valve guide size

[917guy]

Have we always used guides that are 3 thou bigger than nominal?

New 911 guides are offered at 13.08mm, and are called first oversize. I checked a NOS 356 guide which I bought about 27 years ago; it is about 14.075mm in diameter, with my hands warmer than it was, and no gloves to keep all isothermal. Obviously these work for everyone, but does that mean a head getting its fifth set of guides will be happy, and the head getting its first set will be over stressed?

In 1965, I did have a Honda head with looses guides, but that was from repeated over revving, and a very short guide. Honda offered no oversize, so I had a set copper plated, then made new guides when that pair got hit. I do not recall working on any VW, 356, 911, 944S, Triumph or Honda head where guides became loose. Lots of bent valves and damaged plug holes though!

Otherwise I had not paid much attention to this factor. Did anyone ever sell 13.00 or 14.00 mm guides? I do not recall much head material removal when they are withdrawn, when they are PULLED out (threaded, drilled, bolt inserted, and punch hitting the bolt tip). Does anyone bake heads for removal? Al expands more than steel or bronze alloys, after all.

[chris_seven]

We are just in the process of making Colsibro valve guides and I have been looking at the information in the spec books to see what to make.

The early 2.0 litre gives the following sizes:

Std. Guide: 13.049-13.060 Bore in Head: 13.00 - 13.018

1st Oversize Guide: 13.249-13.26 Bore in Head: 13.20-13.218

2nd Oversize Guide: 13.449-13.46 Bore in Head: 13.40-13.418

The SC Spec Book gives slightly different data:

Std. Guide: 13.060 Bore in Head: 13.00-13.018

1st Oversize: Guide: 13.26 Bore in Head: 13.00-13.20

2nd Oversize Guide: 13.46 Bore in Head: 13.20-13.40

The SC Book also specifies press fits as follows:

Std. 0.031 - 0.060

1st and 2nd Oversize 0.06 - 0.090

The expansion shouldn't be an issue but it does depend on the guide material.

A typical Aluminium Casting alloy can have expansions as low as 20-21ppm/degK

Manganes Bronze has a typical expansion of 21ppm/degK so the guide is unlikely to move.

Colsibro and Hudurel (Nickel Bronzes with good heat transfer) are typically 16.5ppm/degK

Phosphor Bronze is around 18ppm/degK