924Board.org

[coup85]

I’m rebuilding the head on my ’79 924 NA. The head had new valve guides installed and the valves/seats were refurbished at a machine shop. I’ve also installed new tappets and a used camshaft that appears to be in very good condition.

The problem is that I’m nowhere near the valve clearances specified in the manual, even with the adjuster screws backed out as far as I think is safe.

Current cold clearances:

L1 Intake: 0.05 mm (1 mark)
L2 Exhaust: 0.13 mm (1 mark)
L3 Intake: 0.00 mm (no mark)
L4 Exhaust: 0.13 mm (1 mark)
L5 Intake: 0.00 mm (1 mark)
L6 Exhaust: 0.00 mm (2 marks)
L7 Intake: 0.00 mm (2 marks)
L8 Exhaust: 0.30 mm (1 mark)

My first question is about the adjuster screw position.

Right now I have the screws adjusted so they just engage the second threaded section inside the tappet (roughly one turn into the second thread). See photo 1.

Does the adjuster screw need to engage both threaded sections, or is it acceptable to run it like photo 2, where it only engages the first threaded section?

My second question is about machining the adjuster screws. The screws have become very expensive and one of mine is already the thinnest version (no marks), while two others are the next size up. Even so, several valves are still effectively at zero clearance.

Has anyone successfully machined/grinding the adjuster screws to recover clearance after valve seat or guide work? If so, how much material were you able to remove safely?

Any advice would be appreciated.






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924 - 79'

[Cedric]

I set my limit so that the screw still have full engagement on both sides. One idea is to buy a scrap head for a few bucks, then you get a full mixed set of adjusters:) i once grinded one down, on very fine sandpaper that i lay down on a flat surface, still holds up though my car isn't doing rounds around the world, but for small adjustment it seems to work.
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1980 924 Turbo
www.instagram.com/garagecedric/

[coup85]

The problem is that some of them are far away from spec; indeed, some of them are still contacting the lobes the entire cycle, so I don't even know how much to take.

I'm gonna try to switch some of them to see if I at least can get some clearance to know where I am.

In any case, has anyone machined this scews? Is it something a machine shop can do? If I'm gonna take material off, I much prefer them to do it.
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924 - 79'

[peterld]

The adjuster screw MUST be threaded into both sides of the tappet, otherwise you will have a catastrophic failure.
The adjuster screw can be machined a little, but you should try moving them around.
The other alternative is to machine a little off the valve stem. If your situation really is that critical, it would seem you have a case of valve recession.
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80/81 932/8 ROW

[coup85]

I was hoping to be able to leave the head in place but it looks like I'm running out of options, I'd need to take out almost 0.5 mm from some of the screws which I suppose is far from optimal.
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924 - 79'

[coup85]

For now I’ve left the adjuster screws at the machine shop. They told me machining them shouldn’t be a problem. For the first batch I’m having between 0.15 mm and 0.40 mm removed, depending on the screw.

While waiting, a question came to mind about valve adjustment.

I’ve never actually reached the point of adjusting the valves with everything assembled. I thought I understood the procedure, but now I’m second-guessing myself. As I understand it, valve clearance is adjusted with the camshaft installed by rotating the adjuster screw through the hole in the tappet. What I’m struggling to understand is the mechanics of it.

Doesn’t the adjuster offer a lot of resistance when you turn it? Doesn’t the flat face of the adjuster tend to bind against the valve stem tip?

What confuses me is that, as the adjuster rotates away from its flat face, it seems like it would push the valve downward and compress the valve spring. Given that we’re only dealing with 0.10 mm to 0.40 mm of clearance, I can’t quite picture how the adjuster can be turned easily with the camshaft still installed.
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924 - 79'

[924RACR]

Who said it was easy to turn?

It reminds me of the old joke about the old carpenter who showed the new kid how to whip the measuring tape back into its spool at speed.

After a week of practicing, the kid comes back to the old hand, his thumb's all bruised and bloody, and he begs the oldtimer, "How do you do it?!?? Every time I retract the tape, it whips back and hurts my thumb!!! I don't understand, what's the secret?!???"

The old guy looks him dead in the eye and says, "Who said it doesn't hurt?"



Old age and treachery...

You are 100% correct, the spring will be compressed until the screw is rotated fully and you hit the flat spot again.

I'm with the comment above, I suspect you have valve recession into the seats.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[coup85]

It hurts, trust me… I have had the car on stands for 6 months now; nothing has been easy. I assume there is some recession; yes, the head, in any case, went to the shop for checking and was sent back ok. I really hope the matter of the screws does the trick and I can finish this once and for all!


Thanks for the tip, I more confident now. I didn't want to end up breaking something.
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924 - 79'