12g295 big valves and clearance to the block
Turbo Mini Forums

hi

Right after reading this thread http://turbominis.com/forums/index.php?p=vt&tid=342805 i decided to check a few things with the engine i am building as i have a small bore block with a plus 40 thou overbore and a 12g295 head with 33mm inlets and 29.5mm exhausts.

After taking some measurements i can confirm that the exhaust valves on my build do indeed overhang the block but i am trying to decide whether i now need to strip the engineto machine pokets or whether i can get away without them.

Now the camshaft i am using is an NA MG metro item. If i have measured the valve lift correctly i make it about 7.66mm.

I then measured the distance from the head face to the face of the exhaust valves which i make as being

cylinder 4 7.97mm
cylinder 3 7.94mm
cylinder 2 7.94mm
cylinder 1 7.99mm

The head gasket i have measures as being 1.16mm thick uncompressed

If i understand correctly i should have a minimum of approx 1.44mm clearance minus whatever the head gasket compresses by.

Now my question is. Is this going to be enough given the expansion of components when hot??

If you have any thoughts or i have done something wrong please let me know, as i don't really want to pull the engine and split it if i can help it.

anyone else have a similar build??
Thanks luke

Luke I went down a different road and had 29mm stainless valves turned down to 27mm.

Morspeed sunk the valves into the head originally and used 1275 inlet valves to aid clearance and so far I've never had an issue with my std head and 1.5's however when I get the head back from phil I'm going to try the blue tac test.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

cheers graham, i will also add that when bolted up it will turn over by hand without any clashes this i assume is because i have clearance as stated above, but i need to know whether it will be enough??

luke

any thoughts anyone?

luke

http://www.firebox.com/product/2505/USB-Snakescope-Camera ??

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

I reckon that will be OK but on the limit.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Hi

Graham that looks interesting but will it fit down the sparkplug hole and be able to return to show the head ?

Thanks Paul,

I'm still not totally confident that when the engine gets hot my clearance won't disappear? when pocketing the block how much clearance is normally given?

I've also done some more calcs and it seems the exhaust valves on my build overhangs the bore by 0.9mm.

I know its not advisable when built up but i think it would help if the edge of the bore had a small chamfer to give a little more clearance, any thoughts

Also has anyone ever done this type of machining on an engine when built ?
where there any repercussions?
ie swarf causing damage

I really would like to avoid tearing it all apart again.

Thanks Luke

Your not talking about the gaffa tape and dremmel trick?

I doubt the snake cam would help in real honesty your talking about seeing a very small clearance. If you have 1.44 mm, whats that as a total percentage of the valve?

I doubt your going to get a "go for it" from anyone TBH.

VMAX do/were going to do a solid copper head gasket. maybe you could get a thickness to give you a bit more expansion room? Its going to drop your CR but needs must?

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Luke just so its in the thread, Kent recommend a clearance of 1.5mm as a general rule of thumb for their cams it was in the back of the book that cam with my new 266..

Good luck mate I hope it flys..

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!