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55 DeSoto Broken Timing Chain
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SharpShifter
Posted 2018-12-26 6:26 PM (#575855)
Subject: 55 DeSoto Broken Timing Chain


New User

Posts: 4

Location: Sacramento, CA
Hey all, just bought a 55 DeSoto with a 291 Hemi which was rebuilt last year. Long story short: the mechanical fuel pump failed by way of the arm pivot pin coming out causing it to lose it's bits inside of the timing cover. One fuel pump change and 20 miles later the car quit again which I was able to diagnose as a failed timing chain. Looks like the bits from the fuel pump compromised the chain causing it to fail.

I've replaced the timing chain (gears are OK.)

Question: Is this engine an interference type motor? I was able to turn the crank all around town while trying to replace the chain with no binding. First time replacing a chain on this type of engine, so I made sure the timing marks on the cam and crank gears were pointing directly toward one another. Is this correct? Thanks for any insight!
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58coupe
Posted 2018-12-27 10:43 AM (#575874 - in reply to #575855)
Subject: Re: 55 DeSoto Broken Timing Chain



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Location: Alaska
Yes, you align the 2 marks toward each other. Not sure if it is an interference motor but you can just do a compression check before you run it. That will tell you if you have any leaky valves.
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ronbo97
Posted 2018-12-27 11:07 AM (#575876 - in reply to #575874)
Subject: Re: 55 DeSoto Broken Timing Chain


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Location: Connecticut

How did you determine that the chain failed ? When you removed it, was it in pieces ? Or was it stretched to the point where there was a ton of slack ?

IMHO, those chains are plenty strong. The fuel pump arm, if disengaged, should just fall away and likely end up in the oil pan. And your car would quit seconds later due to lack of fuel.

Ron

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SharpShifter
Posted 2018-12-27 2:49 PM (#575887 - in reply to #575876)
Subject: Re: 55 DeSoto Broken Timing Chain


New User

Posts: 4

Location: Sacramento, CA
ronbo97 - 2018-12-27 9:07 AM

How did you determine that the chain failed ? When you removed it, was it in pieces ? Or was it stretched to the point where there was a ton of slack ?

IMHO, those chains are plenty strong. The fuel pump arm, if disengaged, should just fall away and likely end up in the oil pan. And your car would quit seconds later due to lack of fuel.

Ron



Good question- when the car quit I determined on the side of the road that the distributor wasn't turning while the engine was cranking. When I took the timing cover off the chain was broken draped over the cam gear There was a definite odd indentation on one of the cam gear teeth (not bad) and I did find the crushed fuel pump lever spring in the front part of the oil pan. You're right in that the arm didn't do the damage and was relatively easy to retrieve once the pump was off.

Now having the new timing set installed and clocked properly (thank you 58Coupe) I can tell that there's definitely no compression on one cylinder, even blowing back up through the carb while cranking... so I've kissed a valve for sure. Compression test is next. The engine doesn't have 5k on it since a rebuild so I've got my fingers crossed that only one cylinder head will need to come off.
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wizard
Posted 2018-12-28 9:18 AM (#575921 - in reply to #575855)
Subject: Re: 55 DeSoto Broken Timing Chain



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You should check if the cam is 180º off first of all.

The timing marks fits together one time during the two revulotions of the crank shaft.


So, first crank shaft revolution starts with the timing marks together in line, next revolution, the crank shaft gear mark will be upwards and also the cam gear will be upwards, still in line.


In my '60 Chrysler, when I changed the timing set, the timing marks was both upwards with the #1 piston in TDC and the rotor pointing at #1 spark plug wire, both valves closed.





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58coupe
Posted 2018-12-28 12:18 PM (#575926 - in reply to #575855)
Subject: Re: 55 DeSoto Broken Timing Chain



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Location: Alaska
Sven, he doesn't need to check the distributor unless it was removed. With the mark on the crank gear pointing straight up, it is the position of the cam whether it is TDC firing position or exhaust. When both marks are lined up pointing toward each other, the valves and the distributor will be timed properly. I was surprised the first time I rebuilt a BB mopar with the marks pointing toward each other it was on the exhaust position but that's the way they built it.
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SharpShifter
Posted 2019-01-02 7:18 PM (#576161 - in reply to #575855)
Subject: Re: 55 DeSoto Broken Timing Chain


New User

Posts: 4

Location: Sacramento, CA
Thank you Gentlemen. The compression check showed a complete compression loss on #1 with the rest all in spec. I pulled the head, and sure enough there is a mark on the #1 intake valve and the #3 exhaust valve where they each made contact with the piston, though there is no compression problem on #3. Great knowing I've got the timing set clocked properly at least. Now waiting for the head to come back from the machine shop...
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Stroller
Posted 2019-01-06 1:18 PM (#576354 - in reply to #575855)
Subject: Re: 55 DeSoto Broken Timing Chain


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Valve meet piston your close friend. Bummer. Valves and pistons occupy the same place at the same time. Be surpised if the front of the cam isn't damaged from the force it takes to bust one of them chains.
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