Skimmed Cylinder head

[johncannon]

I have my reconditioned 12g295 cylinder head ready to fit, which was skimmed to remove 60 thou. Do i need to put packing under the rocker assmebly to make up for the loss in thickness?

[les]

I would say yes if you have removed that amount, I believe 60 thou brings the c/r back for use with a 1098. The rocker adjusters will probably cope, but leverage forces may be higher if the pivot point is lowered. I'm not too sure of the last statement, maybe a 'system of levers' type of guy will comment!

[bmcecosse]

At 60 thou you will be ok. The adjusters can cope - sure for the ultimate last fraction of a HP the rockers will not be acting at their absolute optimum angle - and you may lose all of 5 thou of valve lift. You won't notice it ! I have 295 head on my 1098 - can't say it makes a huge difference, but every little helps. To get the best from it you need a bigger carb (which I have) and a decent exhaust manifold and system (which I don't have). I suspect the restrictive exhaust is losing me quite a bit of power.

[Alec]

Hello John,
this seemed to be common, that on skimming a head shims are put under the rocker pedestals. But the relationship and geometry of the rockers is not altered by skimming the head, and this will be adversely affected if you do use shims. Provided the rocker adjusting screws have sufficient thread to achieve the correct clearance then all is well (and 60 thou should be fine) otherwise the best method of correction is to fit shorter pushrods.

Alec

[les]

Do you know why people do it?

[Alec]

Hello Les,
my only guess is that it was a way of overcoming lack of adjustment. If a higher lift cam is fitted also then that makes the geometry even worse. In theory, the rocker pedastals would need shortening when fitting a higher lift cam although I have never heard of it being done. For an ideal mechanical situation the rocker arm should be horizontal at half full lift, but of course some variation will probably have no practical effect. It's just when extremes are reached that the geometyr suffers.

Alec

[les]

I suppose I'm getting hung up on the fact that the rocker shaft is, in effect, sitting lower in relation to the pushrod tops with a skimmed head and is altering the horizontal position thing and was thinking that shims would restore the status quo, but in practice I'm sure you're right about the adjusting screws.

[bmcecosse]

Yes it is sitting lower relative to the pushrod - but the difference on power is so slight it's not worth worrying about ! If you fitted a re-ground cam with a reduced base circle then that would actually help to correct the imbalance. But take on board my point earlier - that the head is not the main power limiter on the 1098 engine - it's the horrible manifold (with the daft power robbing hot-spot) and the small carb - so to get a worthwhile improvement you need to fit a better manifold and a bigger carb !

[les]

You are missing the point, I am not really concerning myself with the power issue, just good old fashion engineering practice, and thinking if there might be any additional forces involved. Although you might be on your own regarding the head comment, breathing involves a combination of things including the head!

[Alec]

Hello Les,
yes, exactly, it is lower in relation to the pushrods, and normally can be adjusted with the rocker screws. What does not alter is the height between the valve top and the rocker shaft when the head is skimmed. By putting shims under the pedestals the rocker shaft is higher in relation to the top of the valves and marginally reduces the full lift of the valve. I'm sure it is very little and if someone is bored perhaps they would like to do the mathematics.

Alec

[bmcecosse]

Not me missing the point mate ! I am a Professional Engineer - and I understand well the relationship of the rocker/adjuster screw/pushrod. But for 60 thou off the head - it's really not important. However - if it offends you so much - just take 60 thou off the base of the pushrods !! That will put it all back in place. Adding shims under the rocker pillars is NOT the way to correct anything !
And I know from many years of Minor and Mini tuning - the very slight increase in inlet valve size from a 202 to a 295 head will make hardly any noiceable difference - the biggest improvement will be the elimination of the 'hot spot' in the inlet manifold - followed by a better flowing manifold, and the bigger diameter carb. Then the head will start to come into play !

[samheath100]

60 thou! god thats a lot ent it just for a skim. Have you had the bores re sleeved and bored too or just the head skimmed?

[les]

Well thats well and truly put me in my place. MATE!

[Peetee]

60 thou! god thats a lot ent it just for a skim

It's the required amout to restore compression according to accpted practice.

[bmcecosse]

My 295 head had 100 thou off - it was then dangerously close to the oil groove - so this was milled out and a tube brazed in. Bit extreme really.