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MG Midget and Sprite Technical - Big valves in small bore heads

Over the years I have accumulated too many cylinder heads, so I am going through them with a view to building them up and selling them on. I'm starting with a couple of CAM4810 A Plus castings. I have one of these on the car at the moment. Inlets are 1 1/4" and exhausts are 1 1/16". I was told that these are the biggest you can get in without pocketing the block. However I was advised by AC Dodd when I was over there a short while back that he would put in a 1275 inlet valve. I didn't get him to do the job because he spotted that the casting was cracked! Anyway, the question is what size valves are the biggest that will work in this head? I'm well aware that bigger is not always better, because of valve shrouding etc, but the ones I have in my present head are very expensive and if I'm building a head to sell on I'd rather not spend too much.

While pondering all this, it occurred to me that if I can build up another CAM4810 that flows better than the one I have, I'm better off keeping that and selling the existing one. I should add that I will of course be doing some grinding on the chambers and ports.
Les Rose

1275 inlet valve would clear ok, it's the exhaust on small bore blocks that has a clearance issue with larger valves sometimes and needs the block pocketing.
The ideal I/O valve size ratio is between 70/80%
With your 1 1/4 / 1 1/16 valve sizes----- you have at the moment the ratio is 85% so a larger (1275--1.3") inlet should get it back close to the recommended window at 81% without a shrouding problem

Of course you can go bigger than this with both valves but the valve size ratio needs to be right and also you get into shrouding/block interference issue--just depends how excited you want to get

The MG Metro head runs 35.6 mm inlet valves and 30.5 mm exhausts

willy
William Revit

Willy, "The MG Metro head runs 35.6 mm inlet valves and 30.5 mm"
So, if my maths is right that is 85% ratio too?
I have an MG metro head on my 1275.
GuyW

True Guy, very true, so if you were looking at increasing valve sizes it'd be the inlet that needs to be larger --- 38mm-? to get the ratio right
William Revit

Thanks Willy, that's very helpful. I didn't find that in Vizard's book, or anywhere else for that matter. The other issue is valve length. Obviously I don't want inlets and exhausts of different lengths and the 1275 ones are longer. I'll need a 1 1/16 exhaust valve with a long stem.
Les Rose

Back in our old experimental days we used to poke the biggest inlet valve in that we could find then match it up with a decent sized exhaust that'd still fit in the chamber. We were using the best heads available at the time Brownfield offset port head (small block chev)
Best dyno results were from 2.3"inlet matched with a 1.8"exh ---78%
William Revit

2.3" that's huge! I thought large diameter valves were prone to heat distortion and warping ?
GuyW

Stainless do , we were on titanium inlets and inconel exhausts. Inconel is heavier than stainless but stronger so the valves could be machined way thinner for good flow without sacrificing strength on that side-

With the heads Les has I'd go the 1275 inlet and for the exhaust, leave it std. but with the grinding concentrate on getting good flow shape after the valve seat and around the valve guide area.
William Revit

Willie, when you say standard for the exhaust do you mean one inch? That would mean going down to a small bore valve (short), combined with a big bore inlet (long). I haven't heard of anyone doing that.

I assume your ratio is based on diameters not areas.
Les Rose

diameters, yes.
Was meaning just leave the short 1 1/16" valve that you already have, then use .100" lash caps to get them up to close to the length of the 1275 inlet. I think it's about .120" difference in the lengths--Just thinking of the easiest/cheapest way of getting a result, but I guess it's starting to get a bit messy like that
Might be better just spinning a tiddle off the dia of a 1275 exh until it fits the seat that you have----I don't know how big you can go without having to scallop th block for clearance, you'd have to have a measure up.
If there's room maybe just use the 1275 valve and metro inlet but then you'll need springs,caps,keepers to suit.
There's plenty of options, just depends on the budget.?
What size / valves does Peter B. use in his heads, he'd know way better than me what works in these heads/engines
William Revit

I've done a bit more reading over lunch. AC Dodd says he uses 1275 valves in all his small bore heads. As you suggest Willy, he turns down the exhausts to fit. As I have a lathe that will be a lot less trouble than fiddling with mixed valve lengths. So I will have 1 1/16" exhausts and 1 5/16" inlets.

I also read that the CAM4810 casting is in some ways better than the revered 12G295, as it is thicker in the port area near the chamber. This allows a smoother curve for both inlet and exhaust flow.
Les Rose

Ah Les, I go for a sleep and you find answers from ACDodd --Sounds like you've got a plan.---bit weird in my world that someone actually agrees with me--lol, My suggestions are/were based on what I know works in big horsepower chevs. Poking my neck out a bit transferring that over to a 1 litre engine but there you go eh.

Question time-
I assume the groove for the keepers for the valves you have are different to the 1275 valves, do you know if the keeper od and taper angle are the same on both----
If they are you could still use the caps and springs you have and pack the bottom of the spring to get the correct fitted length/seat pressure-

willy
William Revit

The 1275 valves have triple grooves, so I will have to to get new collets, and possibly caps. They are not expensive. As you say, I might have to shim the springs, but Minispares have a set of springs which they say don't need shimming. I need to go through my stock of springs and check them for poundage. Maybe I can reuse some of them.
Les Rose

All good, sounds like you're on top of it----
William Revit

I wonder when the Mini Se7en Racing Club runs its Se7en and Miglia race series as 850cc and 998cc engines respectively what were the max valve sizes and which cylinder head castings were used?

When Mike Garton wrote his book ‘Tuning BMC Sports Cars’ I wonder what max valve sizes he recommended then for small bore engines. Must fish out my copy.

The Morris Minor Owners Club free to join internet discussion forum also has useful chat on small bore engine tuning, e.g. https://board.mmoc.org.uk/viewtopic.php?t=71367

Cheers
Mike
M Wood

I can't find Mike Garton's book right now, but David Vizard recommended 1 1/4" inlets and 1 1/16"exhausts. That's what I have now in a CAM4810 head with minimal work on the chambers.
Les Rose

This thread was discussed between 03/05/2024 and 06/05/2024

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