Engine work
Friend Paul Austin and I have been up to the shop twice in the past month to tear into the motor and try and find the source of the mysterious knock.
First up was to check all the valve springs. After removing the manifolds and valve covers, we removed all the valves, mounting them on a valve stick to make sure we had them in the correct order.
All of the springs looked OK, though one valve spring cage was broken (the cup thingy). Got a replacement for that from Nelson Pease, then had the springs tested. They measured 52-54 lbs each, all in the correct range. End of that theory.
On the following trip, we dropped the pan, removed the head, and pulled the pistons and rods. This motor, a late production version (2 casting plugs) obtained from a junked parts car in 1987, was professionally rebuilt in 1991 and put into the coupe when its original engine got tired at 90K. I ran it for 10 years in the coupe with no problem, then rebuilt the original early production coupe engine (4 casting plugs), swapped it out with the engine now in the cabriolet, and had that engine "freshened up" by a rebuilder. The only thing we really did to it then, after checking all specs, was to replace one valve and put in new .001" rod bearings.
After pulling the pistons, we did find one broken top ring, but no other obvious sign of a problem with the pistons, wrist pins, bearings or rods. There was some minor scoring/scuffing of the pistons, but again, no big problem evident.
Paul carefully miked the pistons and the bores (they are aluminum, .040 over).
The gap between the pistons and bores was a little high - .0068 on average – but not so wide as to cause a problem (we think).
When I got home, I took the pistons to Frank Lamb, an old engine guy here in Portland, and he agreed that they all looked fine and should pose no problem. BUT, he did notice that the rod bearings seemed loose in the connecting rod bores and saw evidence that there was a slight gap between several bearings and their corresponding rod bore. AHA says I, that's the one part we changed, so that may indeed be the problem. We had noticed how easily the bearings separated from the rod bores when removing them.
I'll be taking them to a machine shop to have the bores measured. I also have a NOS GM set of rod bearings and will try them to see if they sit any firmer in the bores.
Feels like we're getting closer. We'll see!
First up was to check all the valve springs. After removing the manifolds and valve covers, we removed all the valves, mounting them on a valve stick to make sure we had them in the correct order.
 |
| From 1936 Pontiac Restoration |
 |
| From 1936 Pontiac Restoration |
 |
| From 1936 Pontiac Restoration |
All of the springs looked OK, though one valve spring cage was broken (the cup thingy). Got a replacement for that from Nelson Pease, then had the springs tested. They measured 52-54 lbs each, all in the correct range. End of that theory.
On the following trip, we dropped the pan, removed the head, and pulled the pistons and rods. This motor, a late production version (2 casting plugs) obtained from a junked parts car in 1987, was professionally rebuilt in 1991 and put into the coupe when its original engine got tired at 90K. I ran it for 10 years in the coupe with no problem, then rebuilt the original early production coupe engine (4 casting plugs), swapped it out with the engine now in the cabriolet, and had that engine "freshened up" by a rebuilder. The only thing we really did to it then, after checking all specs, was to replace one valve and put in new .001" rod bearings.
 |
| From 1936 Pontiac Restoration |
After pulling the pistons, we did find one broken top ring, but no other obvious sign of a problem with the pistons, wrist pins, bearings or rods. There was some minor scoring/scuffing of the pistons, but again, no big problem evident.
Paul carefully miked the pistons and the bores (they are aluminum, .040 over).
 |
| From 1936 Pontiac Restoration |
The gap between the pistons and bores was a little high - .0068 on average – but not so wide as to cause a problem (we think).
When I got home, I took the pistons to Frank Lamb, an old engine guy here in Portland, and he agreed that they all looked fine and should pose no problem. BUT, he did notice that the rod bearings seemed loose in the connecting rod bores and saw evidence that there was a slight gap between several bearings and their corresponding rod bore. AHA says I, that's the one part we changed, so that may indeed be the problem. We had noticed how easily the bearings separated from the rod bores when removing them.
I'll be taking them to a machine shop to have the bores measured. I also have a NOS GM set of rod bearings and will try them to see if they sit any firmer in the bores.
Feels like we're getting closer. We'll see!
posted by Bob Shafto at 8:25 PM
2 Comments:
Check your timing gear. The sound can travel all over and it's hard to tell what or where it's coming from. I've had this on 3 different flathead 6's
Thanks. It's got a timing chain, with gears on the cam and crank. If this piston work doesn't fix it, I guess that's the next thing to tear into.
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