Broken TB Damage Assessment

[jeffnhiscars]

I brought a project home yesterday from the back hills of WV and I'll be setting up a thread about it. Its a true barn find because "a" it was in a barn and "b" it still smells like horse manure even after being towed 325 miles.

I knew it had broken the TB as I had pics of broken rockers before I left so the hope was that it would need a head rebuild and that the pistons had survived. He was driving slow & it "just stopped"...no noise, rattles, grinding etc..it just "stopped".

1st thing I did this morning was to pull the valve cover. The crank turns and the cam doesnt so we can agree its the TB. What I do not understand is why all 6 intake rockers broke and not one exhaust. It also appears that the exhaust valves are stuck open as there is a huge amount of play in the exhaust rockers. I thought perhaps they had backed all the eccentrics out but that does not appear to be the case.

There does not appear to be any evidence of oil starvation as there is no scoring or bluing anywhere.

Opinions before I pull the head ??

[jeffnhiscars]

Couple of pics

[jlevie]

Hard to say why the intake rockers broke but the exhausts didn't. Sometimes things like that just happen. Pull the head and see what the piston tops look like. If they don't look too bad new rockers, valves, seals will fix it. Have the seats ground and at least a minimal surfacing cut done.

[jeffnhiscars]

Hard to say why the intake rockers broke but the exhausts didn't. Sometimes things like that just happen. Pull the head and see what the piston tops look like. If they don't look too bad new rockers, valves, seals will fix it. Have the seats ground and at least a minimal surfacing cut done.

Thanks Jim. Your input is much appreciated as Im sure you have seen more broken TB belts than I have (which is none :-). Of course I fully intend to pull the head, but like most of us who get new toys Im ahead of myself in guesstimating. I pulled 4 plugs and none were damaged so perhaps thats a sign the valves didnt actually break and that the pistons survived.

Do you suggest OE rockers, valves, bolts & HG and to reuse the cam and shafts if they look ok and are there some cost effective refinements that help this particular head "as long as its off".

TIA

[jlevie]

I probably do more to a head than most folks. When I rebuild a head I replace all of the exhaust valves as they'll have the most wear and a regrind makes them too thin. The valves and seats get ground and then I lap in the valves. That is partly to get a better seal and heat transfer and partly to check the shop's work. If they ground the valves and seats correctly it will take very little lapping to fully mate the parts. If either angle is off it will be very obvious. The guides may or may not, require replacement depending on the degree of wear. Most of the time they are okay.

I polish the area of the rockers around the eccentrics to remove stress risers. If the head is high mileage (over 150k) it gets new rockers. The valve springs get tested and any soft or hard ones get replaced. The goal is a set of springs that are within spec and as uniform as possible. It may be necessary to replace the eccentrics if there is any flat spotting, as proved by a micrometer check. A used cam almost always has enough wear to justify having it ground back to spec or replaced with a new cam. A used head always gets surfaced as I've yet to see one that doesn't have some warp.

To keep costs down the head goes to the shop disassembled and clean.

[jeffnhiscars]

We know Im getting at least 6 new rockers and I suspect at least all new exhaust valves since the exhaust springs remain compressed so Im assuming they are bent. From there its not a far leap to just replace all valves & rockers and be done w it given the stresses that occurred.

The engine has a confirmed 98k and my visual of the cam shows virtually no signs of wear.

What's a typical machine shop bill run for the work you describe, given that I can certainly break the head down and have lapped enough valves in my day ?

Do you get them to run any tests on the head ?

[Mastrcruse]

At the point your at, would it be easier and possibly cheaper to source a new head that's already been through? I had a warped head on my m30 and just sourced a head that was gone through completely...it cost me around 400-500

[jlevie]

We know Im getting at least 6 new rockers and I suspect at least all new exhaust valves since the exhaust springs remain compressed so Im assuming they are bent. From there its not a far leap to just replace all valves & rockers and be done w it given the stresses that occurred.

I'll bet the intakes are bent also. If the intakes didn't hit pistons, what broke the rockers?

The engine has a confirmed 98k and my visual of the cam shows virtually no signs of wear.

There is wear, you just can't see it. But at that mileage the cam is probably okay to use. Though I'd want a Cam Doctor report before I put it back into the head.

What's a typical machine shop bill run for the work you describe, given that I can certainly break the head down and have lapped enough valves in my day ?

Do you get them to run any tests on the head ?

If the guides are okay, it costs me about $500 at the local race/machine shop I use. I'll do a dye penetrant check before the head goes to the shop and have it pressure tested if the head is out of an engine that overheated and failed the head gasket. Otherwise I just do a good visual check after I clean the head.

BTW: I clean a head (or a valve cover) by soaking one end in a 5 gallon of carb cleaner for 24 hours and then invert it for another 24 hours. That is after cleaning with mineral spirits, Purple Power, and bushes/scrapers. If there is significant liming of the coolant passages I'll have it hot tanked instead.

[jeffnhiscars]

At the point your at, would it be easier and possibly cheaper to source a new head that's already been through? I had a warped head on my m30 and just sourced a head that was gone through completely...it cost me around 400-500

If I come across one that I can confirm has been recently and properly freshened, then yes, thats a very good option. MR325 offers what seems lilke a stock rebuild for $625 so thats not far off given the assurance of what he did. Call me old school but originality notwithstanding, Id rather repair/rebuild something I know the history of (within reason) rather than grab someone elses headaches

I'll bet the intakes are bent also. If the intakes didn't hit pistons, what broke the rockers?

