is my '84 US S an interference engine - but it's modified
#1
Instructor
Thread Starter
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
Rennlister's
I have an 84 US 928S that has an aged timing belt. Only 12k miles on it, but the belt is 13 years old, so I do need to have it replaced.
Here's my question - I understand the US '84 engine is a non-interference engine. However, the PO had the heads shaved pretty close to the wear limit and installed high performance cams. I'm not sure whose cams were installed, so I don't know the profile. I just know the work was done in 1985 or so and that the car sounds like a boat engine - low rumble!
Anyway, how worried should I be if the belt breaks? Do you think there would be enough clearance between the valves and piston? If the cams were "high lift" would that put them in touch with the piston?
Matt
I have an 84 US 928S that has an aged timing belt. Only 12k miles on it, but the belt is 13 years old, so I do need to have it replaced.
Here's my question - I understand the US '84 engine is a non-interference engine. However, the PO had the heads shaved pretty close to the wear limit and installed high performance cams. I'm not sure whose cams were installed, so I don't know the profile. I just know the work was done in 1985 or so and that the car sounds like a boat engine - low rumble!
Anyway, how worried should I be if the belt breaks? Do you think there would be enough clearance between the valves and piston? If the cams were "high lift" would that put them in touch with the piston?
Matt
#3
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
If you change the belt, remember to watch the tension very closely for the first few hundred miles. The current crop of new belts stretch at first and the factory tensioner won't take it up. I am just finishing up rebuilding my motor that bent a valve after I changed the belt 'just because' and did not watch closely enough for stretching. Of course your best bet is to upgrade to a porkensioner when you do the belt, but now you are talking something like $400 (as I recall) rather than $40. Also I think there may be two brands of belts and only one stretches, but I don't know the detail.
#6
Rest in Peace
Rennlist Member
Rennlist Member
Join Date: May 2006
Location: Bird lover in Sharpsburg
Posts: 9,903
Likes: 0
Received 2 Likes
on
2 Posts
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
If you change the belt, remember to watch the tension very closely for the first few hundred miles. The current crop of new belts stretch at first and the factory tensioner won't take it up. I am just finishing up rebuilding my motor that bent a valve after I changed the belt 'just because' and did not watch closely enough for stretching. Of course your best bet is to upgrade to a porkensioner when you do the belt, but now you are talking something like $400 (as I recall) rather than $40. Also I think there may be two brands of belts and only one stretches, but I don't know the detail.
#7
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I think the 84s are interference. remember, we did a test and there was contact at .3" with the piston, so at near .4" lift, there would be some contact if the belt broke.
Trending Topics
#8
Race Director
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
In theory the 84 US is NOT interference......BUT with the mods you speak of in this 928 I would say it is & I also think you should change the belt and waterpump as a precaution....since $300 in parts in far cheaper than a rebuild
#10
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I am pretty sure Porsche does not make belts. I think the vendors are conti and gates or something like that. If you buy from the dealer you are getting the same thing and probably paying quite a bit more.
#14
928 Collector
Rennlist Member
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I've never seen significant stretch. Skipped teeth, not properly indexed cam timing gears, yes ... but stretch ... not. I've used all the belts available (not yet tried Roger's new one)... I've owned MANY broken belt 928's ....
#15
Rennlist Member
![Default](https://rennlist.com/forums/images/icons/icon1.gif)
I have seen it 1x and perhaps 2x. On my cab, the belt certainly stretched. It was a new belt with less than 500 miles. It was properly tensioned at install, and all of the rollers and pullies and plastic bushings were good. It loosened up enough to skip a bunch of teeth. On my '82 I once had a similar issue, but one of the plastic bushing on the tensioner arm pivot was suspect, so it might not have been the belt.