Valve seat recession and unleaded fuel

 In Engine General, MGA-MGB, Midget

I often get asked what is the best way to deal with this.

The options are:

  • Additives added to the tank when you top-up with fuel
  • In-line tin-based canisters
  • Stainless valve seat inserts.

Stainless valve seats

My personal preference is to fit stainless valve seats as they are the reason modern engines run perfectly happy for ever on unleaded fuel.  I would recommend that if you have the head off for some other reason that you get them fitted then, as it is a relatively inexpensive job – the last one I had done was $200.  However, I wouldn’t suggest you pull the head off specially to do this job unless you use the engine in the higher speed range (see below).

An additional advantage is that hardened inserts greatly increase the reliability and resilience of the head in the event of a burnt exhaust valve.  As series heads in particular are prone to cracking when this happens, but if a seat has been fitted it avoids this damage.

If you have a head with oversize valves, refer further down the page for further comment in fitting inserts.

Additives & cannisters

Add-to-tank additives are generally reasonably effective, and are the officially recommended way to go if the valve seats are not converted.

In-line canisters have caused a lot of debate.  Scientific testing has concluded they are of limited merit but many people swear by them.

There is a school of thought which says there is a “lead memory” and that provided the engine has been run on leaded fuel at some stage then it should be fine.  I’ve seen no evidence one way or the other on this.

Valve seat recession is not significant at lower engine speeds (under 2500 RPM) so if engines are always driven at about this speed you could easily get misleading anecdotal evidence about the merits of various additives or about whether they are needed at all.

Above 4000 RPM, recession becomes much more marked.  The in-tank additives reduce it under these conditions but it is still there.  That’s why I went for stainless seats, as I do quite a bit of competing.

If you do decide to try omitting the additive altogether, all you need to do is monitor the valve clearances, (only exhaust, inlets are no problem) probably after 1000 miles, then if nothing significant every 2000 miles.

If the seats do start to erode, then either revert to the additive or have stainless seats fitted for a permanent cure.  If the seats erode badly, stainless inserts will recover the situation.

If you have oversize valves fitted:

Many head modifiers and reconditioners are reluctant to fit inserts to heads fitted with oversize valves, in case the pressure of the insert results in a crack across the inlet seats between the valves as the strength there is reduced by the valves being so close together.   However, my own experience has shown it is possible to fit seats in these situations & retain complete reliability in this respect.

My MGB big-valve head has the valves only 30 thou apart and has inserts fitted. What we have done to get around the cracking is to insert both inlet and exhaust, with the exhaust insert cut into the outside of the inlet insert a bit. This means that the inserts push on each other rather than just one exerting pressure on the thin bit between the ports. It has been a very successful arrangement, allowing the throat configuration and diameter under both valves to be at their optimum.

We adopted this arrangement after inserting just the exhausts caused one of the inlet seats to crack within a couple of years and I was reluctant to scrap a head that was the best flowing B head we’d tested. Fitting the inserts to the inlet did not affect the flow once they were blended in.

That was in the 1980’s, so I guess you could say it’s been successful.

We didn’t go unleaded in New Zealand until the ’90’s, the reason for the inserts were pocketed exhaust valve seats. At the time, most inserts being fitted were cast iron but the machine shop suggested new-fangled stainless ones (which were on the market here at the time for CNG/LPG conversions which abounded at the time) as they were stronger and so could be machined much thinner, allowing the throat size I wanted while still just avoiding the inlet seat.

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