Minimum valve to piston clearance

[haasad]

Gents please don't flame me since I've searched and not found definitive information.
I've built a 2.8 on a 70.4 mm crank with 92 mm bores and proper S grind cams. The heads are ex 2.7 CIS the pistons are je 10.5 nominal . Cams timed at 5.2 mm lift

I dry built it to check deck at 1.3 mm and am now at clearance checks for the valve to pistons.

I've got 2.0 mm inlet and 1.5 mm exhaust with zero lash.

Question is , is this enough for a street motor and can I "even it up" to get more exhaust clearance by adjusting ( retarding ?) the cam timing a little. If it can be retarded which way would I go. Is it setting the timing lift to the minimum value or the max in the range? I really want to avoid tearing this down again if I can.

[MBruns]

The range on a factory S cam is 5.0 - 5.4mm, try it at 5.0 and see what you have, at zero lash I wouldn't be happy with less than 2.0 mm on the exhaust, you may get away with it but one mistake and its a hand full of broken rockers at best. Its not a big deal to deepen the pockets to make it safe.
Mike Bruns

[haasad]

Thanks Mike, thats good advice. I'll reset the lift at 5.00mm and recheck I too consider 1.5 mm a bit tight but could live with a bare 2.00mm.

I'll check it out this evening.


andy

[haasad]

Re-timed at 5.0mm lift.

Problem is now worse I still have acceptable inlet valve clearance at a minimum known 2.0mm and now its blocked a 1.25mm on the exhaust about 10 degrees before TDC. I took the trouble to check clearance across both inlet and exhaust valves through 2 crank revolutions and it happens in the same place each time. Back out the adjuster and she turns perfectly.

Should I try re-timing again at 5.4 mm on the basis I have nothing to loose.

There has been no dimensional change in building this motor ie the case was not faced or the cylinder lands machined the heads have not been refaced,the valve seats were re cut which will give extra clearance it anything.

The heads are a matched set as are the cylinders.

The pistons are 92 mm diameter 10.5:1 JE's and the cams standard S.

Rather than machine the pistons I would rather swap to a "softer" cam ?

Andy advice appreciated.

[burgermeister]

Advancing the cam takes away intake clearance and gains exhaust clearance, and vice versa. I know cause I just tried it ...

[CleverCog]

Question is , is this enough for a street motor and can I "even it up" to get more exhaust clearance by adjusting ( retarding ?) the cam timing a little.

Those are two questions.

1) Yes, that is enough for a street motor. For what it's worth I've parked many engines @ 0.80mm, my official lowerbound. Fun fact is that your PtoV clearance is reduced slightly after TDC on the inlet because the cam is accelerating the inlet valve down faster than the piston is going down--you'll have less than 2.0mm at that point but no worries.

2) In light of point one, your urge to even it up should be gone--you're more even than you thought at 5.2mm cam timing-nice job nailing the timing.

[haasad]

Ok re-timed it tonite at max advance 5.4 mm I now have a minimum of 2mm on inlet and at least 1.85 on exhaust .

Based on research here and elsewhere I figure I'm good to go as long as I re-time once the chains have stretched a bit.

andy

[Walt Fricke]

Just FYI, Mike is right on when he says having valve pockets cut deeper isn't really a big deal at this stage of the game.

I assume you have sealed up the case, but haven't glued the heads to the cam carriers? And haven't installed more than the #1 rockers? I was in that position last year, and dug out my receipts. The machinist (works mainly on Detroit iron) who flowed my heads cut the pistons I removed for $9 each.

The biggest pain was removing and reinstalling those blasted wrist pin clips from the J&Es. Why can't J&E cut them for, and use. a circlip?

[haasad]

Walt, thankyou . I'm in the position you state other that in my assembly process I did seal the heads to cam carriers. To be honest that was a mistake and if I had thought this through properly I wouldn't have done it. This is the main reason why I don't want to strip it again. I know I could remove the heads and carriers as one but don't have confidence about re assembly giving me the super free turning cams that I achieved first time.

At 1.85 mm clearance i'm only 0.15 mm or 0.006 inch shy of the very safe clearance . I've seen lots of information that 1.5 mm is fine and even down to 0.8 so figure I'll be ok this time ...but a real lesson learn't. I'd never build a modded engine for a 911 but had done several standard engines so this has been a great experience/

I must say I was a bit surprised at the clearance problem given the lack of dimensional change on the parts and that the spec is well understood, Its just possible that my S cams are not original spec but they came from a very reputable source . Is there a simple way to check perhaps by measuring valve lift through the crank rotation and mapping it for inlet and exhaust at say every 5 degrees . I have a degree wheel and can easily set up a DTI on the valve retainer again maybe even on the cam lobe?

Here's a snap of the cam end marking ( I know its up side down)



and a home brew solution to snap ring installation ( not removal)





andy

[Walt Fricke]

Well, there is lift at the lobe, and then lift at the valve. I think the lift figures for cams usually arfe for lift at the valve (if they were at the lobe, the rocker ratio would put the coil spring in a bind). So you have the varying rocker ratio to contend with when making a graph.

Then you may have trouble finding a graph of a "stock" cam of any kind, or of a modified cam. I don't see that kind of information in anyone's catalog, nor for any large number of factory cams.

But max lift at the valve ought to tell you something. And you can measure duration, using several beginning and ending points, and see if those line up with anything,. I don't know what points the factory spec for duration uses.

I have the Stomski tool for inserting the clips. It helps, but doesn't make it really easy. And to remove them I had to make a tool out of a thin punch, carving a groove in its end so when the tool was rotated the clip raised up from the "hole" part and I could grab it with other tools. I eventually found all the ones which sprang out and onto the floor, though I have spares because at one go around I mangled some of them. I still don't understand why not a circlip.

I don't think you would have had issues if you had removed the heads as a unit on the cam carrier, though I know it is kind of a moment of truth as you tighten the last few head nuts, hoping the cam will free up some - as it usually does. My traveling chain hoist goes right over the engine stand (which is how the engine gets there), so I can use it to lift this rather heavy unit. Very handy.

But you aren't going to do that now, I see. Good reinforcement on that decision from others here.

[burgermeister]

I removed a head / cam carrier unit a few days ago, and it bolted back up without issues and happily left me with a free spinning camshaft.