What’s in your Head?

If I can share my information on valve guide materials, I’m hoping to create an exchange of our experiences and the options available.

During the 1980s, leaded petrol was fast disappearing and becoming more expensive. My daily motorway drive was in a 10-year-old MGB GT 1800, with a TD for weekend leisure.

The BGT engine was not running well and I decided that a rolling road check would be wise. I made use of the local facility of Peter Burgess, who has an excellent reputation for preparing MGB and other race engines.

The bottom end, carburettors and electrics were all ok, but the cylinder head was pinpointed as requiring attention. Therefore, it seemed a logical update to have valve seat inserts and copper alloy valve guides fitted.

As a Metallurgist I was interested to know the specific alloy that would be used. However, this was not an easy question to answer as suppliers often used their own nomenclature. So, I volunteered to investigate by doing literature surveys, chemical, microscopic and hardness checks on what was on the market, plus a trip by Peter to the local scrapyards, retrieving valves from accident write-offs.

Just to say at this point, the convention for Brass is Copper/Zinc and for Bronze is Copper/Tin. As my experience was not with copper alloys, it was a learning curve for myself.

The six samples showed four basic alloy types (see Table 1),

Microstructural observations based on the basic phase diagrams for Copper/Zinc, Copper/Tin, and Copper/Aluminium were as follows:

Type 1 This was an alpha structure highly strained, non-annealed with a relatively large grain size.

Type 2 This was an alpha structure with an annealed recrystallised, much finer grain size.

Type 3 These were predominantly beta with a small feathery grain boundary alpha and a reasonably uniform precipitate.

Type 4 This was a duplex structure, fine alpha grains, surrounded by an irresoluable second phase.

Heat treatment and machinability were also reviewed.

Peter’s alloy of choice was the CA 104, but with time and the knowledge that thermal conductivity was similar to cast iron, he moved on to CZ 114. However, occasionally, the valves would be nipped in the guides, so with advice from Delta Bronze he moved on to his current material CZ 135 (HT 20).

The chemical composition of CZ 135 (BS2874) is shown in Table 2 with that of CZ 114 for direct comparison.

The thermal conductivities are shown in Table 3 with Grey Cast Iron and CA 104 for comparison.

With the addition of Silicon to CZ 135 the formation of Manganese Silicide needles give an excellent wear resistance over time.

Both alloys are often referred to as a Bronze, even with large amounts of Zinc; CZ 114 prefixed as a Manganese Bronze and CZ 135 as Silicon Bronze. However, some suppliers use the term High Tensile Brass due to their high Zinc contents, which is more in line with the convention I mentioned earlier.

Just digressing, how many of us call our kitchen foil ‘Tin’, when it is of course Aluminium?

The choice of guide material must be influenced by head geometry, engine operating conditions and thermal expansion characteristics of itself and the valve material. CZ 114 and CZ 135 have a very similar value of 20/21 x 10 (to the power of 6) K.

An article on the MG Experience forum in 2018 By Steven Savage is an excellent read with his comments on the relative merits of the two alloys and valve types.

My old MGB GT is still on the road but I have not been able to speak to the current owner of his experience. The TD misbehaved a few years ago and underwent the conversion by Peter with CZ 135 guides.

I have been in touch with several cylinder head refurbishers, requesting their alloy choices with a mixed response.

CYLINDER HEAD SHOP – (Predominantly motor cycles) use COLUMBIA METALS alloy COLISBRO (Cu Ni 2.0% Si 0.6%) for engines operating below 10,000 rpm and TROJAN (Ni 3.0% Si 1.0%) for above 10,000 rpm.

H.T. HOWARD & Co. Ltd use a spring tempered phosphor bronze.

Other companies either did not know, would not pass on the information, or never replied to me.

I hope this article will generate an interchange of our experiences, enabling an understanding of the options available. Maybe someone has experience of valve materials and matching them to guides.

David Barnes

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