FredR

The original motor fitted to my GTS suffered TBF and i purchased the car knowing this. I currently run my modified S4 motor and with the GTS inlet cam and faster exhaust plus my attempts at sharktuning it seems to run well- especially so since I modified the breather system.

Thinking further ahead as to what the future may hold, I am thinking various options. However, I also have the GTS crank, rubber conrods and original pistons/rings in seemingly serviceable condition.

The crankshaft was damaged during the TBF episode but most of the damage translated into the htrust bearing itself which was ground out and bearing webs in the crankcase were cracked so goodbye original motor.

When we inspected the crankshaft, it seemed to me the ctankshaft might be recovereable so I took it to a good machine shop who specialise in engine repairs and in no time they had the crank on a machine and were able to polish out the damage to the rearward thrust face and also lightly polished the journals.

The only problem I can see [assuming I can guarantee cleanliness of the inner passages from swarf] is that were it to be installed in a motor, it would have an end float of 0.5mm which is in excess of the factory permissible limit of 0.4mm and most of that is of course to the rearward side.

From the rod/piston assembly I visually estimate about 3mm clearance between the small end and piston bosses although I dare say that gap may close to some extrent when hot.

Questions:

1. Would such end float characteristic be a real world problem.
2. Could a stock thrust bearing be modifed [at reasonable price] to take up the inherent displacement [or a custom bearing be made]?
3. Porsche spec not withstanding, what real world float could the engine tolerate before a risk of contact assuming thrust face wear is minimal.

I am reluctant to disturb my motor from the original build but if I did, it would be good to know what options I may have. Whether or not the pistons would fit correct tolerance grouping another matter altogether.

Thoughts please

Regards

Fred

James Bailey

No expert but getting oil to stay on the thrust bearing surface is difficult enough when with in tolerance, open it up and it would be even easier for it to escape....The oil film acts very differently on the large flat bearing surfaces than it does on the shoulders (thrust surface).

Strosek Ultra

Had the same issue with spinning main bearings once. Machined the girdle mating surface slightly off and inline-bored the mains. Had to fabricate a special tool for the job. The crank journals were all OK after polishing but too much end play from excessive side wear at the third main journal. Welded and machined the crank for correct journal width. I should not run the engine with 0,50mm end play. The surface on the side of your main thrust bearing journal is probably bad and may damage the new thrust bearing.

Ake

FredR

The face has been re-ground to a polished finish. However the nitriding is probably not worth a lot and were it to ever be resued would probably require the crank to be re-nitirided.

Fred

FredR

Quick refresh for anyone who may have missed the original query

Regards

Fred

Cheburator

Fred, I don't know whether you have thought of getting an exchange crank from Porsche - comes with 1yrs warranty, all the bearings - mains + conrods and costs at GBP1700 all inclusive of VAT. As for your use - I am sure the "weak" 1R rods will be ok...

FredR

A,
It would be interesting to know if they would accept my crank in exchange. Probably cost more to get the crank to/from the UK though. Still- a good suggestion- thanks

Still interested to know how much end float we can get away with in our motors.

Regards

Fred

Cheburator

look below

Cheburator

Fred, they accepted my GTS crank, which was probably 1mm oversize on the 2/6 journal as the con rod bearings had welded themselves to the journal... I am sure Porsche will accept your crank, which looks like new... All they care is that you give them a GTS crank... In return you will get a shiny crank from Porsche, with 0.25mm undersized bearings in their original individual boxes, caring a time stamp - Made in W. Germany - 1996

Who cares that the Porsche crank is effectively a used one - it comes with a universal 1yr warranty by Porsche... When you consider the cost of a full set of undersized bearings and the actual crank, GBP1700 all in is an absolute steal...

Forgot to add - you can buy your exchange crank via Porsche Oman - I am sure they have a dealership there. The crank will be released from Germany anyway, thus you don't have to ship it here. In fact, I was offered to pick up my crank in Bulgaria at the local Porsche dealership, but it made more sense to have it in the UK and use it when the bores were being cleaned up...

FredR

A,

If I was to go down this route for sure one would fit new bearings and must admit I have not even thought about the cost of the shells and doubtless they do not come free in a packet of corn flakes these days

Strosek Ultra

If your crankshaft is good a solution is to grind down only the third main journal and install a 0,25mm undersize thrust bearing. The undersize thrust bearing is wider so you will end up having an end play within the tolerances specified by Porsche. From measuring new thrust bearings I have found the stock size bearing to be 29,85mm wide and the undersize bearing 30,03mm wide. Of course if you want to spend more money you can grind all the main journals to undersize and install all new main bearings.
I have a new GTS crank (all new no exchange), new bearings both stock size and undersize if you are interested we could make a deal.

Ake

FredR

Ake,

The main bearing journal is OK at stock finish [to my utter astonishment]- a very light polish cleaned it up nicely. The damage [superficially] was to the rear thrust face and logically the nitrided layer. We were able to clean this up but the inherent gap would leave an crank end float [side wall to thrust bearing face] of 0.5mm and of course about .3mm to offset to the rear. Thus the question as to whether this is a real world issue or a compliance to tolerance issue. Maybe a custom thrust bearing could be had with meat to accommodate this offset clearance of .3mm [assuming a good stock set up has 0.2mm- typical end float].

Regards

Fred

Strosek Ultra

Porsche prescribes a crankshaft end play of 0,110 to 0,312mm, wear limit 0,40mm. If your engine will work good for an extended period of time running an end play of 0,50mm not in compliance with the tolerances is hard to judge. If both side surfaces of the thrust bearing journal on the crank are good it will probably be OK but it is up to you to decide. As said before the less costly way to get the end play within the tolerances is to use an undersize thrust bearing which is about 0,2mm wider. I do not know of anyone who can supply a custom made standard size thrust bearing being wider.

Ake

FredR

Ake,

The damaged thrust face was reground/polished to bearing surface tolerances so there should be no problem with this face functioning as a thrust face.

Another thought comes to mind in that if the crank end float is on the high side, preferentially, would one set the crank when it is in the forward position, the rear most position or attempt to set it somewhere in the middle. I have no idea whether the crank wants [naturally] to float towards one extreme or the otherbut if I were to postulate, I would think it tends to move towards the rear during load and recovers to a position further forward when the load is off.

Regards

fred

GregBBRD

You need a crankshaft.

If Porsche has a stock one available, with bearings, for the price that Alex indicates, that would be a decent alternative. Good "used" GTS cranks are very difficult to find....as a huge majority of the GTS automatic engines had thrust bearing failures.

I also make "uber trick" replacement GTS light weight crankshafts (in inventory) with custom Carrillo rods (also in inventory)....certainly not nearly that cheap....but a very nice starting point, for a hot rod GTS engine.