[DeTomaso] Valve seat fail and piston damage
Daniel C Jones
daniel.c.jones2 at gmail.com
Wed Mar 27 17:29:08 EDT 2024
> My gut tells me the piston is trashed and needs to be replaced.
You can have the piston checked for cracks (zyglo) beyond the witness
marks. As long as the crown isn't too thin (it appears the valve notch is
deeper than the gouge so you might be okay) and it passes zyglo, you can
deburr it, check that the bearings and rod are okay and go. Also, check
the bore for scoring and make sure no aluminum is in the rings. FWIW, I've
got an engine on the stand that had 6 of 8 exhaust valves seized in their
guides. Pistons got tagged and it bent a couple of pushrods and damaged a
few rockers. The pistons appear to be reusable but one cylinder bore has a
vertical score mark. That bore will get electron welded or sleeved.
The heads are a concern. In addition to an interference fit, seats in
aluminum heads rely upon dissimilar thermal expansion (cool one part, heat
the other, jam both together and let cool) to lock the seats in place.
Steel or iron cylinder liners in aluminum blocks are typically the same
way. Unless there is some physical interference that damages the seat,
they usually only come loose if the heads are seriously overheated. I'd
contact Blue Thunder and see if they have other reports of issues. I had a
set of TA Performance heads (for a Buick 215/Rover aluminum V8) and TA
contacted me when they found that some heads had been improperly heat
treated. Any head that failed Brinell testing got replaced.
As has been noted, the seats in most iron heads are induction hardened.
The exhaust seats are the ones that wear (recede) the most because of the
higher temperatures on the exhaust side. The width of the valve-to-seat
contact area is important because a wider seat transfers heat better. Seat
inserts can and do fail in iron heads but it's usually due to improper
installation. An experienced machinist can examine the seats for not only
wear but also how many times the seat has been previously cut (i.e. is it
through the induction hardened layer). Some early emissions era engines,
ones that ran hot with retarded timing and lean mixtures, did have issues
with exhaust seat wear. That said, my machinist considers seat inserts for
repair in iron heads only if needed. The cutting necessary to install a
seat weakens the head and can promote cracking, particularly if the head
is subjected to higher than stock pressures or heat (i.e. a high
performance engine) and the ports have to be ground to blend the seat in.
I quote one machinist I know: "You want inserted seats about as badly as
you want a venereal disease."
Before the widespread adoption of induction hardened seats, some iron heads
had Stellite seats installed from the factory. My 1938 Chrysler straight 8
has Stellite exhaust seats but they are in the block (not the head) since
it's a valve-in-block design (a.k.a. a flathead). BTW, many of the seats
installed in iron heads by machine shops are ductile iron and not
especially hard. The hard seats are Stellite (an alloy of cobalt and
chromium) which is very difficult to cut (especially with stones) and much
more expensive than ductile iron.
12 of the 14 Cleveland heads in the attached picture are being prepped for
a trip to the machinist (Dave McLain), along with 2 of 6 straight 8 flat
heads. I didn't see any seats that looked like they needed replacing but
we'll see what Dave has to say.
Dan Jones
On Tue, Mar 26, 2024 at 10:27 PM John McKee <johnmckee at cox.net> wrote:
> I need a recommendation for an engine builder/machine shop in Orange
> County, CA.
>
> Back story:
>
> After sitting for 10+ years I finally fired up my engine a few weeks
> ago. It sounded good for 10 minutes then I had a bad clatter in the
> valve train. At first it appeared to be a loose rocker but there was
> weird interaction with the valves of cylinder #4. I popped the head off
> and found that the exhaust seat had come loose! While dancing around
> the seat also made some deep marks on the piston head.
>
> My gut tells me the piston is trashed and needs to be replaced. I have
> a friend that says I should repair the head and go as is. I don't know
> if anybody on this forum has an opinion.... But, when is a "witness
> mark" a catastrophic failure in the waiting?
>
>
> I posted photos on the picture forum at this link.
>
> [1]https://pantera.infopop.cc/topic/valve-seat-failure-and-piston-damag
> e
>
>
>
> Thank you for your thoughts,
>
> John
>
> References
>
> 1.
> https://pantera.infopop.cc/topic/valve-seat-failure-and-piston-damage
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-------------- next part --------------
> My gut tells me the piston is trashed and needs to be replaced.
