TBF fix
 

I was told I should ask the experts if it's doable.I have a potential TBF and I was told that since the thrust bearing just controls for & aft movement.So I was wondering why couldn't I machine the messed up surface and make a thrust bearing to compensate for the material removed ? .I have a shop full of cnc's ,my guys said putting the block on mill would be a cake walk in aluminum and the bushing wouldn't take long ,both in about 4 hours ,so I figured if the block is shot anyway what have I got to lose! any ideas if this has been tried before? thanks

I will qualify by saying this is purely theory as I do not have any personnal experience with TBF.

A lot of things happen when the crank can move fore and aft as allowed by the failure of the thrust bearing. That being said, how much are we talking about? If the crank has barely gone through the bearing it is one thing but if it has worn into the bearing support it is a whole new story.

At the moment my son and I are tearing in to the car but from what everybody has been telling me TBF is what I am dealing with. I'll know more once we get the oil pan off. It's just the machinist in me to want to cutup something and I'm trying to think ahead and maybe find a method for others out there.

Do some research here.
 

If there's no cracks, you may be able to salvage and actually improve on the original. So, assuming the block isn't really screwed, it is doable.

Most of us don't have CNC access and these kind of options (at anything approaching reasonable cost).:(

There is however a wealth of expertise here on the subject. Hate to say it, but "Search is your friend".........:thumbup:

What is the crank endplay?

before you remove the pan remove the bell hosing lower,
and release the front pinch bolt on the drive shaft clamp,
see how much the clamp slides back then with a screwdriver or other pry bar move the flywheel back and forth check the clearance,
IIRC in inches its from .008 to .016
with .016 being max before the thrust bearing has had it.
Also you can before taking anything apart,
pull the dipstick and drip the oil onto a black piece of cardboard take the sample into the sunshine and look for metallic particles. lots of metal means a bearing is wearing.
If the thrust bearing has had it and it is bad then the block web will usually have one or more cracks in it, this is now a good block for a coffee table

It is easy to do for the experienced engine machinist.

The key is that the bearing has to go back to its original spot, distance, position, so that the rod runs true.

machine the block saddle down
so that a spacer the exact size of the original block web can be screwed to the block web with recessed tapered screws.
the spacer should be around 3/16 thick
The original bearing for the crank will now sit over the spacer exactly like it did before the damage.

You are building a spacer that looks like the bearing, but narrower with the exact OD as the block before the damage.
Line bore it to original spec after its screwed in place.

I have had this done successfully to a 455 olds block to all the webs so that I could fit a smaller 425 steel crank in place thus stroking the engine.

I raced it for four months shifting out of 1st and 2nd at 7200 rpm.

I spun all the bearings and bent the rods when a planetary in the trans blew.
I had no rev limiter and it went past 9000 rpm.
Not much was left to rebuild.

With the right machine shop its a cake walk. IMHO

Brad

BTW Most TBF blocks crack. magneflux the block before you spend any money on it

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Retraction....
 

Sorry, I thought you had the diagnosis already.:rolleyes:
Go back to measuring the endplay first. Before you drop the pan and all the other stuff.
Report back when you actually have an accurate measurement.
TBF may or may not be the issue.

Like Rob said ^^^^

Easy there Brad,
 

No need to yell.:nono:
Yeah we know it's feasible proposition, but the issue isn't defined yet.

BTW
 

Completely agree.:thumbsup:
IF that's the problem, and the block isn't coffee table material.
There's no reason some one who knows what they're doing, (especially with advanced equipment) can't fix the problem in pretty short order.

I would submit that if the thrust web is cracked the block is a coffee table,
but we are getting ahead of ourselves.

Research and inspect what you have then with more info its easier to figure out what to do
Also dont forget that the crank will possibly have been damaged on the thrust face,
so some machine work may needed to clean this up

Good to see that you agree with yourself :D

(heh, I should add IBTR!! (In Before The Randy))

As an engineer who has seen/experienced TBF and anyalysed possible solutions the damage invariably involves crank damage, bearing web damage and cracking of the TB seating surface as the bearing shells spin.

The crank can be polished [I did this to the GTS crank] but by the time it has polished out the axial clearance in a stock TB shell would have ensured that the travel was at least at the factory limit and possibly beyond. To run this would therefore require a custom fabricated TB with one thrust shoulder thicker than the other.

