twin carburettor upgrade

Ive been searching for a while for an inlet manifold to go with the pair of period twin SU carburettors and finally found one going cheap on eBay. To be fair people make these for £85 but I am trying to keep costs down so ebay is my friend. The trouble is with these manifolds that the original one fitted to the car has 8 mounting bolts and the twin carb manifold has just 2 bolts – yes only 2 and they are inside the manifold itself. So an engineering solution is required.

You can see the difference in the picture below – the original one is on the right

So THEORY 1 – “make up a spacer plate to bolt to the head with integral studs to bolt the new manifold onto”. So i crafted a template from cardboard and took it to a machine shop for a quote.

Oh thats dead easy they said, leave it with me and I will give you a quote. 10 days later still no quote, so I called them to see whats up. Sorry they said, bit busy haven’t looked at it yet and might not get round to it for a couple of weeks… Seriously? i said… it really can’t be that hard to price up having to drill 12 holes… so I walked away and considered making it myself out of the 9mm thick sheet of steel I have. But to be honest, the thought of drilling out those 4 big holes (36mm diameter) and then filing them to shape (remember this is 9mm thick steel) made me gulp with the amount of manual effort it would take. So that put me back to theory 2…

THEORY 2 – drill and tap two 8.5mm holes in the cylinder head. Well obviously that is much easier – BUT what is inside the head behind where i need to drill the holes? I put a post on a Riley forum asking this question which raised a storm of outraged responses from crochety old farts about keeping things original – having saved my car from the scrap heap and putting it back on the road which is a good thing in my book –  I dont quite hold with their view so its a good job they dont know what I am doing to the rest of the car then isn’t it (haha).

So after much consideration and to and fro’ing – I thought sod it – I’m going to drill the head, logically it must be okay to doif thats how the factory mounts this manifold to the head. So today thats what I did. Took about an hour to do…

And the end result is this:

Next steps, plugging the old manifold holes in the head (because the muppets at the factory drilled them right into the oil ways – so instant oil leak if i dont plug them…), then gas flowing the manifold so that the ports match the head (improves performance slightly). THEN i have to design and craft the throttle linkage, rebuild the carburettors blah blah… what fun. I’m loving it