You can have the piston checked for cracks (zyglo) beyond the witness
marks.A As long as the crown isn't too thin (it appears the valve
notch is deeper than the gouge so you might be okay) and it passes
zyglo, you can deburr it, check that the bearings and rod are okay and
go.A Also, check the bore for scoring and make sure no aluminum is in
the rings.A FWIW, I've got an engine on the stand that had 6 of 8
exhaust valves seized in their guides.A Pistons got tagged and it bent
a couple of pushrods and damaged a few rockers.A The pistons appear to
be reusable but one cylinder bore has a vertical score mark.A That
bore will get electron welded or sleeved. A
The heads are a concern.A In addition to an interference fit, seats in
aluminum heads rely upon dissimilar thermal expansion (cool one part,
heat the other, jam both together and let cool) to lock the seats in
place.A Steel or iron cylinder liners in aluminum blocks are typically
the same way.A Unless there is some physical interference that damages
the seat, they usually only come loose if the heads are seriously
overheated.A I'd contact Blue Thunder and see if they have other
reports of issues.A I had a set of TA Performance heads (for a Buick
215/Rover aluminum V8) and TA contacted me when they found that some
heads had been improperly heat treated.A Any head that failed Brinell
testing got replaced. A
As has been noted, the seats in most iron heads are induction
hardened.A The exhaust seats are the ones that wear (recede) the most
because of the higher temperatures on the exhaust side.A The width of
the valve-to-seat contact area is important because a wider seat
transfers heat better.A Seat inserts can and do fail in iron heads but
it's usually due to improper installation.A An experienced machinist
can examine the seats for not only wear but also how many times the
seat has been previously cut (i.e. is it through the induction hardened
layer).A Some early emissions era engines, ones that ran hot with
retarded timing and lean mixtures, did have issues with exhaust seat
wear.A That said, my machinist considers seat inserts for repair in
iron heads only if needed.A The cutting necessary to install a A seat
weakens the head and can promote cracking, particularly if the head is
subjected to higher than stock pressures or heat (i.e. a high
performance engine) and the ports have to be ground to blend the seat
in.A I quote one machinist I know: "You want inserted seats about as
badly as you want a venereal disease."
Before the widespread adoption of induction hardened seats, some iron
heads had Stellite seats installed from the factory.A My 1938 Chrysler
straight 8 has Stellite exhaust seats but they are in the block (not
the head) since it's a valve-in-block design (a.k.a. a flathead).A
BTW, many of the seats installed in iron heads by machine shops are
ductile iron and not especially hard.A The hard seats are Stellite (an
alloy of cobalt and chromium) which is very difficult to cut
(especially with stones) and much more expensive than ductile iron. A
12 of the 14 Cleveland heads in the attached picture are being prepped
for a trip to the machinist (Dave McLain), along with 2 of 6 straight 8
flat heads.A I didn't see any seats that looked like they needed
replacing but we'll see what Dave has to say. A
Dan Jones
On Tue, Mar 26, 2024 at 10:27a-PM John McKee <[1]johnmckee at cox.net>
wrote:
A A I need a recommendation for an engine builder/machine shop in
Orange
A A County, CA.
A A Back story:
A A After sitting for 10+ years I finally fired up my engine a few
weeks
A A ago. It sounded good for 10 minutes then I had a bad clatter in
the
A A valve train. At first it appeared to be a loose rocker but
there was
A A weird interaction with the valves of cylinder #4. I popped the
head off
A A and found that the exhaust seat had come loose! While dancing
around
A A the seat also made some deep marks on the piston head.
A A My gut tells me the piston is trashed and needs to be replaced.
I have
A A a friend that says I should repair the head and go as is. I
don't know
A A if anybody on this forum has an opinion.... But, when is a
"witness
A A mark" a catastrophic failure in the waiting?
A A I posted photos on the picture forum at this link.
A
A [1][2]https://pantera.infopop.cc/topic/valve-seat-failure-and-pist
on-damag
A A e
A A Thank you for your thoughts,
A A John
References
A A 1.
[3]https://pantera.infopop.cc/topic/valve-seat-failure-and-piston-da
mage
_______________________________________________
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Members who post to this list grant license to the list to forward
any message posted here to all past, current, or future members of
the list. They also grant the list owner permission to maintain an
archive or approve the archiving of list messages.
References
1. mailto:johnmckee at cox.net
2. https://pantera.infopop.cc/topic/valve-seat-failure-and-piston-damag
3. https://pantera.infopop.cc/topic/valve-seat-failure-and-piston-damage
4. mailto:DeTomaso at server.detomasolist.com
5. http://server.detomasolist.com/mailman/listinfo/detomaso
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