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928 Preventative Maintenance Items that can save lots of money


Buchanan Automotive
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A timely reminder of preventative maintenance in regards to the 928 series that not only saves a massive amount of money if prevented , but also the massive inconvenience & extra cost of breaking down a long way from home 

A )   4.5 L  & 4.7 L ( 2 valve per cylinder engines ) camshaft gear to camshaft bolt needs re tightening every so often , otherwise these engines will suffer a broken camshaft and on a 1984 & 1985 Euro spec engines they will bend valves ( just a tiny amount ) but they will not seal = loss of compression on those cylinders , but worst still is that New genuine Porsche camshafts have been NLA for decades now in regards to the most of the Euro spec engines , be it 4.5 or 4.7 L & good luck buying a set ( or one ) of 2nd hand camshafts for a  M28 / 21 / 22 engine ( 928S Euro spec 1984/5 & UK 1986 )

 

Our general rule of thumb is to tighten these bolts ( one per cam gear ) every cam belt interval change & then again somewhere in the middle as well , so if we replace the cam belt / rollers /  water-pump etc ( say ) every 60,000 kms , then we generally check/ tighten these bolts again as 30,000 kms ,  we have been doing this for the past 25-30 + years and we have never ever lost one , meaning we have never had a customers 928 that we have been maintaining suffering a broken cam shaft 

 

Note } These cams are cast and with a hollow ( threaded ) hole in the centre for the long bolt to go into & will not tolerate the cam gear being anything other that tight / secure on the camshaft , when they break ( because the gear is not held secure ) the broken ( almost shattered ) remains is not pretty and ( this shattered mess ) is not repairable , although saying that I have seen these broken cams attempted to be repaired only to fail a few months or years later ( bent valves again ), that doesn't mean it has not been done successfully somewhere on this planet , just never seen it still successful 10 years later 

 

Note 2 } The reason why the cam gear/s become a little loose on the camshaft is NOT that the bolt has undone itself a bit , but the back side of the alloy cam gear has crushed a tiny amount & this crush of the alloy cam gear is what dramatically reduces the " tightness " of the alloy cam gear to the end stop/boss that the cam gear slides onto and stops against or in other words the length of the centre of the cam gear is now ( after crush ) a fraction of a mm shorter & the cam bolt can not compensate for it , the bolt needs to be tightened from time to time to keep it tight ( very simple stuff ) 

 

This is NOT rocket science , but one ends to know to be able to do something to prevent this from happening , hence why I go to this trouble 

 

Simple periodic tightening of the bolt is all that is required & remember never ( Do Not ) ever apply Loctite to the cam bolt thread , because if you do, and then go to recheck it ( say ) 30,000kms later the bolt head will give the FALSE impression thats it at the correct tightness ,  because you had previously locked the threads with Loctite & the bolt head with its thick washer will not be giving correct force up against the front of the alloy cam gear & eventually the cam shaft will break in two later

 

Note 3 } If we ever remove the cam gears ( say the front of engine is coming apart for oil leak work etc ) then we always install new genuine Porsche cam gear retaining bolts with just a light smear of lubrication BUT NOT Loctite for reasons as mentioned above 

 

Note 4 } The very early 928 engine ( 4.5L only ) had steel cam gears which were not as prone to the crush that the alloy gears will suffer for the obvious reason is steel is a lot harder to crush 

 

Note 5 } The steel & alloy cam gears 4.5 L & 4.7 L ( 1978-1982 ) were square tooth design where as the 4.7 L 1983 - 1985 ( plus the 4.7 L  UK/ HK 1986 version ) were the different round tooth design 

 

Note 6 } The 4 valve per cylinder engines ( 1986 5.0 L engine & the 5.0L 928S4 & GT engines & the 5.4L engines ) can suffer the same fate but its nowhere as common but in the last 35 years I / we have come across at least two with broken cam/s and lots of bent valves and yes the centre retaining bolt was not as tight as it should be and yes the rear of the alloy gear in question was slightly crushed , so because we have come across it we check / tighten the cam gear bolts of these engine exactly the same way & the same intervals & all is good ( very simple stuff ) .

