Timely reminder to tighten some bolts , 928 Specific

[Buchanan Automotive]

This is a timely reminder about a couple of things on the 928 series ( all ) that need to be inspected / tightened / maintained on a regular basis because without this basic tightening of some bolts ( its that basic ) the lack of maintenance eventually results in broken components that are very expensive and completely preventable . 

During this week we had a 928S ( 1984 auto ) that we used to work quite some years ago until it was sold ( we lost contact with this car ) , its one of those stunning garage queens , perfect paint ( no stone chips ) & no fading and near perfect interior , in other words never left out in the Australian sun , anyway with its new ( current ) owner we went over her carrying out servicing and repairs with some nice performance upgrades , but during the servicing and inspecting we discovered that over the last 7 or 8 years since we last saw this car and 14 years since we did a major service to this car , there were a couple of things that were on the verge of disaster 

A ) Engine cam gear retaining bolts ( one bolt per cam gear ) , the alloy cam gears on the 4.5 & 4.7L engines need their ( one ) retaining bolt checked for tightness very very regularly , we check them for tightness every major service ( every 20,000kms ) , the cam gear retaining bolts on this car were loose enough that both Sean & I were pleasantly surprised that the gears had not slipped ( this was a close one because they were not tight enough by a long shot ) , when they slip ( for want of a better expression ) , the alloy cam gear will bash forward and back against the key-way on the ( hollow) cam shaft boss & then that rapidly progresses to breaking the cam shaft in two ( the end breaks off ) and a new camshaft ( if you can even buy Left or Right bank one from Porsche these days for a Euro spec M28/21/22 is debatable ) & naturally is completely avoidable & gets even more expensive in that 90% + of the time a M28/21/22 will usually bend a valve or two at the same time ( not badly bent , but just enough to not seal well & that = low compression , so it ends up be a engine dismantle and some new valve/s , valve guides and a cam - shaft etc etc etc & its all avoidable . But to make it avoidable , one has to have the knowledge that this tightening procedure is needed in the first place and most people and repairer's are simply not aware of it . 

Note } It was an actual Porsche design issue with the height ( contact patch ) of the slide on steel sleeve that buts up against the inside of the cam gear , which Porsche finally made a lot better in the vary last year the 4.7L engine was made ( 1986 ) and not many countries still sold the 4.7L 928s in 1986 

Note 2 } Even with the earlier slide on sleeve , its not an issue if you just tighten the bolt from time to time ( not hard to do ) 

Note 3 } Its even a small problem that's worth checking on the 5.0L & 5.4L Quadcam engines , but a rare problem ,we know of one 5 L S4 that suffered that fate ( broken cam ) and LOTS of bent valves some years ago , so its rare , but must be checked for piece of mind and about every 4 or 5 quad-cam engines we check the tightness ( torque value ) of the single retaining bolt for the alloy cam gear , the torque value ( tightness of the bolt ) is lower than it should be , so like I said they are ALL worth checking 

B ) On this 1984 year model 928S , we also went to check the tightness of the Torque Tube rear coupling inhex bolt & when we moved the exhaust heat shields out of the way and removed the small inspection hole rubber plug ( rear end of the T Tube ) & we were meet with a Red Dust display , meaning there was fine metal particles ( oxidized red rust colour in a dust form ) , we see this quite a lot ( unfortunately ) and this means the male splines on the T Tube quill shaft inside the female ( split in 3 places ) trans input shaft are not tight enough and are moving ( this is the very fine metal particles ) & just to see what the torque value was of this coupling bolt was we could turn it at least half a turn to get near the torque setting required . Now because the rear coupling was not tight enough I / we knew full well that the quill shaft had probably migrated forward , so we removed the ( front ) bell housing lower section & we could plainly see the flex plate pushed forward , so like we have had to do many many times over the decades we do the following } 

We removed the front coupling bolt ( it was quite tight ) , like they normally are 

We removed the rear coupling bolt ( this was the one that was way too loose ) , this bolt ( head and shaft ) was completely covered in the red dust & on the flank of the inhex bolt you could see where it was just starting to be dragged over ( yes over and into ) the male splines on the quill shaft 

Note } this rear coupling bolt always stretches from acceleration & probably engine braking events 

We then reset the TT quill shaft back into its center position and installed 2 x new genuine Porsche inhex bolts with greased threads ( not loctite ) & torque up & that's it  

Note } because this engine has been on 20w-50 and / or 15w-50 for decades , there was NO engine thrust bearing wear ( even though the flex plate was pushed in ) because the oil film strength protects the engine thrust bearings 

