Packard Forum Index » Pre-War (1899-1942)

1940 - 356 engine ring job

Postby Bill Butterworth ? Fri Aug 16, 2013 9:23 am
In the process of doing a ring job, what is the clearance spec. between the pistons and cylinder wall? Also, what is the allowable "rocking motion", if any, for the wrist pins? What is the best procedure for removing and then replacing the pins? Should I replace the pins or the bushing?
Thanks, Bill

Dear Bill:

It depends whether you have original or replacement pistons.

The cylinder taper is the important spec. If you haven't removed the piston yet you can get an idea by cleaning off and measuring the ridge. Typical taper tolerance is .005 and roundness .003.

Original pistons were running under .001 clearance.

New cast or forged pistons typically .002-.0025 clearance.

Pin bushings typically under .001 (.0005-.0007) or a push fit at room temperature. Pins in pistons are an interference fit at room temperature (push fit when heated).

Don't plane the head unless it fails when checked with a calibrated straight edge as cutting it can result in knocking. Store the head on it's side to prevent bending. Also, when checking it can be laid on a machinist's table and checked when pressing a palm in the center if suspect.

Hope this helps.

Tim, yes, this does help. One piston clearly "rocks" on the wrist pin. Removed pin and it measures .878" which I believe is .003"over size. This was done I assume when the engine was rebuilt in the past. The pistons are not original and are .020" over size. The question I have for the "rocking" wrist pin is, do I try and find a .006" over size wrist pin and if I do will heating the piston in hot water allow the pin to fit. Or do I have install new bushing and have it reamed to fit the current .003" over pin? How are bushings removed?
Thanks, Bill

Hi Bill:

Those bushings can be replaced, but I would take all the rods to a good machine shop and have them checked for alignment.

You can also get an estimate from EGGE or Terrill which are two old line shops that do Packard work. They can send you back the rods and pistons with rings matched to the grooves.

They may also knurlize the skirts to take up any noise.

Hi Tim, thanks, will follow through and contact AGGE for a quote. They rebuilt the oil pump for this engine and did a good job. I guess fitting the pins, bushing with the pistons is more than a replacement parts job, it requires some actual machining. For future reference, where is Terril located?
Bill

Quote:

Terrill Machine, Inc.
Engine Parts For Antique Cars
100 County Road 454
De Leon, Texas 76444
Phone: (254) 893-2610
Fax: (254) 893-4841

Hi Bill,

One thing that may have caused your problem is that parts don't always come back clean and there may have been dirt stuck in the oil feed hole on that rod. So check all parts for cleanliness when you get them back. And of course make sure the oil hole is actually feeding the bushing.

Tim, thanks again for the information. I believe the problem with #6 cylinder started with a leaking head gasket at #6 cylinder (white exhaust and low compression). The two oil passages in the # 6 rod checked out clear. I was concerned about just replacing the head gasket and not checking the head. It was cut .013" to make it true. The mistake I made was not checking the head clearance before re installing and bent #6 exhaust valve. Have sense found out via a copy of the Packard parts manual that two head gaskets were available (not counting the "extra thick" export gasket). The replacement gasket I used was the .060" not the .091", which would have worked. Since then I have another head (uncut) and have done a valve an guide job. Was able to stat engine one time after valve job then engine wouldn't restart. Did air leak down test and found all cylinders had 60% leakage into crankcase, no air lose at tail pipe or carburetor. Added 2oz oil and recheck showed good compression again. Pulled pistons and found rings collapsed into groves and very difficult to get out, lots of black carbon. Rings are now out. Rod bearing were previously replaced with .001" over size when I assume the.020" pistons and hardened seats were also installed. It also appears that over sized pins were installed at that time as the wrist pin bushings look quite worn. That's the whole story.