How much effort to turn cam in head off engine?

Post Wed Jun 22, 2005 11:10 pm

Just finished off my head rebuilt (new cam & springs) and in setting the valve clearances tonight, i was amazed how much effort was required to turn the cam. Remember, this is a head on it's own, off the engine, sitting on a table (propped up at both ends to allow valves to move). With a normal 8" long socket spanner on the bolt at the end of the cam, i could turn it over, but only just. Now i'm no muscle-man it has to be said, but at times i had to hold the head down to stop that turning over as i was trying to turn the cam. Is this normal? Strikes me that there's a hell of a lot of power required from the engine to move this.

I do have uprated springs in now, which i guess would be matters worse.

Post Wed Jun 22, 2005 11:30 pm

Sounds normal to me m8 & the uprated springs would'nt help matters.
If the head didn't try to turn over, I would be very worried.
(I used to be an engine builder for a motor sport firm by the way)
HTH
Iain S

Post Wed Jun 22, 2005 11:32 pm

Mine was the same. Dont forget to have the head 'up on blocks' I had a 1" book at the front and back to give room for the valves to protrude when you turn it. If the head is flat on the table it is much harder to turn ( and may risk damage to the valves?)

Do the clearances twice! I'm about to do mine for the 3rd time and its so much easier off the car!

Post Thu Jun 23, 2005 7:25 am

Thanks guys - just wanted a bit of reassuring before this goes on the car tomorrow.

I spent two hours doing the clearances last night - setting them, checking them, then checking them again.

What made me think though is that these uprated springs must sap a lot of extra power from the engine to turn the cam over. It's an uprated cam, so i'm gaining power from that, but surely losing a fair bit through the extra effort required to move the cam?

Post Thu Jun 23, 2005 7:41 am

What i have done in the past is to fit the cam without any of the valves / springs etc, nip the bearing caps down slowly checking the rotation of the cam as it goes. If it then gets too tight to turn you need to investigate if one or all of the bearings are clamping down too much, if so investigate.

What you are aiming for obviously is a good tight fit, the cams / cranks etc should turn over evenly with no loose or tight spots.

Some advice for the future! as i guess you ain't going to strip your head down again!

Best of luck with the rest of the build.

Post Thu Jun 23, 2005 7:56 am

Cheers Mark, though i think the procedure you described isn't applicable to the M20 head. The cam slides in from the end and there's no bearing caps to tighten down. I was a little surprised it's done this way as i was expecting it. The cam seemed to turn over very smoothly before i put the two rocker shafts and all the rockers back in, so i guess it's all ok.

All will become clear on Friday night when i get the car back hopefully!

Post Thu Jun 23, 2005 9:11 am

This all sounds normal. I wouldn't expect to be able to turn the cam on the bench wthout holding the head down.
A problem can occur with heads that have warped at some time and been skimmed. The skimming may flatten the deck of the head, but it doesn't put the cam bearings back into line.