As stated earlier a semi circular segment could be machined to fit into the web face [but leaving it thinner/weaker] fitted with counter sunk screws. However, machining the faces would presumably be very difficult unless some pretty sophisticated tooling is available.

Lastly, to resolve the cracks in the bearing seat faces of the block, presumably the whole block would require line boring or reaming and thus this would effect every bearing dimension unless there is a machining technique where this can be done on just the thrust bearing journal.

Add to the above that any machining on the crank presumably negates the nitriding layer it is not hard to see why recovery would be prohibitively expensive for most owners.

If you have the capability to resolve all of the above all well and good but whether you would have the original integrity of design may be another matter.

Regards

Fred

At least a couple of 928's have apparently been repaired successfully after thrust bearing damage - one in Canada, and one in the UK.

I don't recall the exact details, but somewhere along the line it was mentioned that using the thrust bearing from an early 32V engine (US 85/86) was part of the solution, although I haven't physically seen one to see how different they are.

Some excellent suggestions make here, however, it requires to be "eye balled" step as you strip down the engine to determine the damage in a logistical manner.

1. As Stan has said release the forward flexplate clamp and measure the migration.
2. Bill said then measure the end play "bump clearance" of the crankshaft, as this will give you the end play measurement to see whether it exceed the specification limit. If it does then you have probably worn the aft thrust bearing thrust face.
3. Expose the the crank shaft bearing to view the thrust bearing thrust face and see the extent of the damage and to see if there are wear marks on the webs of the crankcase from the crank throw.
4. This inspection should let you know whether you need to remove the crankshaft to expose the thrust bearing pocket to see whether the bearing has rotated in the pocket and whether the crankcase web in way of the bearing pocket has cracked.
5. At this juncture, if not before you, will know whether you need to strip down the engine completely.
6. The crankcase can be checked for alignment with a "jig borer" in a tool makers shop to determine whether the thrust bearing pocket and web need machining and whether you need to have an oversize bearing made and fitted.
7. If the short block is cracked in way of the TB pocket, this can possibly be repaired by a "metal lock" tradesman. "Metalock" is a trade name, so do a google on the web to find a representative to see whether they would accept reparing it. Usually they "dye check" the area to identify the extent of cracking to see whether it is a viable repair.
8. If it is repairable, then after the metal lock repair again fit in "jig borer" to have the pocket machined true to the crankshaft axis (lined bored) and the faces of the pocket web machined where the crank throw has touched the web.
9. A new thrust bearing can be manufactured to meet the dimensions required to satisfy Porsche's specifications for end play and bearing clearance.
10. If the bearing has not rotated and the crankshaft bearing surface has not been scored or thermally cracked the crankshaft may either required to be ground or polished and you may get away with just fitting a new thrust bearing.
11. When a new bearing is fitted after repair the alignment of the thrust beaing need to be checked to ensure that the thrust face is in fact at right angles to the centre line of the axis of the crankshaft.
If is no in perfect alignment then the actual thrust bearing surface will not be sufficient of prevent boundary lubrication and the bearing could fail again when and if the end loading from the the flexplate breaks down the oil film.
12. If there is some wear on the bearing pocket web then it is possible to maching and fit a collar spacer recessed slightly into the bearing pocket web held in place by countersunk set screws.

You can see by the above it is a stage by stage process of strip down and appraisal and a stage by stage build up and appraisal including blueing the various surfaces for full contact, bearing clearances and end play.

Hilton has made note that there has been a couple repairs done.

I have done repairs of this nature on much larger internal cumbustion engines it just a matter that if man can design and make an engine then man can repair it in service by numerous methods with the repair materials available today.

Hope this helps.

Tails 1990 928S4 Auto

AHHH WOW!!!!!
gotta say that took the enthusiasm out of that idea.
found a motor with only 68,000 miles out of a 5 speed so it wouldn't have had any tbf potential for $3,000 and since I only paid 7 for the car which otherwise is mint, I think maybe just throwing in a newer motor might be the way to go.I'll just piece out the other engine on E-bay.

note the lifters and any other parts that run with oil will be contaminated in the TBF motor with metallic particles so lifters are toast

Sorry Rob.

I thought CAPS was yelling and bold was just easier to see.