Machining of a 2.25 head

[John-2a]

Hi Guys and Gurls

Can anyone here advise of a correct skimming/machining depth for increasing compression on a 2.25 (original) engine, I was advised to add extractors a webber carby and machine the head, just cant remember for the life of me what depth was suggested...

If anyone can shed some light it would be most appreciated

Cheers

Johnno

[JDNSW]

Well, the first thing to do would probably be to check whether it is a 7:1 or a 8:1 head to start with, and also check whether, if it is a 7:1 head it is one of the later ones that have enough material to be safely skimmed.

The heads that can be skimmed have a rectangular boss on top between the rocker cover and the carburetter that should have either a 7 or 8 on it. No boss = no skim. According to my calculations 1.5mm should raise it from 7 to 8:1. It should be OK to go a bit higher than that, but you may have problems running it on ordinary unleaded. Any outfit competent to do the machining should have access to the correct data.

Raising the compression should improve power and economy without affecting driveability at all. Extractors, and skillful port machining will increase maximum power, but will possibly reduce flexibility. I'm not too sure about the Weber though - I have no experience with them, but understood they were preferred for economy rather than power. Someone on here was recommending an SU for power, but that needs a special intake manifold.

Regardless, you need to make sure that whatever carburetter you use, that it is in good condition - which if it is the original, it probably isn't, whether Solex or Zenith. Both can be overhauled to new condition, though.

You need to understand that this engine was designed primarily to function reliably, flexibly, on steep slopes, and on poor quality fuel if necessary, with power and economy pretty well down the list of requirements. Certainly these can be improved, but you are likely to lose some of the other design properties doing it.

John

[Michael2]

There's a "thumb print" recess on one corner of the head, don't machine beyond that depth.

The early heads can't be machined as much because that thins the wall to the water galleries.

It's all a bit more complicated than just - "skim the head."

The greatest improvement will come from doing the cam. I've done the cam, extractors, head skimming and weber on the 2.25. Grinding the cam to the 6cyl cam specs which open all valves (inlet & exhaust) 5 degrees sooner and close them 5 degrees later, allows the motor to breathe a whole lot better. In fact after the cam grind, the weber hardly made a difference. Save the money and time it takes to fit a weber (and modify the manifold) and spend $100 - $150 on getting the cam ground.