16 replies to this topic

[KernowDel]

I came across a 12g295 in virgin condition a little while ago and set about having it overhauled by Kieth Calver.

Now the spec of my engine as i was lead to believe was a 1098 with +30 re-bore and a 997 cooper cam.  I gave these specs to KC and he worked his magic to produce a great bigger valve version ready to fit bar a skim to get the right CR.

 

Now, it turns out that the engine in fact has only had a 20 thou bore and there is a small chance that the valves might touch the block as they are larger than the originals.

 

I have the 997 cooper cam with a measured lift of 0.256

A set of 1.3 rocker which for arguments sake and for the reason of tolerance (worst case) i will class as a true 1.3

This gives an exhaust valve lift of 0.333.

 

Now the head has been skimmed i have an exhaust valve to head face clearance of 0.350

 

Ignoring the gasket thickness (0.032) and the backlash in the rockers (0.015) is this enough tolerance for a safe build? 0.350 - 0.333 = 0.017

 

I am thinking that it should be enough of a gap once the gasket and rocker backlash are factored in.  I am going to do a dry build minus the gasket to see but i wondered what people thought.

[nicklouse]

some reading for you http://www.theminifo...head-clearance/

[ACDodd]

Del, have you actually measure the lift at the cam lobe?

Valve lift= (total cam lobe lift-follower clearance) x rocker ratio.

This will give the theoretical lift. Actual lift will be 6 to 10 thou less as the springs cause bending.

Ac

Edited by ACDodd, 05 November 2018 - 07:36 PM.

[KernowDel]

I measured the cam lift with a dti on push rod. I figured that there would be some extra clearance due to those other factors

[Retroman]

Do a dry build with just the head and see if the valves over hang the block,

 

You can even do it with some very light springs and no gasket....you will feel it stop if they are hitting

 

I don't think they will with a 295

[KernowDel]

I agree, if the standard valves were used they wouldn't touch but they are larger on this head thanks to KC.

Looking at the measurements and what AC has said I am fairly confident that it will be clear. I will do a dry build to make sure though.

[sledgehammer]

a bodgy thing I do sometimes , if my brain isn't up to the task , is ...

 

clay / blue tack on the valve face most likely to touch - (tiny bit of oil on surfaces to stop sticking)

 

dry assemble engine , torque H/G - do rocker clearances

 

do several rotations by hand - not starter !
 

disassemble , & check thickness

 

a better description here -

 

edit -

 

also when bolting down head , make sure all pistons are down a bit - away from head

It would be easy to bend something while torquing head down & not even notice it

Edited by sledgehammer, 07 November 2018 - 10:33 PM.

[KernowDel]

Del, have you actually measure the lift at the cam lobe?

Valve lift= (total cam lobe lift-follower clearance) x rocker ratio.

This will give the theoretical lift. Actual lift will be 6 to 10 thou less as the springs cause bending.

Ac

So i am actually looking at 0.017 theoretical gap plus the follower clearance of 0.015 plus the head gasket safety margin plus a 6 to 10 thou extra caused by the springs.

So i make that around 80 thou which is looooads of room.

 

 

 

 

I'm still going to try it by hand first though lol

[ACDodd]

Follower clearance is 0.010".
Always best to do it by dummy build.

Ac

Edited by ACDodd, 06 November 2018 - 08:14 AM.

[KernowDel]

Yep my bad i was thinking rocker clearance

[Retroman]

 

Del, have you actually measure the lift at the cam lobe?

Valve lift= (total cam lobe lift-follower clearance) x rocker ratio.

This will give the theoretical lift. Actual lift will be 6 to 10 thou less as the springs cause bending.

Ac

So i am actually looking at 0.017 theoretical gap plus the follower clearance of 0.015 plus the head gasket safety margin plus a 6 to 10 thou extra caused by the springs.

So i make that around 80 thou which is looooads of room.

 

 

 

 

I'm still going to try it by hand first though lol

 

 

Don't forget  -0.015 for rod stretch

[ACDodd]

Rod stretch won't be a problem.the edge of the block is the beast to watch out for.

Ac

[Cooperman]

Always, always do a 'trial build' to check everything before final assemble using plasticine to check any suspect clearances. Use an old head gasket so that you won't waste a perfectly good one and torque to about 75% of final torque figures. Set the valve clearances to 50% of final figures, i'e around 6 thou. Turn the engine slowly by hand and stop if any resistance is felt.

[sledgehammer]

This seems obvious , but so easy to forget ...

 

When bolting down head ,

 

make sure all pistons are down a bit - away from head area

 

It would be easy to bend something while torquing head down & not even notice it , if pistons are at TDC

 

I haven't done this on a rebuild - YET - , but I know it will happen one day , my memory is getting worse 

 

best of luck

Edited by sledgehammer, 07 November 2018 - 10:38 PM.

[KernowDel]

So I put it all together and turned the engine by hand. No dramas there, I then took to step of turning it over on the starter and again no dramas. Looks like the calculations were all ok in the end. Next I just have to decide where I start needle wise to get it running right.

1098 +20 stage one kit 1.3 rockers 997 cooper cam and a large valve 295 head....any suggestions? I had a custom AAU with the standard 202 head (thanks AC).