Need Help. Wanted to install 3.8l 3 SU's on 4.2L

I wanted to install a 3 SU carb set on a 1968 S1.5 replacing the dual Stromburgs.
I have the 3 SU’s and the 3.8l manifolds.
I now understand that the cooling systems are different, in regards to connections for water pump and radiator.
So, has anyone done this type modification, and if so, how did you do the interconnection? What parts or hoses used.


Hi Mark I believe you need to scrounge up hoses to connect the small pipe on the inside of the large outlet pipe on the manifold to the fitting on the return pipe on the water pump. It looks like you have a hose on that now that’s a bit short. You then need to connect the big hose off the top right of the rad to the big curved outlet pipe (after the thermostat). The small hose is I believe a bleed hose to let air escape from the block when you fill the system. There is no place on the 3.8 manifold to hook it up, but I believe that if you use a thermostat that has a small hole drilled into the flange,(say 1/8") and placed at the top of thermostat that you install, it will fulfill the same function, and you can just block off the outlet on the rad.

It’s essential that you use the right thermostat, that blocks the return to the water pump otherwise most of the water will not circulate through the rad when hot.

It’s unfortunate that you’ve started with the 3.8 manifold, the 4.2 manifold is much easier to work with. You’re going to have to find a long flex hose for the top radiator hose. You can block the small fitting on the radiator. The bypass has to go to the pump, but the hose sizes won’t be compatible. You could switch to a 3.8 pump, but that will cause plumbing issues elsewhere. You may have some luck finding a hose with different diameters on either end. To locate suitable hoses, you can use the Gates identification guide, which lists hoses by ID and length. it also has photos of every hose they make, so you can judge whether it’s molded correctly for your application:

As for a thermostat, you really don’t have any choice. The only thing you can use is SNG’s 3731/1.


I’m doing this right now
I can see from the photo that you seem to have some plumbing that is not original

There should be a joiner, pipe and joiner that run from the water pump to the firewall installed hanging from the bottom studs of the manifold in two. Places fore and aft

There can be a connection to the bypass as per my photo …… I haven’t yet addressed the overflow tank or the large pipe yet …but that should be relatively easy

Thank you. So no worries on overall cooling by using the S1 thermostat?
It appears the later S2 type provides better flow perhaps and more thermostat choice?

This may be mostly about hoses but expect the 4.2 to have two water ports that have to be plugged or covered. I once made an aluminum plate like an intake manifold to cover those holes, Most recently, I had core plugs put into machined bores when the head was off. My blank off plate worked but I was only running 4 psi at the radiator cap.

I used a brass adapter threaded in to the bypass port. Measured what I wanted for a long curved outlet hose and found something close at a local auto parts. Made my own adapter for the vent hose (probably not necessary)

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If you had used an S1 4.2 manifold, you could have used the S2 thermostat extension and cover. That would have allowed you to use the double poppet S2 thermostat, which has good availability and a wider choice of options. It would also have been compatible with the standard S2 hose set. Unfortunately, the S1 3.8 manifold can only work with a sleeve-bypass thermostat. That gives you only the one choice. It’s not a tragedy, as long as 3731/1 remains available.

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Shorter Michael: Buy three!