9 replies to this topic

[daenesh]

hey guys, i would like to know if there is a difference in LENGTH between an A+ triple collet 998 inlet valve and a A+ triple collet 1275 inlet valve? I know the size is bigger 998 is 25.5mm and the 1275 is 33mm.. i'm planning to make my inlet valves bigger on my 998 head and was looking at the 30.93mm ones but they are expansive so a friend said i could get the 1275 ones at 33mm and cut the holes bigger on the head.. maintaining the standard 25mm exhaust valves.. any comments on if this setup will work?

[Dusky]

hey guys, i would like to know if there is a difference in LENGTH between an A+ triple collet 998 inlet valve and a A+ triple collet 1275 inlet valve? I know the size is bigger 998 is 25.5mm and the 1275 is 33mm.. i'm planning to make my inlet valves bigger on my 998 head and was looking at the 30.93mm ones but they are expansive so a friend said i could get the 1275 ones at 33mm and cut the holes bigger on the head.. maintaining the standard 25mm exhaust valves.. any comments on if this setup will work?

Please read into this.

You can't fit the 30.93mm valves without modifications to the cylinder head, the valves wouldn't seal otherwise.

With a standard cam you can use a 12G940 head on a 998 engine. But I don't think you can fit 33mm valves into a 998 head, because there simply isn't enough room to fit them ( you would end up cutting into the exhaust valve).

 

With a non standard cam and a 12G940 head you'll need to pocket the block.

[daenesh]

i am going to cut the valves to make them bigger ..machine the valve seats bigger.. but from what i've read 30.93mm is the maximum size that can be used for a 998cc.. correct me on this.. also im using a 1098 camshaft which i have determined has a higher lift than the standard 998 one

[Dusky]

Welk,you have limited amounts you can cut ;) a 12g202 hard woud be a bit of an improvement for you

[ACDodd]

Perfectly possible, but you also need to open out the chamber to avoid valve shrouding. In fact this is the only way I do sml bore heads. You need the change the exhaust valves too as the A+ 1275 valves are longer.

I usually aim for 1.26 to 1.28" for the intake size and 1.05" for the exhaust valves. The intake ports needs opening a little to get the best from the new bigger intake valve.

Ac

[daenesh]

great replies.. now 1 last question.. if i use the 1275 valves would it hit the block as its longer? do i have to pocket the block or anything like that?

[ACDodd]

No, but you will need to shim up the rockers by around 0.080" to accomodate the extra lenght of the valves.

AC

[daenesh]

ah ic ic ..i get what you mean.. what happens if i don't shim it?

[carbon]

If you use the longer valves then you need to shim under the rockers to get the correct geometry for the rocker pad and the valve tip.

 

You can get the shims in sets of four (one per post) from Minispares. They are about 1mm thick, so I think you may need two shims under each rocker to get the right geometry.

[daenesh]

brilliant stuff i'll look it up.. thanks a bunch guys