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torque spec for 1956 exhaust bolts

Started by Caddy Wizard, August 11, 2010, 10:40:44 AM

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Caddy Wizard

On my 56 60S, the shop manual says that the bolts that hold the exhaust manifold to the head are 3/8 thread and should be torqued to 25-30 ft-lbs.  But the bolts are actually 7/16 thread.  Usually, 7/16 bolts would normally be torqued to 40-50 ft-lbs.  25-30 seems way too low for these larger thread bolts.


Anybody know what is the right torque spec for these 7/16 exh bolts?
Art Gardner


1955 S60 Fleetwood sedan (now under resto -- has been in paint shop since June 2022!)
1955 S62 Coupe (future show car? 2/3 done)
1958 Eldo Seville (2/3 done)

J. Gomez

Quote from: cadster49 on August 11, 2010, 10:40:44 AM
On my 56 60S, the shop manual says that the bolts that hold the exhaust manifold to the head are 3/8 thread and should be torqued to 25-30 ft-lbs.  But the bolts are actually 7/16 thread.  Usually, 7/16 bolts would normally be torqued to 40-50 ft-lbs.  25-30 seems way to low for these larger thread bolts.


Anybody know what is the right torque spec for these 7/16 exh bolts?

Art,

Although the data on the Service Manual is incorrect for the bolt size, I would strongly suggest in staying with the torque specs showing. I’ve checked other repairs manuals and they all show the same torque.

FYI http://www.modifiedcadillac.org/documents/Multiple_Years/55-64%20Motors/tn/55-64%20Motors-4.jpg.html

Granted the bolts can be torque higher but the thickness of the exhaust manifold material would not support the extra torque, and one need to care for a bit of leg room for expansion.

Good luck..!
J. Gomez
CLC #23082

Caddy Wizard

So far, the only reply is that the lower range (25-30) should be used, even on these bigger bolts.  I am not so sure about that.


I am concerned about this because if the bolts are too loose, they will back off due to vibration and thermal cycling and then lose their clamping force (causing leaks).  On the other end of the spectrum, if they are too tight, they can fail (break or stretch beyond their elastic limits).  So it is very important to get it right.  On my 56, all of the exhaust gaskets had failed (even though this is a 45,000 mile car).  So the big question is what is the right spec.  I have to believe that the 25-30 number is wrong.  I think it IS the right number for a 3/8-16 bolt.  But it seems that it must be wrong for a 7/16-14 bolt. 


Commonly available engineering data suggests that for a "Grade 5" 7/16-14 fastener, the torque should be in the 40-50 ft-lb range and that the maximum torque for such a bolt is somewhere in the 55-60 ft-lb range.  For "Grade 8" fasteners, the numbers are even higher.


Here is a little info from the web (edited some by me):  A properly tightened bolt is one that is stretched such that it acts like a very stiff spring pulling two mating surfaces together. In the case of exhaust manifolds, the bolt acts like a spring clamp, to clamp the manifold to the head.  The rotation of a bolt (torque) at some point causes it to stretch, applying a clamping force. Several factors affect how much tension occurs when a given amount of tightening torque is applied. The first factor is the bolt's diameter. It takes more force to tighten (stretch) a 3/4-10 bolt than to tighten a 3/18-16 bolt because it is larger in diameter. The second factor is the grade of the bolt. It takes more force to stretch an SAE Grade 8 bolt than it does to stretch an SAE Grade 5 bolt because of the greater material strength.


Art Gardner


1955 S60 Fleetwood sedan (now under resto -- has been in paint shop since June 2022!)
1955 S62 Coupe (future show car? 2/3 done)
1958 Eldo Seville (2/3 done)

John Washburn CLC 1067 Sadly deceased.

Art,

The exhaust bolts, with the flange, seems to vibrate loose, tightening them down more does not seem to stop this aggravating problem. I even found new bolts and tried them, same symptoms.

I, now, use permatex thredlocker, which keeps these guys snug against the manifold.

That is my story. The 56 CDV has been driving me nuts for over 30 years, but someday I might just get everything working back to normal.

The Johnny
John Washburn
CLC #1067
1937 LaSalle Coupe
1938 6519F Series Imperial Sedan
1949 62 Series 4 Door
1949 60 Special Fleetwood
1953 Coupe DeVille
1956 Coupe DeVille
1992 Eldorado Touring Coupe America Cup Series

Art Gardner

John,


What torque spec have you tried?  Did you try the published 30 ft-lb?  Did you go as high as 40 ft-lb?


Art

Glen

Correct torque on exhaust manifolds is important.  As the engine heats up the manifold get hotter than the block and will expand more than the head.  Conversely when the engine and manifold cool they contract at different rates.  If the torque is too tight it pins the manifold to the block and creates strains that can crack it. 

I would stay with the lower torque rating. 

