Issue with a cam bearing cap

[RoadsterTech]

Okay here is the scoop...

While I was changing out my timing belt, water pump and front end seals on the soon to be new motor....disaster struck.

I was teasing out the cam seal and the top part of the bearing cap snapped off...

Here is a picture:



Looking closely at the cap you can see that this cap had a flaw or crack in it ... see the discoloration (looked like oil seepage).

So here is the problem...
I have another cap from an old head. It went on very easily with no binding and the cam shaft moves freely.

Do I check this with plasti-gauge and if within 0.006" use it, or do I bite the bullet and get the replacement cap line bored in?

Anyone repaired that little seal surface before?

Just for grins here is a picture of the head:

Its not cleaned up and is just as I opened up the valve cover.... The new engine has 49K miles on it.



Oh and I was already admonished for not taking the cap off in the first place.

[— kung fu jesus —]

Plastigauge it, for sure. Decide after the results.

[RoadsterTech]

I did a plastigage clearance test on the top of the bearing surface and on the sides ~ 30 deg down from the top...

Here is a picture of the compressed plastigage on the tight side.


I got 0.0025" clearance on top, 0.003" clearance on the outside measurement and 0.0015" clearance on the inside measurement. The cam bearing surface measured out at 1.021" the cap at 1.023"

Those scuffs on the cam shaft were not from me... I used one of our members seal removal tool... they were not leaking on the head prior to disassembly though.

Doing some CAD work to extrapolate the clearances comes up with this...



While there really is no crash point there is a point where the clearance is closer than 4 decimal places can show.
I am not sure this leaves enough room for oil flow around the bearing...

So here are my options.

1. put the old cap on and hope it works... (not my favorite)
2. Epoxy the lip back on the cap
3. Pull the head off, pull out the valves and cams and send it off to be line bored. $100.00
4. Put the cracked cap on my old spare head and trade it in on a rebuilt head. $375.00
5. Look for a decent used head. $?

Here is my take on these options:
1. This is a true crap shoot... Will the cam polish the race down before it oil starves and galls the bearing?

2. Before I get blasted on this option here are some thoughts:
- the crack is not on the bearing surface
- the only pressure exerted on this is the seal
- the Valve cover seal presses down on this lip holding it in place.
- it is on the outside of the engine proper.

3. This is a good but time consuming option... the only thing that holds me back on this is, 1 time and 2 the condition of the cam seal surface.

4. Good but costly solution I get a 3 angle valve job a perfectly straight head and a set of cams that should not be as badly dinged up... and I don't have to spend rebuild time I can't spare. I could then save the old head have it line bored latter and possibly put in some more aggressive cams in it at a later date but as stated before it is costly...

5. Not so great, I might be buying someone else's problem..

I hedging toward either option 2 or option 4 due to available time...


What do you guys think?

[— kung fu jesus —]

Were you able to find what is spec for the clearances? Your findings don't seem bad.

I'm thinking that unless you are game for building a performance head, epoxy that piece back on. If it fails and the seal walks out, you're going to know it pretty quick. Still, even if it does fail, I think you have enough material around to keep the seal in place.

If you are pulling the head to line bore, you might as well go for a shave, polish and new guides, right? Your current head looks very clean. My B6 was very strong from P&P, gasket matching, 3 (or 5, can't remember anymore) VJ, and a nice shave. My compression readings 70k after installing that head were between 195 and 205, suggesting about 10:1. I think reassembling a head is actually pretty easy, YMMV.

[RoadsterTech]

On the advice of one of the TBM guys. I made a little jig for the mill to hold a sanding cylinder parallel to the bearing and polished it out slightly off center and got these results.

Head on shot:


outside and middle in view:



Inside to middle view:



The interpolated results are, Inside (.0017" to .0018") Middle (.002" to .0018") Outside (.002" to .0019")

The original inside edge must have been either keeping the cap from going down or slightly pushing down on the cam.

I could only find a maximum clearance value of .006".
Does anyone know if .0017" is good enough of a clearance?

[RoadsterTech]

Two of the TBM guys found that typical min clearance is 0.002" for street and 0.004" for race.
I was able to polish them to ~.0035" on all three measurements.
I call this a success!
I saved myself a boat load of cash. My thanks to Loren from TBM. For the suggestion