I acquired (for free) a 1977 F-250 with a 351M, C6. The problem with the engine is that it has a broken camshaft at the timing gear (below is a link to a photo of the damage known so far). I don't know anything more about the failure. My next step is to pull the valve covers and see if there are any bent pushrods or valves.

Has anyone seen this happen before? What might have caused it, other than just fatigued metal? Is there likely greater damage to the engine internally?

Thanks

http://rides.webshots.com/photo/2235843680101530015nwOXwi?vhost=rides

 

It probably has about 1/2 million miles on it. I would prepare myself to pull the motor and rebuild it to a 400.

 

hi danlee paul, i am curious, did it BREAK or SHEAR. cams do fail in this manner but not as a rule. i am interested and curious. if for now other reason than to further my own education on the stress involved in an engine UNDER LOAD. i am curious any more details. later me.

 

It is not a real high mileage engine; I'll put in one of the 460's I have in my garage before I go through the expense of a rebuild. But if I can just put in a new cam that would be best.

Unfortuantely I don't know the details of the failure other than what I can see. Are the valves in danger of getting hit by the piston(s) if the cam suddenly stops turning?

 

YES. Did the accident happen while the engine was running? Or, did the cam snap when trying to start it after it sat without lube for too long? I have never seen something like that in a stock motor...

 

If the motor was under load when the cam broke, it probably damaged a lot, because the drive train would keep the crank turning against open valves etc.
If the motor was only idling or the tranny in neutral, then the first valve would stop it, and there would be less damage.

 

Unfortunately I don't know when or how the failure occurred. I will let you know what I find after I dig into it further...

 

hhi prr, absouletely, you know your self the chain determines the relationship of the cam to the crank. why not jerk the heads off quick like and look for VALVE to PISTON CONTACT. you may get lucky. i did one time with a 1970 ranchero that the time chain jumped a bunch of teeth because it was so worn. silly me i knew it was coming and i did nothing about it. it was a daily driver, i was young short on cash but in the end i paid three times over. god i loved that truck. a friend of mine eventually totaled it BEYOND repair. just a war story but true later paul. k md

 

Update: Pulled the valve covers, managed to turn the camshaft and the valve train seems to work. Loosened all the rockers, no bent pushrods. Crank turns freely as well.

Borrowed a leakdown tester. 7 of 8 cylinders leak 55-65%, but number 8 cylinder leaks about 85%. Cylinders 6 & 8, I can hear the air leak through the exhaust pipe. Of course this was done on a cold engine. The rings are leaking, as I can hear the air excaping into the crankcase.

Just trying to get the engine running, not looking to spend a lot. If a valve were bent, it would not seal and I would get 100% leakage, correct? Only thing troubling me at this point is #8 cylinder, but it may be good enough to get the motor running?

PS: all the plugs looked OK, no major carbon or oil fouling.

 

I don't believe the stocker is an "interference" engine. i.e. the pistons are so far down and the valve lift so small they don't hit even at full lift with the piston at TDC.

Your leak down is extreme however.

 

Are you sure about that? It could be, especially with the low compression ratio... I have never checked before!
Can anyone confirm this?

 

Beartracks, you are probably right about that, unless all or part of a valve has entered the cylinder.

It could be that is what broke the camshaft in the first place.

I think that the motor has to be torn down, before any attemt is made to run it. The clean plugs indicate that it was running normally and stopped suddenly.

 

hi bear, paul, in my nobody eyes an outstanding mechanical point, you see why i like this fr and the folks on it. i for one will be thinking very hard about your post for a while. if this is the case, then maybe like others said another reason. wish i had a spare midland around to investigate with. again very very good mechanical point in my op only. lot to think about i love it. later us. got to go think now.

 

Thanks to all for the input. Here are my thoughts so far:

Yes, the leak-down is extreme, but remember 2 points.
1) The engine was cold and has not run in some time (i.e. no oil on the cylinder walls); it is recommended to warm the motor up before a leakdown or compression test is performed. Interestingly, the right bank (pass side) had the better readings and no air noise out the exhaust.
2) 7 of 8 cylinders are in the same readings range, which according to the gauge I used, is "moderate" leakage. I think #8 cylinder has a leaking exhaust valve, and/or excessive ring wear.

My main intent of the leakdown test was to prove there are no valves bent or stuck open. (a broken or partially open valve would cause to leak 100%, right?) I think that has been proven even with the problem on #8 cylinder. I was able to turn the cam and valve train. Remember, I just want to get the motor running. If it were to be a daily driver, then yes a complete teardown & rebuild might be in order. But can I really go wrong putting in a new cam & timing chain set?

My latest theory on the cam failure: it is broken where the timing sprocket bolts on; the sprocket I think was still attached by the bolt & pin, but maybe that through it out of time just enough that it would not run. The previous owners tore it apart before I got it.

 

Oh yes, it will definitely have to be timed again after losing the sprocket...
cam timing will have to be set from scratch again, but it is a quick job with a degree wheel, pointer, and runout gauge/ travel dial indicator and a piston stop.