Thinking a chain is as strong as its weakest link, I figured theres a chance the rockers lost and the valves won. As I said though, Im prepared to replace them all.

There is wear, you just can't see it. But at that mileage the cam is probably okay to use. Though I'd want a Cam Doctor report before I put it back into the head.

If the guides are okay, it costs me about $500 at the local race/machine shop I use. I'll do a dye penetrant check before the head goes to the shop and have it pressure tested if the head is out of an engine that overheated and failed the head gasket. Otherwise I just do a good visual check after I clean the head.

BTW: I clean a head (or a valve cover) by soaking one end in a 5 gallon of carb cleaner for 24 hours and then invert it for another 24 hours. That is after cleaning with mineral spirits, Purple Power, and bushes/scrapers. If there is significant liming of the coolant passages I'll have it hot tanked instead.

It will be interesting to see where this goes. If the pistons are done, head work is academic as I'll probably look to do a 5spd S5X swap and if the pistons are good Ill be $$ ahead to do whats necessary to make the head right. We are talking about a 98k car, even if it does smell like horse shit

[jeffnhiscars]

I finally got the head off and the good news is that the pistons are not chunked (see pic). The bad news is that in the process of breaking all of the intake rockers that rocker shaft seems to have bent with a resulting crack to the rear support tower.

The exhaust shaft easily slid out by hand once I got cam load off of it but the intake side needed more encouraging. Even after cremoving the 6-I rocker the shaft did not want to slide further so I drove it back in and it clearly does not align with its guide hole. Looking dead on from the rear of the head (so along the axis of the shaft), I'd estimate the gap to be .020...huge !! The only way I see to get the shaft out at this point without destroying the rest of the head is to cut the shaft in to pieces to eliminate the stress, but I suspect that's academic since the cost of repairing that tower is probably out of line.

What's interesting is there's a parts for sale thread with an M20 head that has the identical crack so evidentally this is not uncommon.

I'm thinking I need another head and while I would intend to test, deck & reface etc any head, the one part of head recon jobs I dont see discussed is guide & valve stem wear. With a 98k car it make no sense to install a head with 200k in stem\guide wear....or do those just last forever which is why you dont see them replaced often ?

Thoughts ?

[jlevie]

Valve guides do not last forever. If the engine had decent maintenance, the guides are usually okay at up to about 150k. The 200k and over heads I've done have needed new guides. Stem wear usually isn't an issue as the stems are very hard as compared to the guides. But every high mileage head I've done has needed new exhaust valves.

[jeffnhiscars]

Thanks for the input, Jim.

I did manage to get that rocker shaft out and complete the tear down. Since I had the shaft out a couple of inches I was able to rotate it with a screw driver in the lube hole. That made it obvious that the shaft was bent so by rotating it often I was able to gradually drive it free.

Here's what I have:

1 bent rocker shaft
12 bent valves
6 broker rockers
1 cracked rocker support tower
a partridge in a pear tree

The guides were very tight but given the cracked tower I don't think the head is not worth repairing just to keep 100k guides IMO. Maybe it is.

Otherwise, I have to get a rebuilt head, find a good core and rebuild it with new or scrounged parts or get a "good used" head, deck it, lap the valves and worrying about it later after I get the rest of the car sorted.

If anyone threw a rod 3 weeks after installing a fresh head or WAS going to rebuild a head and now has a drawer full of valves and rockers...yada yada... let me know.

Meanwhile, I'm going for a ride, top down, in the GOOD Cabrio :)

[TobyB]

For a street car, I think I'd have that bridge welded up, check the alignment
of the rocker support rod holes,
and be happy. I had one with 2 bridges popped out, and a friend did it for whiskey.
It worked fine- that part of the head's only REALLY stressed when the belt breaks.
Don't know if I'd race it- but he said that it welded very cleanly, and that in his estimation,
it's no weaker than the surrounding metal, as he got good penetration and was able
to increase the surface area of the weld boundary into the casting. And he's pretty
competent at welding castings (and re- breaking them).

Kinda depends on what your comfort level is, and how adventuresome you are!

t

[jeffnhiscars]

For a street car, I think I'd have that bridge welded up, check the alignment
of the rocker support rod holes,
and be happy. I had one with 2 bridges popped out, and a friend did it for whiskey.
It worked fine- that part of the head's only REALLY stressed when the belt breaks.
Don't know if I'd race it- but he said that it welded very cleanly, and that in his estimation,
it's no weaker than the surrounding metal, as he got good penetration and was able
to increase the surface area of the weld boundary into the casting. And he's pretty
competent at welding castings (and re- breaking them).

Kinda depends on what your comfort level is, and how adventuresome you are!

t

Thanks for the feedback. I actually ran the good shaft in that side and it slid in perfectly so the bore alignment is still good. I've welded iron heads that were cracked between combustion chambers with good results so the concept of doing this doesnt phase me. You just have to find someone who knows how to do it right which would involve grinding a small "V" and filling it without warping the whole thing. It might even make sense to bolt it to a block while doing it.

In this case, the weld would sit under the edge of the head bolt washer so it would have to be ground or milled flush (or even filed) but then it would surely have some good back support.

I'm looking at a used head that might just provide the parts I need to get this one back together.

There's hope yet :-)

[TobyB]

Yup- that's what Jon did. But since the towers are pretty thermally isolated, he didn't
have to clamp it, nor did it warp. Preheat, tig, postcool. I did the machining afterwards.

SO don't toss it if you don't use it- it might just save someone's car...

t