1275 head on 1098 engine ?

[jameswallace]

Hi,

I'm looking to change my current leaded head for an unleaded, and whilst at it, considering a 1275 (12G940) head for extra power. Would this work ? and if so, what else would I need to do ? And if it does work what would it do for power and economy ? I've been trying to make sense of Vizard's book on this sort of thing and couldn't find enough info.

Jim

[bmcecosse]

It works a dream - but you are best to have an HIF 38 (or even 44) SU carb on an alloy inlet manifold - and the best exhaust manifold and system you can afford - but it will work with standard. Must use 1275 head gasket and file a bit off the top edge of the water pump - and be sure to line up the rockers nicely over the valves ! Problem with this head is that the exhaust valves will bang the block on a 1098 engine - you MAY get away with it on a 948 engine cos the miserable camshaft in that engine has less lift on the exhaust lobes!! You can either sink the exhaust valves into the head by 40 thou (not sure if this is ok with 'unleaded' inserts - OR, grind little pockets (40 thou deep) in the block top face under the exhaust valves to give them clearance. My advice - get a normal 'leaded' 12G940 head - and sink it's exhausts by that 40 thou - and then just run it with 15 thou valve/rocker gaps and keep an eye on them. It will likely last for a good 10,000 miles before the valves even start to 'recess' - and then you can just keep adjusting the gaps- for a good while anyway, while you look out another head! Obviously - use of a decent 'additive' will help to keep recession at bay - the only one with Lead is Tetraboost, the others are pretenders with sodium/potassium compounds which don't work nearly as well as real lead!

[winger300]

On my 12G940 head it didn't have the water bypass hose that connects to the water pump, so in the end I drilled some holes around the thermostat to allow water to pass even when it's closed... and of course replaced the water pump with a suitable one without the bypass connector.

As an extra safety check, I painted the bottoms on each valve, fitted the head without gasket, set the valve clearance to zero and turned the engine by hand. After that, removing the head gives you an idea of where the cylinder makes contact with the valve. On mine a couple of valves made contact before they were recessed, and afterwards there was no contact. The added valve clearance and head gasket raise it enough to give you peace of mind.

Why can't an unleaded head have the exhaust valves recessed? My head was from a 90's mini, sold as 'unleaded' but to be honest I never checked. I took it to the local engine shop to sink the valves, and they did the job, never mentioned it.

[bmcecosse]

Ahhh - good news then - I just wondered if the 'hard' unleaded seats could be easily deepened. But - was it really unleaded? The seat inserts would be obvious when you removed the valves. Excellent to be without the bypass - one less thing to go wrong - but don't drill too many holes in the stat if you want a good heater - in fact NONE if you want an excellent heater - but then you MUST keep the heater valve open. So - how do you assess the performance improvement with this head - and what other mods does your engine have ?

[Matt]

There are 2 sorts of unleaded though... the original rover ones apparently dont have inserts, they are hardened some other way and if you recess the valves you would need to run unleaded.

Hardened seats should be able to be recessed but i guess you need equipment to cut valve seats

Just what ive picked up from information from this and other forums, not sure its 100% accurate

[bmcecosse]

Well - i really doubt if the early heads are truly 'unleaded' - I think they just uprated the valve material slightly and crossed their fingers. But later heads DO definitely have obvious seats fitted.

[winger300]

I didn't remove the valves left that to the engine shop.

My only way to assess the performance difference is by driving. It certainly feels more spritely. I don't have any more mods, I think it all cost about £100 in the end, and I needed a new Cylinder head in any case.

Actually, since I replaced the HS4 carb with the HIF38 the car feels a little less powerfull, but I think that is down to the difference in the throttle linkage. I now have to depress the pedal further to get the same throttle opening, and it just feels like there is less power. I was so used to cruising on the motorway at 65mph and barely touching the pedal, but at full throttle there is no difference, and of course there is more control of the power at lower speeds which helps in traffic.

[jameswallace]

Thanks for your thoughts on this issue gentlemen - It's good to know that there is that amount of experience and know how out there. Unfortunately things have moved on from my first query and am now looking at changing the whole engine which includes the possibility of an upgrade to the 1275 plus gearbox - hence the new question submitted above - if any of you have any thoughts on that then they would be appreciated.

Jim