2.7 Carrera engine getting tighter
 

Had my 2.7 RS engine rebuilt at 130k by a very experienced 911 mechanic 7k miles ago. The crank showed no appreciable wear. 3k miles ago, the RH hydraulic chain tensioner failed and was replaced. Now the RH on has failed, popping off its relief valve in the process. The engine is now almost impossible to turn over using a wrench on the crankshaft pully - hot or cold. Otherwise the engine starts and runs fine. My feeling is that the engine has been getting tighter as time goes by. My mechanic suggests that some older crank cases suffer from the number of heat cycles and that the bearing housings can change shape! If the cam bearings are getting tighter, that could produce a chain back-lash sufficient to wreck the tensioners - but how does this tie in with a possible crank case problem?
Any ideas?

Was the case line-bored during the rebuild? If it was, there shouldn't be binding issues. It's hard to assess "difficulty" when turning an engine by hand. Have you pulled the spark plugs when you do this?

Given the relative young age of the rebuild, I think your first port of call is with the engine builder and have hm check out his work.

No, the case was not line bored. As I said, the crank looked 100%. The stiffness is such that there is no slack at all at the crank nut in either direction. My mechanic's own 2.7 RS was nearby and this turns easily up against compression and rebounds slightly - as you'd expect. I'm awaiting the results of the tear-down and just wanted to hear if anyone was familiar with such a problem.

Your mechanic would(should) know this so I suspect something was lost in the translation. With mag case motors "Past performance is not a guarantee of future results." In other words, if the crank spun freely before the motor was disassembled and the bearings looked good it doesn't mean that the case is straight (now). As soon as you disassemble them the case can go "boing" and be twisted.
-Chris

a 2.7 that turned over just fine with it's old bearings, will bind and be very difficult to turn over with new bearings installed. expecting the tight bearings to "wear in" is not a good call, as the engine could seize in the process. sounds like he needs to warranty the job and align bore the case.

Hmmm. Last time I checked. Experienced "911" mechanics know that the line bore should always be verified as straight before rebuilding one of these motors. I'm by no means a pro and I've known this for quite some time.

Tightness on 'start-up' I can see, but at 7,000 miles? If that happens then line boring won't solve this issue either, because the case has twisted since re-assembly.

I'm no expert compared to JW, but I'm thinking it might not be the crank. I would say it needs to come apart given the failure you've seen.

I am having my '73 RS motor rebuilt now also and when the case halves were split at the initial teardown they sprung apart and would not go back together. I wonder how your mechanic was able to bolt the case halves back together after 130K miles.

The machine shop (Competition Engineering) had to re-machine each case halve to true them up and then of course line bore the case following this. This will straighten up the twisted Mg case and keep the bearings standard size. This would be a good opportunity to install case savers also if you have not done so.

Hello all.

Remember 360 RS is in Old London Town..

And this side of the pond it was once common practice NOT to linebore mag cases..although this is becoming much commoner these days..

In fact, the need is very variable IMHO..I have the same policy with all engines, Porsche or not..bolt the crank in and it should literally spin by hand with nice slick oil on the bearings..

If the case is bent, as seems likely, it will be MUCH cheaper to get another case than buy alinebore and oversize bearins..I know no-one in UK who is currently doing the full, correct recon as done by CE and others Stateside..which trues up all the cylinders etc to the crankline and allows use of standard bearings...though it is only a matter of routine machining really..I guess they will have some nice jigs and borers etc to speed up the work..

PS, I happen to have an excellent 7R 2.7S case here..

Kind regards
David

"Good" 2.7 mag cases are very difficult to find... US/Blighty/Australia where-ever. The fact is these cases are getting very old and the chances of finding a good one is getting harder. That's why it is sometimes easier to machine the case you have. If I was doing it all over again, I'd probably ditch the mag case and look for a later aluminium case to work with.

Yes, they are getting worse.

But I still have one here in my barn!
Temeprature is everything with mag as we know..IIRC it begins to soften dramatically around 120 degrees centigrade..there is a distinct infexion or knee in the cureve..and undergoes plastic deformation in normal use..just from the fasteners even..

But some engines round here never see those temepratures..why, I'm still wearing my thermals in July..

Kind regards
David

Kind regards
David

Pretty cold eh? Where are you?

David,

That is very interesting information about the state of machine work available in the UK. Surely there are shops in Europe that can do that work, if not then he should ship it to one of shops here to have it done or, as has been suggested, use another case. That case will need to checked also though.

That is the big choice with a collectible car like the Carrera RS, do you keep the original case in the car or do you find a substitute and put the original one aside. I chose to rebuild the original but I will certainly not take it to the track and put large amounts of stress on the engine.

Agreed.

There is nothing difficult about the machining as far as I know..all simple stuff and easily within the capacity of thousands of precision engineering companies in UK.

But I'm not actually aware of anyone doing it all like in the US...just linebores.....

But that does not mean no-one is doing it I guess!

Kind regards
David

They don't list case resizing, just the (common) line boring.
-Chris

does this happen on aluminium cases?

Line bore problems should be evident upon rebuild and the engine, if it can even turn over with a bad line bore will loosen up with time. Sounds more like glass beads in the crank to me. One of the biggest causes of engine failure is the use of glass beads to clean things. They get lodged in places that are impossible to clean and eventually work their way out as the engine is run. The result is the engine gradually siezes up as more and more glass beads work their way loose and jam up in the bearings and grind everything to pieces.

I don't believe it does.