Advice on oversized exhaust valves?

[twenty valve]

Hi all,

Going to be fitting a new set of valve seats and valves into an ABY head. Thinking I may as well go oversized as its a high power engine. I'm looking at valve seats from Serdi (https://www.serdi.co.uk/engine-compo...-seat-inserts/)

I was thinking to go 1mm bigger on the OD and 1.5mm on the ID (making the wall thickness of the seat slightly thinner to preserve the material between the inlet and exhausts in the head), then run a 1.5mm oversized exhaust valve. Inlets I'm going to keep stock.

Does anyone have any advice/recommendations or experience on the subject? I'll update when I'm finished for reference

[twenty valve]

Update incase anyone is interested;

These are the measurements I took from my stock ABY head

Inlet:
Cylinder head bore diameter: 32.53mm
Valve seat OD: approx 32.66mm
Valve seat ID: 27.00mm
Valve seat depth: 6.5mm
Valve seat wall thickness: ~2.83mm

Exhaust:
Cylinder head bore diameter: 27.53
Valve seat OD: approx 27.66mm
Valve seat ID: 23.00mm
Valve seat depth: 7.0mm
Valve seat wall thickness: ~2.33mm

I don't know what the stock material types are for each seat, but there is a difference; the inlets seem to have a copper/bronze element added whereas the exhausts dont (the stock exhaust seat material is definitely harder).

I decided to be conservative with regards to enlarging the OD of the valve seats because I wanted to preserve material in the cylinder head. As many will know, these heads are renowed for cracking between the valves seats, and between the valve seats and the spark plug threads (I happened to have 9 cracks on this head; 3 deep cracks into the spark plug threads, and 6 superficial cracks between the valve seats).

I therefore enlarged the OD of the exhaust valve seats by 1mm and the inlets by 0.5mm. The ID's have been finished at 1.5mm and 1mm respectively. This might not sound like a lot, but this equates to a 12% increase in surface area on the exhaust valves, and 7% on the inlets. I bought the seats from Serdi (as per the previous link), both inlet and exhausts are sintered, but the exhausts were a slightly harder material.

I'm still looking for valves so if anyone can recommend a supplier for oversized valves that would be appreciated.

[twenty valve]

Machining out exhaust seats

[twenty valve]

Machining out inlets; colour of the swarf material noticably different

[twenty valve]

The worst of the spark plug thread cracks ground back

[twenty valve]

other cracks ground back ready for welding

[twenty valve]

Welded

[twenty valve]

head bores remachined

[twenty valve]

Machined - just have to clearance and re-tap plug threads. Then hand finish combustion chambers and balance volumes after valves are fitted

[SteveH]

Nice work. I've not seen anyone on the forum do this before. Certainly not any pics of the process. Hopefully worthwhile as I bet it would cost a fair bit to have this done by a machine shop.

[AndyP]

Have you done all this work yourself?

[mac2226]

What andy said^^ as I have a head what needs this work doing. And thanks for sharing pictures

[Error404]

Nice work. You’ll need to weld up and reshape the ports to make the most of them. Also, seats will need recutting to suit the valves. Ferrea make 0.5mm oversize
HTH

[twenty valve]

Thanks for replies, glad you've found it helpful. Yes I did the work myself, I used to do machining and my mate has a workshop with a bridgeport that he lets me use.

It's hard enough to find cylinder heads these days, leave alone crack free ones. And in reality, even if you find a crack free one (such as a 7a) and build it with a big turbo, it will almost certainly crack (I've seen it happen numerous times). So aside from time and expense, you may as well repair one like this. Although I would imagine it's an expensive process if you pay someone to do it. I'll update again when it's all finished and looking shiny

[Error404]

Id imagine you needed to clock each seat individually when you bored them out? Time consuming job with a boring head like that!