Note 2 } The reason NOT to use Loctite on the coupling bolt threads is because the bolts never ever undo ( they stretch & Loctite can not do a thing about stretching of a metal ) , but the worst thing about Loctite in this application is when you go back to check the tightness of the rear coupling bolt ( say ) 20,000kms later , the loctite on the treads will instantly give you the impression that the bolt is tight , but the bolt may well be stretched a tiny amount , but the threads are held tight and the bolt head will not move & the impression is that's it tight when it actually is not ( its a real trap for the unwary ) 

Note 3 } The T Tube coupling inhex bolt is only threaded for first 1/3rd of its length ( 15mm ) , the un-threaded part of the bolt length 30 mm is the section that mostly stretches & the head of the bolt reduces its clamping force against the coupling ( very basic engineering stuff ) 

Note 4 } The only Torque tube design on the 928 that was really quite good was the 3 speed auto version ( 1978-1983 ) , it had two ( solid ) male shafts connected by a sliding sleeve with 2 x same inhex bolts , why Porsche did split in 3 places hollow input shaft on the 4 speed auto ( 1984 > ) and the manual trans ( all 928 year models ) is / was and always be a built in weakness , but we have to live with the design & they can be maintained but it has to be a constant thing 

Note 5 } The cut in 3 places hollow input shaft on the 4 speed auto version quite often breaks ( metal fatigue ) if the rear coupling is allowed to move / chatter on its splines with insufficient tightness of the coupling 

Note 6 } The cut in 3 places hollow input shaft on the manual trans version ( much more expensive to replace ) suffers the same fate if neglected , we had to purchase a new 5th gear set ( that's the input shaft on the manual ) on a 1986 928S a few months ago ( broken ) through neglect from a long time ago . 

Regards 
Bruce & Sean Buchanan 
Buchanan Automotive
 

[Gavn]

Thanks for the great post - always love reading your material & will be going over mine tomorrow with the above points in mind.

[9fan]

Thanks Bruce and Sean, any similar problems with 968 Torque tubes ?

[Buchanan Automotive]

In regards of what I was mentioning about certain 928 models , that all centered around Porsche's decision to NOT to use what the 931 / 944 / 951 / 944S / 944S2 and the 3 speed ( Auto Trans  928 1978-1983 ) , that is the transmission has a solid splined male shaft coming out of the transmission to meet the solid male splined shaft from the Torque Tube & of course there was a female splined sleeve that joins the two solid shafts with two inhex bolts , it was a very sensible in design and worked very well and even more amusing would of been a bit cheaper to produce in production as it was already being made for the cars mentioned above 

So in essence its the Hollow Input shaft ( cut in 3 places so it can be clamped ) designed implemented on all the 4 speed auto trans  928S / S4 Auto / GTS Auto  1984 - 1995  and the same concept ( different part  naturally)  the manual transmissions input shafts ( all version 1978-1995 )  , its not a designe that can be left to its own devices over a long time , it has to be cared for , it has to be maintained , basically speaking it has to be tight at all times and as I mentioned with only 1 ( one ) lousy bolt , it will cause expensive repairs if neglected , its as basic as that & remember I am talking about the results of a V8 engine with its V8 Torque that will give this single bolt a real work out , if it was a low torque low output engine with less overall weight , say it was a 924 Auto , in this case ( low output ) it would not be an issue 

Now the 968 Manual trans ONLY , has a very nice , very strong splined female sliding sleeve with 4  ( four ) bolts = excellent

Now the 968 Automatic trans version ( Tip )  , this has the same single bolt rear of torque tube clamp ( slides over and cut in 3 places)  as the manual trans 928 has ( very similar to the 928s 4 speed auto one ) , so if the 968 Tip had a V8 engine it would eventually cause some issues , but being a 4 cylinder NA , its not really an issue , but that said , with the Tip , its not a bad idea to check the tightness of this single bolt from time to time & naturally the 968 Tip T Tube is completely different to the manual trans version

Note }  Like the 944 2.5L Auto , the 968 Auto Trans( Tip ) has the the 944 116 065 04 rubber Dampner bolted to the engine flywheel , its designed to soften out the 4 cylinder firing impulses , this in turn protects the single rear coupling quite a bit , in a way its the very very distant cousin of the dual mass flywheel & like I said only the 944Auto had one & the 968 Auto has one ( different part number ) & yes these are a bit of a weak link in old age , meaning they do break eventually  , so the 968 Tip Torque Tube / drive line from the engine flywheel to the auto trans the only ( in old age ) weak link is the rubber dampner

Regards
Bruce Buchanan
Buchanan Automotive