I have a pair of pre pollution heads that have bigger valves, enlarged ports etc that have been sitting in the shed

for years that only travelled around 2000 miles.

My question is will the larger 1.94" valve hit the top of a standard 3.625" 253 bore.

I also plan on fitting a mild street cam and a rebuilt quadrajet with standard inlet and outlet manifolds. Has factory style dual exhaust fitted.

I have been getting differing advice. Some saying the 1.94" valve requires milling a 2.000" dia relieve on the block deck offset in line with the valve centre. No mention of the depth of cut into the block deck.

Others saying this isn't needed unless running more then .500" lift.

Some also mention shrouding problems with these larger valves on a 253 opposed to a 308.

Want to keep the 253 as it's the original motor to the HG but not keen on milling into the block deck.

Am I wasting my time contemplating using these heads or can they be used safely with a bit of a increase in performance?

Thanks in advance

Jim

I agree with Dr Terry. These heads are designed for a 308, so on a 253 you don't need bigger valves. Not on a street engine anyway. A 253 with pre-HX manifold, Quadrajet and dual exhaust will be fine with standard heads.

Make sure you get the right Quadrajet too. If you are going to use an early one (pre 1/75 Holden Quadrajet) use the right one for your gearbox, for an auto use an even numbered carby and manual an odd number. They have different vacuum advance and you do not want an auto's vacuum advance on a manual car and vice versa. Once you get to 1/75 LH and HJ the carby is the same for both applications (7045281) and has both ported and full vacuum for the dizzy. What is probably easiest for you unless you want it to look totally original/authentic is to use a VC/VH/WB 4.2L carby, these will have primary metering already setup for a 253 and have the choke mech on the carby already rather than having to plumb up the exhaust manifold to the choke mech on the manifold. Just buy an electric choke operator for it. 17059596 CXL is the blue motor 4.2L Quadrajet. You have to block up the original choke mech on the early manifold if going down this path and non essential but a good idea, replace the driver's side exhaust manifold with one from a HX or HZ if the choke pipe in the original HG exhaust manifold is leaking.

That is a HJ or LH Torana manifold that has been drilled for the extra water hole for a HT-HG. The drilled and tapped boss at the front just behind the water plug is for a HJ or LH cable throttle support, not drilled on a HT-HQ. You'll need another drilled and tapped thread at the back for the return spring on a HT-HQ - you can see it in the first photo just near the 8, look on HQ manifolds and you'll see it drilled and tapped.

The carby is a pre 9/72 HQ auto carby. Will be fine for a HT-HG but you'll need to get it built for a 253, these have different parts in the primary circuit, secondary is the same from memory.

That's what I get for relying upon memory! You are right the front one is drilled for both pre-HJ and for HJ, it is the back one that isn't drilled on pre-HJ. So probably those manifolds you have are both HT-HG, or at least HQ ones that have had the water hose point drilled and tapped. That single drilled and tapped mounting point at the back for the cable throttle return spring bracket is the only difference that I know of between HQ and HJ/LH manifolds, and then the only difference between HT-HG and HQ is the HT/HG one has the boss drilled and tapped for the water hose. The different casting marks and casting numbers move and change over time but none of that is really relevant to you as a 253 never had a 4BBL anyway! That one you have in the first picture is perfect for what you want to do. f you really wanted to go super anal and make it look like a real HG 308 you'd want a 7029282 carby.

You might have to fiddle with the carb's internals for a 253. Get hold of a WB or VC-VH parts catalogue and you can see what is different between a 4.2 and 5.0L Quadrajet - there isn't much, mainly primary needles IIRC.

Try it as is as you have to pull early ones apart to change it, but when it is all setup on a dyno you'll probably have to make some changes, but if the engine has been modded the WB/VC-VH tune may not be exactly right. 253-4.2 changed a little over the years mainly in compression ratio (via pistons) and the dizzy settings, so take all that into account too.

I'd just try the carby as it is. Check your ignition timing versus a WB or VC-VH auto 4.2, I think these had slightly more compression than a HT-HG 253 so may have less total advance. Cam should be the same but retarded a bit in the blue engine. GMH didn't change the carby specs for single or dual exhaust. So the same carby setup in the WB/VC-VH 4.2 carb would probably work, but just ignore the idle circuit components as these started to be leaned off beginning 9/72, just focus on the primary and secondary main fuel delivery items that differ between 4.2L and 5.0L on the blue engines.

But i'd still just try the carby as is, get it all running OK, run your cam in and and dyno it where the dyno operator understands how to tune Quadrajets, then make whatever changes are needed.