Buchanan Automotive Posted 4January, 2022 Report Share Posted 4January, 2022 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 furtherNote } 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 numbersAfter 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 tomo, 550Spyder, OzJustin and 3 others 4 2 Quote Link to comment Share on other sites More sharing options...
Stepo Posted 4January, 2022 Report Share Posted 4January, 2022 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 Quote Link to comment Share on other sites More sharing options...
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