Transmission vacuum modulator leaking?

[gordon gibson]

So the verdict is in - hairline crack in the left cylinder head (#3) between the intake and exhaust valves. I guess the big question is why? Engine
rebuilt 10,000 km ago (new rings , bearings, timing chain, etc., heads done but not magnafluxed) Really sweet running mill, burned less than a 1
quart of oil over the 10K km. I did experience a stuck thermostat at 2,500 KM, but thought I caught it quick enough before the digital temp reading
exceeded 225....and it ran without a hiccup for 7,500 km afterwards,

Matt mentioned short cycles in cold weather, and it was below 0 centigrade when I started it up and saw a puff of white smoke for the first time. But
I'd done a bunch of driving in Northern Alberta at the 5,000 KM mark in -10C with no problems - driving another 5,000 problem-free kms after that.
And surely lots of Toronados got driven to the grocery store in the dead of winter....

Frank Borrmann has some J heads. We are going to magnaflux one before doing the machining just in case, as well as doing the same for my "good" head
and my manifold before reassembling just to be safe.

If anyone has ideas on what should NOT be done to avoid a repeat, I'd love to hear them.
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada

 

[Jim Hupy]

Don't use 40 year old heat cycled castings. This stuff is ageing out.
Careful inspection and Magnaflux are your friends. Cautious torque
procedure all help. Luck figures in there, too I think.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> So the verdict is in - hairline crack in the left cylinder head (#3)
> between the intake and exhaust valves. I guess the big question is why?
> Engine
> rebuilt 10,000 km ago (new rings , bearings, timing chain, etc., heads
> done but not magnafluxed) Really sweet running mill, burned less than a 1
> quart of oil over the 10K km. I did experience a stuck thermostat at
> 2,500 KM, but thought I caught it quick enough before the digital temp
> reading
> exceeded 225....and it ran without a hiccup for 7,500 km afterwards,
>
> Matt mentioned short cycles in cold weather, and it was below 0 centigrade
> when I started it up and saw a puff of white smoke for the first time. But
> I'd done a bunch of driving in Northern Alberta at the 5,000 KM mark in
> -10C with no problems - driving another 5,000 problem-free kms after that.
> And surely lots of Toronados got driven to the grocery store in the dead
> of winter....
>
> Frank Borrmann has some J heads. We are going to magnaflux one before
> doing the machining just in case, as well as doing the same for my "good"
> head
> and my manifold before reassembling just to be safe.
>
> If anyone has ideas on what should NOT be done to avoid a repeat, I'd
> love to hear them.
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
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> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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[Matt Colie]

Quite some time back, Jim Miller identified a mis-match between the water pump and the timing cover that could not be easily seen in normal assembly.
He believed it to be the cause of the left head cracking that he found.
The only way to see this is to compare the water pump to the timing cover off the engine. I did not see it in the two passcar motors that I tore
down.
Those yielded a pair if G and a pair of J heads. My visual inspection (that has not yet be proven wrong by an MPI) did not locate cracks in those.

Matt


--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[gordon gibson]

James - proper torque certainly makes good sense, but I'm not sure I understand the comment about avoiding heat-cycled castings. Any used head is
going to have experienced lots of "heat up to operating temp. - cool down" cycles. Are you suggesting using new aluminum heads? Does anyone have
experience - positive or negative - with these?
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada

 

[Jim Hupy]

No I was just stating what should be obvious. These engines are 40 years
old, they have been "rode hard and put away wet" to coin an old phrase. If
you have a choice and know the history of the heads you intend to use, then
the fewer heat/cool cycles they have experienced, the better for you. All
we can do is the best we can. No guarantee about cracks, the previous
history of the heads is largely unknown.
Jim Hupy

> James - proper torque certainly makes good sense, but I'm not sure I
> understand the comment about avoiding heat-cycled castings. Any used head
> is
> going to have experienced lots of "heat up to operating temp. - cool down"
> cycles. Are you suggesting using new aluminum heads? Does anyone have
> experience - positive or negative - with these?
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>