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24 July 2003 |
There is no specific large-bore (1275cc-based) head
casting number that will fit the 998 engine any easier than any other. They all
have the same problems - generally requiring exhaust valve relief cut-outs
machined into the block and re-alignment of the front water gallery transfer
ports. However, I would avoid using heads with valves bigger than 35.7mm on the
intake since these are too big for most 998 engines unless they are absolutely
full-race spec where top end power at high rpm is all that is required.
First of all it is worth checking that you actually
need to make the relief cut-outs since some heads have a big enough head face
to exhaust valve face clearance to allow fitment without the cut-outs when the
standard cam and rocker gear is retained.
If possible, measure the actual valve lift you are
getting with the current fitted set-up. Carefully set the valve lash clearance (clearance
between valve tip and rocker) to 0.012-in./0.30mm. With an exhaust valve in the
fully closed position, position a dial gauge (DTI) on the valve cap, zero the
gauge and then rotate the engine until maximum valve lift is achieved -
counting the amount of lift indicated by the gauge. This is likely to be around
0.270-in./6.86mm. The maximum you are likely to see is 0.280-in./7.11mm if you
have a particularly good set-up since the standard 998 camshaft will only have
0.235-in./5.97mm lift at the lobe, the maximum rocker ratio is likely to be
1.25 - the valve lash setting being taken from the result. So 0.235-in. x 1.25
= 0.294-in., less the 0.012-in. lash setting gives 0.282-in. BUT it is rare
that the standard sintered steel rockers give anything like this lift ratio.
From personal experience the best to be expected is 1.22 - so the actual nett full lift will be less.
If you can't check the valve lift because the head has
already been removed, you will need to check the lift at the cam using a DTI positioned
on a push rod placed in one of the exhaust valve followers riding on the
camshaft whatever reading you get, multiply this by the rocker ratio and minus
the valve lash setting detailed above.
With the 1275 head upside down, accurately measure the
depth of the exhaust valve face in comparison to the head face. This
measurement needs to be at the very least the same as the exhaust valve lift
registered as above - preferably greater. It is possible to increase this
distance by cutting the exhaust valve seats a little further into the head
and/or removing material from the face of the exhaust valve. I would only look
to gain around 0.030-in./0.76mm by doing this though. Alternatively, going to a
race-spec exhaust valve may nett
some more clearance since the valve seat edge to valve head face on race valves
tends to be less than standard OE type valves. If doing this when using the
standard cast iron guides, it is essentially you use either a chromed-stem type
competition valve or one of the later nitrocarborised/Tuftrided
type valves (charcoal grey/black in appearance.
All of the above is totally affected by camshaft type
and rocker gear used. 1.5 ratio rockers simply don't work in general on
road-going 998s in my experience so should be avoided. The probable maximum cam
lift you may be able to entertain using the above criteria would be
0.250-in./6.35mm as used on the more 'performance' standard profiles such as
the MG Metro, standard 1275 Metro and older 998 Cooper and S cams. In which case you'd be looking for around 0.293-in./7.44mm
clearance between exhaust valve face and head face to avert using cut-outs.
The head gasket is your safety margin - so don't add
this into the equation at any point. Crushed as fitted it gives around
0.028-in./0.71mm to 0.032-in./0.81mm thickness - just enough for that safety
margin. Don't ever over-rev the engine or miss a gear-shift though!
And talking of head gaskets - you will need to use the
1275 item to get a reasonable alignment for sealing both the combustion chambers
and the front water gallery transfer ports in the head. The 998 head gasket
over-hangs the 1275 head combustion chambers on the exhaust sides and doesn't
give as good a line-up of those water gallery ports.
It is prudent to plug the front water gallery transfer
port holes and re-drill them using a 1275 head gasket as a template. This is
pretty easy even with the engine in the car by tapping the holes then fitting
suitably sized grub screws, which are then drilled to allow water through in
line with the 1275 gasket. Crude but effective and means the block face doesn't
need re-surfacing afterwards. If this realignment isn't done, the head gasket
is likely to leak water out and down the front of the engine.
If cut-outs are necessary in the block, the easiest
way to establish their relative position is to use the 998 head gasket. Place
on the block using head studs for alignment. Scribe around the outer exhaust
edges of the gasket onto the block surface. Use a fly-cutter of suitable size
to machine the cut-outs in. It is possible to use a die grinder to make these
cut-outs, but only in desperation! The depth needs to be sufficient to clear
however far passed the head face the exhaust valve will be when the valve is at
full lift. To establish this simply subtract the exhaust valve face to head
face measurement from the full lift measurement. To be safe I would add
0.040-in./1mm to this depth to ensure no valve to block contact despite using
the gasket as a safety margin. And avoid going so deep as to intrude on the top
ring land.
Useful part numbers:
|
C-AEG105 |
29.5mm
competition spec exhaust valve triple collet groove type |
|
C-AEG106 |
29.5mm
competition spec exhaust valve single collet groove type |
|
TAM1061 |
29.2mm
standard OE spec exhaust valve triple collet groove type |
|
CAM4601 |
29.2mm
standard OE spec exhaust valve single collet groove type |