4 replies to this topic

[Biggles42]

Good Afternoon.
I have finally got round to stripping my 1098 to upgrade it a bit and have got most of the parts. Only a few questions that i need clearing up before i Begin,
I am planning on starting a project thread as soon as i get it re-bored.
The engine plan is as follows:

1098 Block with 40" overbore (already 20")
SW5 Cam
Duplex timing chain
12g940 head
Electronic ignition.
Stage one kit

The questions i am asking is:
How would I work out how much pocketing to do?
what cc's I am likely to get (if anyone has done and knows)
What is the best gear ratio. (Complete novice when it comes to gearbox

Any help would be most welcome.

Chris

[Cooperman]

To work out how much block pocketing is needed you first need to know what the maximum valve lift the cam gives. this should be given by the cam manufacturer.
Then measure from the valve faces to the head surface. If that dimension is more than the max. valve lift no pocketing is necessary. If it is less, then pocket by that amount plus 0.040" for safety. Ignore the gasket thickness as that's is a bit of safety margin. All you need is a digital vernier caliper, but measure all the valves as the seats may not all be at the same level in the chambers.
The new engine capacity will be 1131 cc. This is 282.75 cc per cylinder and you'll need this figure for doing the compression ratio calculations.
Bear-in-mind that when measuring for block pocketing you'll need to have first done the final head skimming for compression ratio setting.
Don't know what compression ratio you are going for, but about 10:1 would be good on your spec. engine.

[bmcecosse]

The SW5 is a very soft cam with unusually high lift - it's really not suitable for use with the 940 head where the exhaust valves will hit the block..... FAR FAR better to use an MG Metro cam (or the very similar 266) - and sink the exhaust valves into the head as required (40 thou or perhaps less - measure it! ) to prevent the touch.
If you pocket the block enough to avoid collision with the SW5 cam - the top rings will be exposed to chamber heat. Don't do it.....

[Biggles42]

Thanks for the advice. I am hopefully picking up the new pistons next weekend and the cam. Ill get the engine re-bored and then decide whether to sink the valves or pocket the head.
The engine is all stripped ready for the re-bore!
Is it better to sink the valves or pocket the block>

[Cooperman]

If you can get clearance by sinking up to around 0.035" just sink them and blend in the chamber face. If not pocket the block. With a 266 cam you might not need to pocket and will almost certainly get away with a 'valve sinking'.
As Roy says, the SW5 cam really is very mild and relies on high lift rather than good overlap, so the Kent 266, or the almost identical MG Metro cam, would be much better.