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924S Engine Rebuild or Replace?

 
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RickF  



Joined: 03 Aug 2015
Posts: 44
Location: Howell, NJ 07731

PostPosted: Tue Dec 08, 2015 9:40 am    Post subject: 924S Engine Rebuild or Replace? Reply with quote

The timing belt let go on my 87' 924S. It occurred at idle with clutch disengaged. I know I have at least a couple of bent valves but not sure of piston damage yet. I'm pulling the head to inspect everything. My question to the Forum is, "At what point should I decide that it's more prudent to just replace the whole engine or completely overhaul the one I have?" I would really appreciate all help, input, experiences and expertise of this group before I proceed.

Thanks for your help.
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fiat22turbo  



Joined: 18 Jan 2006
Posts: 4040
Location: Portland, OR

PostPosted: Tue Dec 08, 2015 9:54 am    Post subject: Reply with quote

A head rebuild isn't too bad.

Not sure why you'd need to replace the engine for that, especially since they are known for going to 200-300k with proper maintenance.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)
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RickF  



Joined: 03 Aug 2015
Posts: 44
Location: Howell, NJ 07731

PostPosted: Tue Dec 08, 2015 10:05 am    Post subject: Reply with quote

I'd have no problem rebuilding the head if bent valves and damaged guides are the extent of the of the damage. However, if the pistons are damaged or holed then wouldn't that necessitate splitting the cases? I ask as this would be the first time I've worked on a water cooled Porsche.
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Fifty50Plus  



Joined: 28 Feb 2008
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Location: Washington DC area

PostPosted: Tue Dec 08, 2015 10:12 am    Post subject: Reply with quote

Most likely the pistons aren't damaged at all as long as it didn't occur at 6000 RPM. I just did one and you can barely tell that the valves tapped the piston top.
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1979 924 NA race car
1982 924 NA race car - Sold
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RickF  



Joined: 03 Aug 2015
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Location: Howell, NJ 07731

PostPosted: Tue Dec 08, 2015 1:15 pm    Post subject: Reply with quote

I can only hope there is little or no damage to the pistons. The car was well maintained so I have my fingers crossed.
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jason c  



Joined: 13 Jan 2014
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Location: Nwi

PostPosted: Tue Dec 08, 2015 2:23 pm    Post subject: Reply with quote

Fifty50Plus wrote:
Most likely the pistons aren't damaged at all as long as it didn't occur at 6000 RPM. I just did one and you can barely tell that the valves tapped the piston top.


I agree. Busted pistons are rare in the 2.5 with a broken belt. Very unlikely at idle.

Just have the head redone with new valves & let it roll.
Bet you won't skip another timing belt service.
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11723
Location: PacNW

PostPosted: Tue Dec 08, 2015 11:33 pm    Post subject: Reply with quote

If you have time on your hands (winter project) and your are committed to the car, I'd recommend pulling the motor to: 1) do the belt service, 2) inspect the clutch, 3) replace rod bearings, and; 4) reseal the engine. Also a great opportunity to work on the wiring harness.
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RickF  



Joined: 03 Aug 2015
Posts: 44
Location: Howell, NJ 07731

PostPosted: Wed Dec 09, 2015 12:13 am    Post subject: Reply with quote

Good News! No damage to the pistons! Couple of bent valves, etc. So, I'll do a complete head overhaul and service to the cam, etc. I was rather surprised the timing belt let go as it only had about 20k miles on it and it was about 4 years old. I'm thinking the belt was old when installed by to PO. Just as an afterthought, is there available a timing belt to chain conversion? I haven't researched it yet but in light of the inherent problems this engine has with timing belts I would think a timing chain setup would be more sturdy, longer lasting and more desirable.
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Rick
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jason c  



Joined: 13 Jan 2014
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Location: Nwi

PostPosted: Wed Dec 09, 2015 10:20 am    Post subject: Reply with quote

RickF wrote:
Good News! No damage to the pistons! Couple of bent valves, etc. So, I'll do a complete head overhaul and service to the cam, etc. I was rather surprised the timing belt let go as it only had about 20k miles on it and it was about 4 years old. I'm thinking the belt was old when installed by to PO. Just as an afterthought, is there available a timing belt to chain conversion? I haven't researched it yet but in light of the inherent problems this engine has with timing belts I would think a timing chain setup would be more sturdy, longer lasting and more desirable.


Age of the belt is as big of a factor as mileage. Iirc, the factory recommended interval is 2 years or 30k miles.
The timing belt design isn't the issue, the owners lack of maintenance is. In 20+ years of owning these cars (I have owned several at any given time), I've never lost a belt. Maintenance is key.

Timing chains need to be encapsulated in covers within the crankcase so they are constantly lubricated, this is not possible on these engines without a significant design change. An unlubricated chain is a disaster waiting to happen.
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fiat22turbo  



Joined: 18 Jan 2006
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PostPosted: Wed Dec 09, 2015 11:14 am    Post subject: Reply with quote

Porsche did screw the pooch with the timing belt design to some extent given the number of similarly sized engine from the same time period that utilized timing belts without similar issues.

That said, proper maintenance is key and they'll last a good long time. More importantly, drive the damned cars, sitting doesn't do these cars any good.
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1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)
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RickF  



Joined: 03 Aug 2015
Posts: 44
Location: Howell, NJ 07731

PostPosted: Fri Dec 11, 2015 10:39 am    Post subject: Reply with quote

With the head removed a close inspection of the head gasket and the crankcase surfaces revealed some clogging of the galleys for the engine coolant between the two. I've seen that before on motors that have been sitting a while without the coolant being drained. Stefan, you are 100% correct.... sitting really doesn't do these cars any good! Thanks all for the help and insight.
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