Exhaust gas take-off holes in V8 heads

Just been getting a pair of heads ready that I've had lying around and notice that on cylinder numbers 3, 4, 5 and 6 (the inner ones on both heads) there are some threaded holes above where the exhaust manifold is bolted on and going through into the exhaust tract.

On the earlier 3.5's I know some had the pulseair system utilising these holes on all the cylinders but I was unaware of them on later heads. I can't say exactly what age the heads are, but they're probably early P38 / Disco 2 era being definitely 28cc heads but not as well finished as those from the later engines.

Anyone any ideas what the holes are, and more importantly what thread they are in order to block them off?

Thanks.

They are likely to be discovery 2 heads with holes for the secondary injection. They are a really odd thread size. On the discovery 2 there are inserts that screw into the head and a union screws on a pipe to that that is pumped full of outside air on start up. When I deleted my secondary air I welded some bolts in the end of the inserts just to plug the holes for now. I have purchased an engine that doesn't have those holes in the heads.

Hope that helps

D100 said:

They are likely to be discovery 2 heads with holes for the secondary injection. They are a really odd thread size. On the discovery 2 there are inserts that screw into the head and a union screws on a pipe to that that is pumped full of outside air on start up. When I deleted my secondary air I welded some bolts in the end of the inserts just to plug the holes for now. I have purchased an engine that doesn't have those holes in the heads.

Hope that helps

Click to expand...

Thanks, sorry for delay. Confirms what I'd found out that it's the secondary air emissions system. Closest thread I could find was 3/8" BSPF so have drilled them out and re-tapped to take metric. Didn't fancy welding as knowing my luck I'll distort the heads and given the time I've just spent porting them thought that might be slightly annoying. Anyway, they're on now and the next problem needs solving!

I didn't weld them on the heads I removed them and done them on the bench. I take it you took all previsions to stop swarf falling on top of the exhaust valve and then into the bore.

Just re read the first post and realised you had them as spares. Apologies.

No worries, bits of aluminium swarf into cylinders would be interesting... Yes they're one of the spare sets that I've accumulated and decided that they are the least warped so are going on to the rebuild and I had all the valves out to re-cut the seats and port out.

I admire your ability of welding aluminium, the chances of me not blowing holes or warping them would be slim! Have a couple of petrol tanks to create soon and debating what the minimum thickness of aluminium is that I can safely weld without just making a colander!

No I didn't weld aluminium. There are steel I steers that screw into those holes with holes in them and them the sai system screws into the inserts. For now all I did was weld the inserts closed and wind them back into the heads if that makes sense.

Wish I could weld Ali!

I understand now, makes sense and matches what I did on my old pulse air heads on another engine. As I didn't have the original fittings I couldn't modify them this time though hence trying to find out what they were used for and what the thread was originally. Problem solved though now thanks. Now I just need some time to drop the engine back into the Disco!

Ah fair play. There not too bad to remove and refit though.