's, why wouldn't it work on a 140hp boxer 6?

Any opinions on this?

Thanks!
BK

[fred cook]

The process you are talking about is called "knurling" the valve guides. Does it work? A qualified yes. Qualified because a valve job done by this method will not last as long as replacing the guides with new ones. Basically it is a cheapo way to do things. Unless you are planning on getting rid of the car right away, I would replace the guides.

Good Luck with your project!

Fred Cook
'80 911SC coupe

[Scott Clarke]

Because 300 hp V-8s are watercooled. 911 exaust valves get really hot. The path for the dissipation of that heat is through the stem of the valve to the valve guide to the cylinder head to the cooling fins cast into the head, and finally to the air being forced past the fins by the fan. Since knurling reduces the area of contact between the valve and guide, it reduces the potential for heat to travel along this path. The result could be a valve head breaking off of its stem, subsequently likely to raise some heck the very next time TDC rolls around (kaboom). The fact that it is a T may not be of great benefit if no external oil cooler has been retrofited to the car.
-Scott

[cstreit]

I'll second that. The valve seat and the valve guide are the two places that the valve sheds heat back into the heads. If you remove 50% of the material from this area, you will remove 50% of the cooling capacity... Sounds like a disaster waiting to happen on an air-cooled motor.

[BK911]

You guys are good! I went by a local shop yesterday afternoon and asked the same question. Pretty much got the same answer.

But... if you knurl the inside of the valve guide, aren't you adding more surface area? The valve doesn't actually touch the guide, does it? It would be kind of like adding fins to the guide.

I know, if it worked wouldn't Porsche have done it?

[Scott Clarke]

The valve stem does contact the guide by means of a film of oil, so more contact is possible with a smooth surface.
-Scott

[Jay Auskin]

I'd be more than happy to show pictures of what happens to your 911 engine when the valves aren't properly cooled. It's definitely not for the weak-of-stomach.

[Wayne 962]

There are indeed shortcuts everywhere in this world. There are typically downsides to these as well...

-Wayne