Date: Wed, 15 Jul 1998 10:09:02 -0400 (EDT)
From: "Isca"
Subject: 998 pistons
i have heard that the cut outs in the tops of the pistons need to be bigger if you are using a big valve head.
is this true?
but surely it is also affected by the lift of the cam. or not?
therefore.................
(i) how can i tell which pistons i have got?
(ii) can they be machined for larger cut outs or must i buy new pistons?
any help will be most gratefully received
many thanks
martin bristow
devon, uk
Date: Wed, 15 Jul 1998 17:40:07 +0100 (WET DST)
From: Simon Trickett
Subject: Pistons
Hello,
998cc pistons came in two types....... ones with cut-outs for sport size valves and ones that had cut outs for the 1.4" Inlets and 1.125" Exhausts. The goods ones should have 11/16" pins as well.
You can have the smaller ones modified and it is a good idea to have them a little deeper as well. Ian Carter is the man for this, his details are as follows:
I.R Carter
Unit 14
Westbury Close
Townsend Ind Est
Houghton Regis
Dunstable Bedfordshire
LU5 5BL
01582 604728
Simon Trickett
Date: Thu, 16 Jul 1998 02:54:02 +0100 (WET DST)
From: Paul Greville
Subject: Re: 998 pistons
Hi Martin,
Increasing the size of the valve cut outs is a very good move as it removes the shrouding effect caused by the small cutouts around the larger valves and will stop the valves and piston tops meeting. I would also suggest that you check very carefully the clearance between the top of the piston and top of block. If the block and head you are using have been machined a few times you WILL have clearance problems and an increased compression ratio.
I would suggest that you take the pistons, block etc to a good machine shop and check the above details before assembly. I have succesfully machined quite a few thou off the top of 998cc pistons to provide clearance and lower compression and have increased the cutout size and depth without any concerns being created, however unless you are very competant and have access to a good machine shop this work is best left to experts - IT IS CRITICAL TO CAREFULLY MEASURE THE THICKNESS OF THE PISTON TOPS BEFORE MACHINING COMMENCES.
Regards,
Paul Greville
Kalgoorlie, Western Australia.
Date: Thu, 16 Jul 1998 11:31:22 +0100 (WET DST)
From: Simon Trickett
Subject: RE: 998 pistons
Martin,
Yep what Paul says is correct......I have just had to have 0.012" taken off the top of my pistons to bring them to the correct height in the bores.
This is especially important if you are running Wills Rings as they can give a compressed height which is half that of a Reinz gasket The compressed height of a Reinz gasket is 0.032" The compressed Height of Wills Rings is approx. 0.018" to 0.020" (on a good day!)
If you are running Wills Rings, the pistons should not come higher than 0.010" down form the block face.
hope this is of some help
Simon Trickett