928 Preventative Maintenance Items that can save lots of money

[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 year alone we had "New to us " more than usual number of 928 and 928S ( 1980-1985 ), 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 ) ,if the cam gear retaining bolts on any of the above mentioned 928 engines are not tight enough, 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 ( Left or Right bank )one from Porsche these days for a Euro spec 4.5 or 4.7L are mostly no longer available ) & naturally is completely avoidable & gets even more expensive in that 90% + of the time a M28/21/22 ( example only ) 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 ( that you not buy new ) - 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 ) With the beginning of the 4 speed auto trans version 1984 year model 928S onwards , meaning 928S2 928S3 , 928S4 ,928GTS , we also want to check the tightness of the Torque Tube rear coupling inhex bolt & we move the exhaust heat shields out of the way and remove the small inspection hole rubber plug ( rear end of the T Tube ) & if we are 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 ) transmission input shaft are not tight enough and are moving ( this is the very fine metal particles )
. Now if the rear coupling is not tight enough ( bolt stretched ) ,then the quill shaft will have probably migrated forward , so you then remove the ( front ) bell housing lower section & you can see plainly see if the flex plate pushed forward , so like we have had to do many many times over the decades we do the following } 

We then remove the front coupling bolt ( they are normally quite tight ) , if the front flex plate is pushed in then you will see the flex plate will now try to straighten out , but check for cracks as we had a 928GTS recently that the flex plate was pushed ( deflected ) to the MAX and was cracked quite badly ( Avoidable )

We then remove the rear coupling bolt ( this is the one that is not tight enough because of neglect / stretched bolt ) , this bolt ( head and shaft ) , if the index bolt head is covered in the red dust & on the flank of the inhex bolt then there is the danger that the bolt shaft is being dragged over ( yes over and into ) the male splines on the quill shaft & naturally this damages the male splines of the T T quill shaft even further

Note } this rear coupling inhex bolt always stretches from acceleration & probably engine braking events where as the front coupling index bolt ( if it does stretch , will be far less than the rear one )

With the 2 old coupling bolts removed and the flex plate back to being straight again , we install our special tool to connect a workshop dial gauge to measure the engine crankshaft ( forward and back movement  / thrust bearings wear ) , if its within acceptable wear specification , we write down the figure for future reference & we then reset the T Tube shaft back to its correct position within the T tube  & remember there are 2 completely different thrust bearing wear specifications , one set of numbers are for the 4.5 L & 4.7 L engines and the 5.0L and 5.4 L engines have a completely different sets of numbers

After we 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 } Whats interesting , over the decades we have noticed "IF " the said 928  engine has been on 20w-50 and / or 15w-50 ( High Oil Film Strength engine oil ) for decades , there often NO engine thrust bearing wear ( even though the flex plate was pushed in ) because the high oil film strength protects the engine thrust bearings , but if the 928 engine has the misfortune to be on a this century engine oil , like a so called synthetic 5w-40 or a 10w-40 ( low oil film strength oil ) then in these situations thrust bearing wear can be way too much and often the engine becomes an economic write off & its Avoidable 

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 threads 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 & wears the male splines on the T Tube quill shaft ( expensive and avoidable )

Note 6 } The cut in 3 places hollow input shaft on the 928 manual trans version 1978-1995  ( much more expensive to replace ) suffers the same fate if neglected , this is important these days because 5th gear assemblies for some year models are No Longer Available new from Porsche & as the engines became more powerful and cars kept putting on weight this is a real issue and its Avoidable and Expensive 

If the above are maintained , they will last and last & last & costs very little in that maintenance , if however the above is not maintained the costs are quite high depending on new parts availability & 2nd hand parts are often pre fatigued /internal cracked  due to lack of above maintenance  ( meaning already damaged and just waiting to fail )

Regards 
Bruce & Sean Buchanan 
Buchanan Automotive
 

 

 

[Stepo]

Good on you for putting the time into writing this for the 928 owners Bruce/Sean.

I look forward to your summary on the 944S2  , I remember the dialogue on brakes...............have you ever thought about writing a book