7 replies to this topic

[AreyParey]

Hi all,

Just a quick question..
I'm looking at a 295 head which apparently has "1300 valves" on it.
I have a standard 998 and understand the block would need to be pocketed to accommodate larger valves(?), and I don't really want to do this..
So I am wondering if it is possible to fit smaller valves? And what it needs to be done?

Sorry if this question is a no-brainer ;P

[CMXCVIII]

How would you expect to fit smaller valves into larger holes?

[ginsters]

Get yourself the vizard book,as the last answer you can't put small things in big holes.

[AreyParey]

Didn't think you could! Thanks for the answers

[ACDodd]

That's no strickly true, that depends on how well the work has been done. How do I know, I have done quite a few downsizing jobs. It is not cheap. Seat Inserts can also be used, Again been there done that.

With the 295 head, you need to measure the diameter of the exhaust valve. it is usual to machine down the 1275 exhaust valve to 26.5mm to do this conversion. SO before you do anything measure what you have first this will lead the way to what needs to be done.


If people are going to answer questions please enure that you have the necessary experience to answer or state in your answer if its only your opinion. Many thanks.

AC

Edited by ACDodd, 03 January 2013 - 01:46 PM.

[Wil_h]

Fitting 1275 valves to a 295 is pretty common as the 295 valves are NLA. So in most cases it's done for practical rather than performance reasons. Sounds like someone is trying to make you think the had is something special, when it might not be.

As it happens 29 and 33 valves in a 295 works great on any 998 and will fit within the bores with no need for pockets.

[ACDodd]

Again the 29mm exhaust won't fit without pockets, they need to be reduced in diameter before fitting. Please stop with the misinformation on this thread.

AC

[Gr4h4m]

. I run 27.5mm exhaust valves that had been slightly sunk into the head (These are turned down 29mm valves minispares 1275 stainless jobbie's. IIRC I had to use 3mm shims under the rocker posts to correct the rockers as the valve s are taller than the original 295, therefore it might be a good idea to replace all of them for taller 1275 versions. After some discussion with Paul Ivey when I was trying to purchase some rimflow's he recommended not going over 26mm if on a N/A engine. however the larger valves work well on my supercharged engine.

Edited by Gr4h4m, 03 January 2013 - 10:46 PM.