33 replies to this topic

[kyle9832]

I have recently started testing out everything for when I start to build my engine in a few weeks and have come across a strange problem, when I push the valves down,  about half way down the travel of the valve it hits the top of the block, it is a 12g295, im not sure what it has been skimmed to but has a chamber volume of 23.5cc, it has 32X29 valves, I am just wondering if anyone else has had this problem on a 998? using a 998 head! and how to overcome it! many thanks:)

Edited by kyle9832, 11 June 2015 - 09:56 PM.

[blacktulip]

Measure the thickness of the head. Standard heads are 2.75" thick. The 12g295 head should have 28cc chambers. Yours must have had a fair old chunk taken off to achieve 23.5cc. I'm surprised you haven't gone into the oil gallery. Are you testing the lift via the cam lift?

[kyle9832]

Measure the thickness of the head. Standard heads are 2.75" thick. The 12g295 head should have 28cc chambers. Yours must have had a fair old chunk taken off to achieve 23.5cc. I'm surprised you haven't gone into the oil gallery. Are you testing the lift via the cam lift?

well the head was done by vmaxscart all work carried out by himself, I was just pushing the valve down with my hand and it doesn't even reach halfway down the lenghth of available movement, and I have a 276 cam going in which is relatively high lift so am a bit worried about it hitting the block, even if it does not with the cam in, the gap will be very small!

[Spider]

It does happen from time to time due to the coff coff precision manufacture & machining of the castings.

 

You may have to pocket the block, but should be done on all cylinders to keep them all the same.

 

If you haven't invested too much in that block, try another, though I suspect the head is more off than the block.

 

Another possible way around the problem is to fit blank guides fitted and have them machined such that the valves 'angle' towards the centre of the cylinder, the seats will also need to be re-cut too of course and it usually has a HP benefit as well, but that depends on what's been done to the ports.

[sledgehammer]

Are they wider than standard 12g295 valves ?

[Turbo Phil]

It's because of the valve sizes used. The 29.4mm ex valve will catch.

Phil.

[kyle9832]

Are they wider than standard 12g295 valves ?

yes, standard inlet is 29mm I think, these are 32

standard exhaust is 25 I think! and these are 29..

didn't realise the standard ones were that close!

[kyle9832]

It's because of the valve sizes used. The 29.4mm ex valve will catch.

Phil.

after trying again yes I have realised it is just the exhaust valve.. what should I do about it?

[blacktulip]

What size pistons have you got?

[kyle9832]

It does happen from time to time due to the coff coff precision manufacture & machining of the castings.

 

You may have to pocket the block, but should be done on all cylinders to keep them all the same.

 

If you haven't invested too much in that block, try another, though I suspect the head is more off than the block.

 

Another possible way around the problem is to fit blank guides fitted and have them machined such that the valves 'angle' towards the centre of the cylinder, the seats will also need to be re-cut too of course and it usually has a HP benefit as well, but that depends on what's been done to the ports.

I have had the block skimmed, bored out and honed so I don't think another is an option, also the head has been ported very well and has new valve seats ect

[kyle9832]

What size pistons have you got?

+60 nural flat top pistons

[Turbo Phil]

Your options are either pocket the block, reduce the valve size, recess the valve or reduce valve lift.

Phil.

[kyle9832]

Your options are either pocket the block, reduce the valve size, recess the valve or reduce valve lift.

Phil.

what would you recommend in this case?

[Turbo Phil]

By far the cheapest/easiest would be to recess the valve into the head. But you can only do this a limited amount, so it will depend how much clearance you require.

Phil.

[nicklouse]

First I would be assembling it and make some measurements to check that you do actually have a real and not perceived issue.

then there is the other option of sell head and get another more suitable.