Race Engine Forum UK

Guy,
On my 1977 Fiat / Lancia 8 valve head, the exhaust valve seat has an inside diameter that is considerably smaller (almost 2mm) than the stock port diameter. Is it safe to increase the inside diameter of the valve seat to the diameter of the port? It appears that doing so will require using a larger valve, maybe a 37.5mm. What is the minimum wall thickness of the valve seat insert that you recommend? What about the minimum finished exhaust valve seat width? I did a search on the site and saw information for intake seats but not exhaust seats.
Thanks in advance,

Tom, hi.

Good question.

As a simple rule of thumb, whatever you do to the throat by way of enlargement the insert must be kept 0.5 wider on radius than the valve or, with ferrous OE or sintered aftermarket materials it's going to crack in service. And again, as a rule, just like it says in my book I still recommend 0.042" contact face width per inch of valve dia on with stainless or allow steel valves and 0.030" per inch of valve dia for sodium cooled valves. I know Fiat contact face widths (I was doing some yesterday on a 105TC head) can be as narrow as 1mm, but you're not going to get any performance or reliability gain from going much - either way - from my dimensions.

So, 36mm ex valve multiply 36 by 0.042 and you get, as shown 1.5mm contact face width.

If you take a look at the 2 dwgs I've done on one you can see a parallel throated insert and one with a 70 deg top grind/cut (I use 70 or 75 blended into a parallel lower section on the TC heads - unlike the inlets where I always leave the 70 deg grind with a sharp edge where it meets the contact face and lower section). Now although the OE seat has a fully parallel throat, they did not grind the seat right out to true valve diameter, which means that there is plenty in hand for an overhaul regrind. If you 'blueprint' the seat, ie: take it right out to 36mm (or whatever the valve dia is) and do a full parallel with a 1.5mm contact face there will be no spare material left to 'touch up' the seats on overhaul at all. So go with the 70 throat (or at least some angle to allow for overhaul) every time. You have to assume the seat, in service, will go slightly oval, get some embedding and corrosion etc, never mind that if you change the guides everything wil be slightly out of alignment and you might be able to generate a fully concentric contact face at all. If you're running a highly boosted engine the amount of distortion on ferrous ex seats can be huge..

I mention that in passing because I see an awful lot of what I'd call 'maxxed-out' seats where this has been done and sometimes there is so little metal left you have to put a new seat insert in. Very often the port has been 'hogged' right out too and even with the new insert it can be a real problem. Not good practice. As I said in my dvd, you still get folks who think that their prep/build is the only one the engine will ever see! Never happens. Get something like a stone under the dry sump belt or a detonation event and that engine will have to come apart straightaway - maybe even after its first practice run!

Compare your port section diameter below the insert with the drawing of the 70 deg throat. Fitting a bigger valve might allow you to move the contact face outwards a bit and enlarge the throat, but you'll probably end up with less than my required 0.5mm minimum edge thickness, so it might be better to fit bigger inserts.

Hope this helps,

GC

Guy,
Thanks for the thorough reply. I will take some measurements of the valve seat and see what I need to do. Curious that the stock valve seat, inside diameter, is so much smaller than the port itself. Seems like there is some free horsepower available just by matching the port ID to the seat ID, surprised the factory didnt do it from the start. From the look of things that may mean a new larger seat, oh, and then a larger valve to go with it, maybe thats why.
Thanks