 

If the Cam Gear Bolts are tight ( correct Torque ) they can not break a camshaft 

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The next one is the dreaded rear of Torque Tube coupling bolt , this single bolt was never ever good enough for a V8 , but thats how it it ( it has to be maintained because of this under engineering ) , worse still the 928 models with manual trans ( all years ) has the input shaft that is part of the very expensive 5th gears ( yes 2 gears as an match assembly and input shaft as one unit ) assembly & this female splined shaft is hollow and then cut in 3 places ( every 120 degrees ) , quite spectacular engineering but if not kept to MAX tightness & the expensive gearbox input shaft LOVE to Break into bits ( kidding this is not expensive because its all part of the entire 5th gears assembly  ) , but as its in the throws of destroying itself , its also destroying / wearing away the male splines on the rear of the T Tube quill shaft &  this single ( clamping ) bolt never every turns its self loose , it just stretches & in stretching it can not hold the clamp holding everything together tight enough and expensive destruction is guaranteed , but it can be maintained( looked after ) , but is needs to be checked a lot , at least every 20,000 kms or even more depending on the amount of hard acceleration the driver does , its acceleration from a standing start that is the major contributor to the stretch of this single bolt

 

The 928/s Auto Trans , 3 speed ( 1978-1983 ) has a much stronger and better set up ( much like the Porsche 944 series in manual trans version ) , the 3 speed auto has a sold input shaft with male splines which goes very close to the solid male spilled shaft of the T Tube quill shaft & over the top of both shafts slides a totally seperate "female" splined clamping sleeve with two of those bolts and because both shafts are solid and the sleeve has two bolts its gives little or no problems at all , its such a good design it even makes removing of the auto trans much easier , its just a fantastic design ( near perfect ) 

 

The 928S 4 speed auto trans (  1984-1995 ) , I have no idea what Porsche was thinking but they must of thought it would be a good idea to not use the ( strong ) excellent rear of the T Tube coupling design of the 3 speed  928 /s Auto( and 944 as well ) and instead used a similar set up as the manual trans 928 , but this time the hollow and cut in 3 places( weakened ) transmission input shaft is part of the Torque Converter Flex Plate ( rear flex plate ) , so many of these have destroyed them selves over the decades  , because the bolt stretched and no one every tightened it because its out of sight and out of mind & often damaging or destroying the male splines on the rear of the torque tube quill shaft  , for some years now you can no longer buy a new input shaft from Porsche for a 1984 or 1985 or 1986 928S with Auto Trans & like the manual trans is has a SINGLE BOLT ( how Stupid ) , but it can be maintained , but at the same time we reset the front( engine end ) Flex plate position via the rear coupling , because its the rear coupling that always ends up with less "Tightness" as compared with the front coupling which almost never stretches as much as the rear coupling bolt .

 

So for quite a few decades now we check ALL 928 T Tube coupling bolts and replace the bolts on regular intervals & Never Ever use Loctite on the bolt threads , because they do not undo , they just stretch & Loctite will give you a false ( torque ) reading when you go back in to check the tightness later because the threads will be locked but the bolt will be stretched 

 

Note }  The 944 / 951 series are similar to the 3 speed  928 Auto set up in principle , meaning two solid male splined shafts that come together with a female splined ( slide over ) sleeve / coupling with 2 bolts , then the ultimate set up from Porsche ( they were learning )  , the Porsche 968 ( 944 series 3 )  1992 to 1995 , these cars had a 6 speed manual trans with similar two male shafts coming together , but this time with a special female slide over sleeve/ coupling , but this time with 4 bolts ( Four Bolts ) , this is what the next generation of 928 would of received if Porsche did not run into serious financial difficulties in 1995 , like other high end car companies around the world due to very very very high interest rates which resulted in low sales volumes 

Regards
Bruce Buchanan

Buchanan Automotive  

 

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Great info Bruce, 

Hope you are putting all this info on these cars on a memory stick or disc for future access, to us and those mechanics that never learnt this in the modern age.

Hope 2021 is a great year for yourself and Sean.

Cheers, Mark.

 

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