924Board.org

[j03k64]

I completely rebuilt the engine and have reinstalled it, yet of all things, I'm having issues timing the engine by syncing the head and block using the indicators. The cam indicator is always 0.5 of a tooth off (one total tooth width, approx 4mm).

The cam gear is oriented correctly, deep side toward the windscreen. Tensioner pulley inspected and all good. The flywheel shows the numbers, indicating that it's properly installed. I have 3 belts floating around (2 used and I think 1 new). I picked up a brand new belt to be 100% sure.

I followed this process:

Align crank pulley to oil pump housing indicator mark and put the car in gear to help avoid it moving.

#1) Align cam dot with indicator, install belt, and tension as shown/instructed in the workshop manual; doing this causes it goes to the red mark.

#2) Align cam dot with indicator, advance cam 1-1.5 teeth clockwise (viewed from front of the car), install belt, while having someone ensure it doesn't slip on the crank, reverse cam counter-clockwise to remove slack from the upper portion of the belt, and tension; it ends up at the black mark.

#3) Align cam dot with indicator, attempting to install the belt causes the high part of the tooth to rest on the high part of the cam gear teeth perfectly, thus forcing you to move the belt one way or the other to sync the teeth and leading to the same outcome/s above.

It's driving me totally crazy.

All I can think of that might be causing it:

The machine shop had to resurface the head; shaved approx. 0.003 (if I remember correctly). I rang the shop this afternoon and am awaiting confirmation from the machinist as to just exactly how much was removed as it's not recorded on my shop receipt.

Head gasket thickness? I swear that I confirmed it to be the same Goetze PN (stock thickness) as the head gasket that I replaced on my 1980. My 1980 has a fresh engine as well, but the head was NOT machined and I had no issues installing/syncing the head and crank.

Any ideas? Am I doing something wrong? Has anyone else experienced this after having a head resurfaced?



Post revised to improve clarity.
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1979 Porsche 924
1980 Porsche 924

[jacobroufa]

I had this exact issue when timing my 931 last year. It took me three tries.. I had to purposefully set it in the opposite direction to get it directly on.

At the beginning of this post in my thread here (which links to a different thread where Vaughan talks about this issue): https://www.924board.org/viewtopic.php?p=400737#400737

Good luck!
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1980 Porsche 931
1981 Porsche 924 Weissach

[safe]

In the picture you look spot on, the angle of the camera being a little off.

Have you rotated the engine to see how it lines up? If you are a full tooth off then, just move it one tooth.... If you are somewhere in between, good enough, that's the tolerances without an adjustable wheel.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe

[Fifty50Plus]

It's a tad retarded which happens whe you shave the block. I'd advance it one tooth and your torque comes in a bit sooner. Nicer for street driving and not worries about interference between parts.
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[j03k64]

jacobroufa: I reviewed the thread/s. To clarify it's habitually off by half a tooth at tension (or one total tooth width); updated the initial post to improve clarity. I tried syncing it 20+ times with replicated results.

If I advance the cam to slip it on, it goes to the black line (Scenario #2). I tried preemptively advancing it and using the tensioner. I tried actively meshing the belt, while rolling the cam back from advanced position. I had someone hold it at the crank to ensure it's not doing funny stuff down there. All result in the black mark.

In the photo, the pointer is dead on with that black line. Phone lens height won't let me photograph straight down the valve cover.

The high points of the belt and cam match perfectly at 0-0; I cannot put the belt on. If I set it dead on, the tensioner drags it to the red line and takes the crank with it as it removes the upper slack.

I think this is occurring as a result of resurfacing the head. That changed the distance between the cam and crank, thus impacting required belt length or head gasket thickness which I failed to account for during rebuild.

The head gasket failed, causing a leak on an exhaust port, creating a hot spot and forcing the resurfacing. I saw 1.1mm head gaskets (stock) and a 1.4mm head gasket for 'machined heads' while browsing around last night trying to figure this situation out. The 1.4mm gasket set is seemingly unavailable; unless you have a copper one made by Cometic. I also read posts elsewhere that taking approx 0.010 off a head or block results in a full tooth needing to be removed/tuned out of a belt/chain. Anything around 0.005, would result in this half tooth stuff and inability to sync the crank/cam to original timing marks as well. It would now also be running at a slightly higher compression. At least it's a non-interference engine!

I thought about an adjustable cam gear, but those are apparently very scarce for our cars, which also lines up a bit with what safe is on about above!

I found this post by Ideola last night (https://www.924board.org/viewtopic.php?t=42177).

I guess if it's off by half a tooth, it's a 'free performance cam gear' of sorts? Turn a problem, to an opportunity?!

Set it to the black line (advanced) for higher low RPM torque, but less high RPM HP

Set it to the red line (retarding) to lower low RPM torque and increase high RPM HP.

Fifty50Plus replied while writing; totally thinking that's it!

Cheers for your replies!

I'm afraid this might become a more common issue as time passes for our cars. I would love to know other's thoughts and experiences as that might help others in the future.

Will I have to account for this in ignition timing? I zeroed the crank and put the distributor in earlier today.
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1979 Porsche 924
1980 Porsche 924

[Fifty50Plus]

Don't worry so much. Set your ignition as you normally would after replacing the belt. Having the cam advanced slightly makes it more drivable on the street and won't hurt it at all. When we race these cars, we set the timing so that at full advance we are running about 41 degrees. This provides the most power. That equates to about 20 degrees at idle. Street settings are fine for a stock engine with a slight cam advance.
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[morghen]

[j03k64]

20 Degrees is heaps! I figured it's a free $300 two setting adjustable cam gear at this point based on the commentary. Lemons to lemonade!?!

The pulley position; is that why most guys are using the flywheel to set TDC (more accurate)? I just use the crank pulley and cam gear, then set timing using a light and gently moving the distributor.
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1979 Porsche 924
1980 Porsche 924