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MG Midget and Sprite Technical - Cylinder head swap


Went out for a blast yesterday - blue skies, relatively ice free roads, bit chilly - to test the new head. This is I believe an MG Metro head - 35mm inlets and no bypass, a little porting work but single valve springs - as opposed to the fully ported and polished previous set up - which had 33mm inlets and was otherwise pretty standard.

Some observations

It revs a little more freely and possibly has a little more acceleration and top end speed.

Very slight hesitation when floored. Not much, but there...Nothing previously..

Slight step change - for the better - at around 3500 rpm in terms of acceleration and torque. Previous was smooth all the way through rev range.

Possibly a liitle less torque low down, but very difficult to perceive.

Some Qs

Would timing need to be adjusted? I though it was pretty good beforehand, and saw no reason to adjust it.

Adjusted fuel mix slightly - probably fiddle about a little more after more trials to ensure it is spot on

Plugs - any reason why these should change?

Anything else which may require adjusting to suit the new head?

I'll check the tappets again after another few runs.

Mark O

Timing change:
Depends on CR change
Depends on timing and CR
Depend on CR

So as you see with the changes you made most is dependant on the possible CR change.

As for tuning It sounds like you are in the ball park.
Try advancing the timing a bit and see if she pinks.
If not do some more ;)
Then finetume the mixture as should be done on your carb (weber SU?)

Onno K


Cannot imagine CR has changed that much. Carb is HIF 44.


Mark O

"Cannot imagine CR has changed that much."

That would depend on whether the head has been skimmed.
Dave O'Neill 2

as Onno and Dave

and I'd check the tappets straight after fitting the new head and later after a good few miles

if you adjust tappets then you'll need to check/adjust (CB points) - plugs - timing - carb in that order
Nigel Atkins

It dosnt take much ither way to seriously change the CR...

With the single valve springs your probably experiancing valve spring bounce at the 3500 rpm level for the step down ... im just guessing thats probably also where your harmonics kick in, so you would notice the valve bounce at that point more then any other point in the rpm range...agian, pure conjecture on my part

Prop and the Blackhole Midget

If you did not check the volume of both heads you will not know the CR change until you do.

So it is all a bit guess work.
No problem but since you are the only one tinkering and driving it is hard to give any more advice than basic tuning advice.

So just change 1 thing at a time and take notes.
If it does not work reverse and try something else.
Onno K

I measured the volume of a chamber of each head with oil beforehand - virtually indentical. Head skimming - well unless they have really gone at it with a large band saw, it usually does not make an appreciable difference. Block and pistons as before.

Yes - appreciate the valve bounce - fair point....

I do think it is a question of fine tuning rather than anything major....
Mark O

What arrangement do you have on the carb intake side? It may now benefit from induction bellmouths to smooth the air flow. Moving to larger valves will get more fuel mixture into the engine, but can also slow the gas flow speed which will upset the air flow and possibly weaken the mixture a little, so it may now benefit from a needle change.
Guy W


Yes...been umming and ahhing about bellmouths. ATM, simple K&N pancake filter on HIF 44 inlet....needle change - standard BDL in at present and perhaps BBW may suit better - what about enriching a tad?


Mark O

This thread was discussed between 09/02/2015 and 10/02/2015

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