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-   -   TBF fix (https://rennlist.com/forums/928-forum/634431-tbf-fix.html)

vundba 05-19-2011 10:52 PM

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

WICruiser 05-19-2011 11:01 PM

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.

vundba 05-19-2011 11:09 PM

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.

928 at last 05-19-2011 11:13 PM

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:


Originally Posted by vundba (Post 8569934)
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


Bill Ball 05-19-2011 11:20 PM

What is the crank endplay?

Mrmerlin 05-19-2011 11:33 PM

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

928mac 05-19-2011 11:47 PM


Originally Posted by vundba (Post 8569934)
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


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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928 at last 05-19-2011 11:50 PM

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.




Originally Posted by vundba (Post 8569990)
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.


928mac 05-19-2011 11:56 PM

Like Rob said ^^^^

928 at last 05-20-2011 12:06 AM

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


Originally Posted by bwmac (Post 8570103)

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

.
.
.
.


928 at last 05-20-2011 12:10 AM

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.


Originally Posted by 928 at last (Post 8570168)
No need to yell.:nono:
Yeah we know it's feasible proposition, but the issue isn't defined yet.


Mrmerlin 05-20-2011 12:46 AM

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

Hilton 05-20-2011 01:28 AM


Originally Posted by 928 at last (Post 8570174)
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.

Originally Posted by 928 at last (Post 8570168)
No need to yell.:nono:
Yeah we know it's feasible proposition, but the issue isn't defined yet.


Good to see that you agree with yourself :D

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

FredR 05-20-2011 06:20 AM

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

Hilton 05-20-2011 06:31 AM

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.


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