9 replies to this topic

[austinmac2]

Can anyone help with their experience on this one.
998 engine rebuild with 940 type head and gasket.
For a veriety of reasons I may need to let the pistons come above the block deck height.
Max revs 7500 with limiter fitted.
Vizard states 0.015" rod stretch at 9000rpm, the 1275 gasket being 0.040" fitted should give enough to play with.
However I have had conflicting advice on this.
I have been told Vizard is way off the mark and in fact should keep the pistons below the block deck by 0.006" otherwise I will hit the head!
As many seem to deck the block and pistons flush this advice seems at odds with this practice.
Has anyone out there got solid advice based on actual builds?
Please help.

Paul N-M

[Ethel]

I can't see an issue, valve clearance is the problem with putting a 940 on a small bore, when the valves are that open the piston will be nowhere near the head, and a head for a bigger bore surely has more piston clearance at the circumference?

[Cooperman]

You'll be fine so long as the pistons don't protrude by more than about 0.005". That will give you 0.020" nominal minimum clearance with a standard head gasket, assuming 0.015" rod stretch, and you wouldn't want to go much less than that.
The head design is such that the heart-shaped chamber does mean there is a flat part of the head surface over the piston face, but so long as David Vizard's 0.015" stretch figure is correct there will be no conflict.
One assumes you have a pocketed block.

[austinmac2]

Thanks for input.
The doubt is wheather Vizard is correct and his rod stretch 0.015" was for engines running to 9000rpm. I am proposing 7500, a hell of a lot lower and even less stretch I would think.
The block is pocketed.
The main reason for going above the deck height is to get more c/r if required after all pocketing, head work, block releiving etc has been done.
I was wondering if anyone out there had tried this and knew from experience the minimum clearance needed at 7500 or thereabouts?

Paul N-M

[Cooperman]

Can you not skim the head a bit more to get the necessary C.R.? That is the normal thing and the 940 casting can be skimmed quite a lot really.
What sort of CR are you going to run?

[bmcecosse]

You won't be struggling for CR with a 940 head on a flat top piston 998! I ran a 998 regularly to 9000 with an S head - and they didn't smack the head.

[austinmac2]

Thanks guys,
My problen is a little more complicated than just skimming the head.
My haead is a 940 type but American made 5 port aluminium. The chamber volume with valves and plug fitted is 21.5cc (even though the maker states 23cc ihave measured several times and keep getting 21.5) However the chamber depth is only 7.2mm and with exhaust valve fitted 5.7mm from valve to head face!!
There is also some shrouding round the valves which could do with opening out increacing the chamber volume to about 23.5cc at an educated guess.
Add the block pocket volume at 4mm deep (kent 286 with 1.3:1 rockers), gasket and any block releiving for inlet valves.
I am looking for 10.5:1 not any more, bore is std 998. The engine is a 948 bored to 998 using A+ rods and pistons.
If any of you knowledgeable types out there can advise on what the c/r is likely to be please do. I calculate I will need some piston above the block to get near 10.5:1 though.
Any help appreciated.

Paul N-M

[carbon]

The exhaust valve lift with 286 cam and 1.3 ratio rockers will be about 10.7mm

The 'working clearance' you have for the exhaust valve before it clatters the bottom of the cut-out is 5.7 + 1 + 4 = 10.7mm

Have you done a dry build using plasticene to check the valve-block clearance?

[bmcecosse]

So - it's a MInor engine ?? I hope you have put cam bearings in the block.....!!!!!. Better to use a 1098 block that already has the bearings... Don't use the high ratio rockers - the 286 has plenty of lift for a 998. And you will struggle to make the pockets deep enough as it is without exposing the top rings - I know - been there/done that!! So don't skim the block down.

[austinmac2]

Thanks for comments.
As yet I have not machined the block for pockets as I need to work out how much depth is required. It looks as though a 286 with 1.3 rockers is going to be very tight!
A very carefull measure is now required.
Hammill when referring to pockets in 998's recommends not chasing c/r over pocket depth.
Much appreciate all input so far.

Paul N-M