390 low compression

[coop]

Thanks for the vote of hope Papabug. I know people dont always do what they say. I will keep you all posted on the rebuild.

[Ranchero50]

Cheap and easy check would be to remove the valve springs on the suspect ports, then lower the valve 1/2" and rock it around. It shouldn't have any movement. Sounds almost like they goofed the guide centerline to the seat centerline or the seat fell out but you'd be able to tell that right away. Can you pull the exhaust off and look down the port to see if anything obvious is screwed up?

Jamie

[coop]

Well I stopped by the machine shop to see the damage. The valve guides were wallowed and the seats were egg shaped and the valve stems were worn and burnt. We pondered what might of happened he asked me to bring in the ex manifolds to see if they had leaked. The heads showed some signs of leakage and so did the gasket. He ran the manifolds on his surfacer and both ends of each manifold were slightly warped. He smoothed them out. He thought that they might of leaked causing the valves to go bad. Could this be the culprit . He said because of the hardened seats the ex leak could of burned them.

What ever the case he is going to replace the bad valves and seats and guides at no charge. What is a good method of keeping the darn manifolds tight. When I removed them they weren't real tight, but I believe they only torque to 12lbs. I used lock washers and new bolts and copper spray on everything. Do you have to re torque them. The gaskets I used were Felpro the matte type. Is there a specific type anyone would recommend?

Thanks all

[70_F100]

I checked the Ford factory service manual, and sure enough, it specs the torque at 12-18 ft-lbs.

Something has to be wrong with that spec. That's more like the torque for a 5/16 bolt.

For a 3/8-16 bolt, such as those used on FE exhaust manifolds, the standard torque should run around 35 ft-lbs.

I've never used a torque wrench on FE manifolds, I just get them as tight as I can by hand. Using a standard 3/8 drive ratchet, that should put them at about 35, maybe a little over.

I've always used the stainless steel gaskets that have the spark plug heat shields incorporated.

As far as an exhaust leak causing a burned valve, especially in hot weather, IT AIN'T GONNA HAPPEN!!

With the low mileage you have on this engine, I still say it was substandard machine work.

One other thought came to my mind while reading your last post. If he put new valve guides in, but didn't ream them to the correct dimension, you could potentially get the same results you have. Generally, there is only about a .002" tolerance on the clearance of a correctly sized valve guide. Too tight, starts galding to the valve stem, causing excessive wear. Too loose, flops around too much, causes excessive wear.

[coop]

Well I picked the heads up they look good. He replaced 2 ex valves and redid the seats and guides. He uses grinding stones to cut the seats with a gudie to center it in the seat. Is this a good way to cut the seats? After doing some reading doing a good valve job is like doing heart surgery you have got to know what the hell you are doing.

I put the whole thing back together and it fired right up. It is running a lot smoother than it did before. I am going to do a compression test on it this weekend and see if everything is in line. I used the ex manifold gaskets with the spark plug heat shields, torqued them to 20 lbs, they seem to be holding. We will see what happens.

Thanks for everyone's input

[70_F100]

Forget the 20 ft-lbs torque.

I still maintain that's not enough to hold a 3/8-16 bolt properly.

Tighten them with a box end wrench and get all the torque you can on them.

Most technicians I've seen use an impact wrench on them with the engine out. That's probably too much, but it's been done for years.

[19ford71]

I've had trouble with my exhaust manifold gasket too. The last time I replaced one, I drove it just long enough to fully warm up, then let it cool and several of the bolts were able to be turned in a little more, then I ran it again, and again, several bolts could be turned in a little more but not as much as the first. So, maybe, running the motor and retorqing until you can't do retorque anymore is the trick. I'm not 100% sure but something I've noticed.