Glen Houlton CLC #727 
CLCMRC benefactor #104

Caddy Wizard

Glen,


The question is whether the 25-30 ft-lbs is the right torque spec.  Just because it is printed in the book doesn't mean it is true.  There are often errors in shop manuals, particularly when something has changed from one year to the next (like in this case where the bolt size increased from 1955 to 1956).  If it said to torque it to 125 ft-lbs or 5 ft-lbs, everyone would KNOW that the spec is a mistake and would not do that.  The problem is that 25-30 ft-lbs might merely be a carry-over number from the 49-55 engine spec using 3/8 bolts.


I agree that the correct torque spec should be followed.  I'm trying to find out what that is.


Art G.
Art Gardner


1955 S60 Fleetwood sedan (now under resto -- has been in paint shop since June 2022!)
1955 S62 Coupe (future show car? 2/3 done)
1958 Eldo Seville (2/3 done)

Rod Dahlgren #19496

Can't help myself... Come On Guys, wasn't one of the original SAE founders one of our own Cadillac Engineers???

The SAE came up with standard bolt markings and torque specs long ago... Here is a chart  http://www.engineershandbook.com/Tables/torque2.htm

Now, that said, I gotta add.... Bolt stretch is the "goal" but you can measure that on most bolts, so ya gotta use the SAE standard torque of the darn bolts will come loose....

I can say a lot more about mounting and securing exhaust manifolds so they don't leak, but I doubt that it will change the way some of you deal with it..  Gotta say, mine don't leak.  ;D
Did You Drive Your Cadillac Today?

Caddy Wizard

Rod,


You obviously see the problem -- the shop manual for '56 recommends a torque level that is correct for a 3/8 bolt and seems wholly inadequate for a 7/16 bolt.


I haven't heard any compelling information to use the 30 ft-lb torque spec for these larger bolts, so I am going with what would normally be correct for a 7/16 bolt (40-50 ft-lbs) and torque them to 40 ft-lbs.


Thanks to all who provided comments.  It is still a mystery to me, but at some point I have to stop fretting over this and just get the darned car back on the road...


Art
Art Gardner


1955 S60 Fleetwood sedan (now under resto -- has been in paint shop since June 2022!)
1955 S62 Coupe (future show car? 2/3 done)
1958 Eldo Seville (2/3 done)

Chris Conklin

... "There are often errors in shop manuals, particularly when something has changed from one year to the next (like in this case where the bolt size increased from 1955 to 1956)..."

Hi Art,

Just curious, I have had a loosening issue with my '66 Eldorado as well and the recommendation is 30 ft-lb. You mentioned it is a 45K mile car so I would guess you're fairly certain those bolts weren't changed by a previous owner (possibly to resolve the same problem you've experienced). Have you been able to confirm that they were changed by Cadillac from the '55 to '56 model year? Do you plan to just go with the higher torque alone or do you plan to apply a chemical fix also as John Washburn mentioned he had done? I've been tempted to try some Loc-Tite but am leery of the cure being worse than the ill. Thanks.

Chris
Chris Conklin

35-709

Possibly if someone has the 1956 Cadillac Serviceman bulletins for 1956 this issue has been discussed.
Geoff N.
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible - Sold
1950 Packard 2dr. Club Sedan
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

Art Gardner

Lou Commisso of the Cadill-Ikes group looked into the Serviceman to see if there was any mention of this and apparently there wasn't.


The bolts are the original, factory bolts with the integral flanges under the bolt heads -- special lengths, not hardware or parts store replacements.  They might be Grade 8 (I don't remember).  They don't use lock washers under the integral flanges.


As for Loctite, I guess I could do that.  But in an exhaust bolt one would worry about never getting them back out.  If I do use Loctite, I would only use the Blue, of course.  Maybe I'll do that on the driver's side and not do it on the passenger side.  On the driver side, you have to remove the head to remove the manifold (or even some of the bolts) so it is even more critical to keep the darned bolts in the thing.  Heck, you can't even torque at least one of the bolts on the driver side with the head on the car (unless you remove the steering gear or column).


I was a professional mechanic for 10 years and have a mechanical engineering degree.  So I tend to be a stickler for torque specs most of the time (especially on anything that can leak oil or fluid!).  It is that experience that made me suspect that the 30 ft-lbs was just plain wrong.


Art

Roger Zimmermann

When I overhauled my engines ('56 & '57), I certainly torqued the exhaust bolts according to the shop manual, without problem. 2 years ago, when I replaced the head gaskets on my '56 de Ville, I installed the exhaust manifolds when the heads were on the engine. It's just impossible to torque most of the manifold's bolts due to the lack of space, so I used a wrench and turned as far as I could. I have no problem...

Roger
1956 Sedan de Ville (sold)
1956 Eldorado Biarritz
1957 Eldorado Brougham (sold)
1972 Coupe de Ville
2011 DTS
CLCMRC benefactor #101

The Tassie Devil(le)

Whenever I get these bolts that I cannot get a Torque Wrench on, tighten up everything, then I torque up what I can, and then with a Ring Spanner, or whatever, I get a feel for the "Previously torqued up ones" and then do the rest by "Feel".

Bruce. >:D

PS.   Plus, I get to "Talk" to each one as I go.
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe