View Full Version : 478M piston dilema.

March 9th, 2013, 07:59 AM
Hello everyone, finally made it over to the new board. As some of you may recall I had a project on the go last year to install a 478M into my '68 2500, I've been meaning to post an update but ran into some serious problems that I hoped to resolve by now but I'm at my wits end here with this situation.

The engine I purchased had been rebuilt, test run and put into storage for about 25 years so I completely disassembled it to clean and inspect everything plus I decided to have everything balanced since I was using a 379 flywheel and front hub. One thing that was apparant was that the pistons had been draging the cylinders so I measured the clearance and they showed .004, I figured no problem I'll just hone them out a little more to .005 and that should be more than enough according to the factory spec. So upon finally getting the engine installed and running I soon discovered that after shutting it down for the first time the engine was locked up to where I couldn't turn it over, even with the flywheel tool I had, but after letting it cool for an hour or so it was once again free and started right up. The engine never exceeded normal operating temperature so I knew what had to happen.

So out came the engine again, completely dissasembled, honed out to .0065 somewhat reluctantly because I knew this much would begin to create some audible piston slap. Engine back in, started and ran and once again after shutting it down the starter still wouldn't turn the engine. This time the drag was less though as I could get it to turn with the flywheel tool. So third times a charm I thought, out with the engine again, took the clearance to .008 now, there was no way I was going to do this again! Put it all back together and all seemed fine except for the now prominent piston slap at low rpm especially when taking off, which was really dissapointing since I know how quiet the 478 in my '63 water truck is which is still standard bore by the way.

Anyway everything else was great, incredible power and torque for a pickup, exceptional smoothness at idle vs. the amount of power I had, and was getting 14 mpg on the road with 4.10 gears.
I put on about 5000 miles with it last summer, but near the end of the season I was working it moderately one day towing the dump trailer with gravel up some steep grades and noticed the drivers side exhaust started puffing blue at idle. So I pulled the plugs and looked down the cylinders and could see #1 had some pretty good scratches on the walls.
So I started a quest to locate some different pistons, I remembered hearing about Mechanica Pederosa in Puerto Rico and sure enough they had a set in .060 that I needed and I now have them. I planned on having the skirts coated and built up a few thousanths with an anti friction piston skirt coating to bring me back closer to orginal clearance.
But now here is where I have come to my dilemma. I have the engine apart again and decided to do an expansion comparison with the two sets of pistons first. I heated and measured one from each set in an oven at 50 degree incriments up to 250 f and they both had almost identical rates of expansion, about .001 less on the new set. The new set is TRW and the casting mark indicates '78 dom. The original set has an SP in the casting mark dated '84. Honestly they look like they are almost from the same mould which is probably about right since TRW and Sealed power were affiliated. I wish I had an original GMC piston to compare as well.
This whole scenerio is way too reminicent of the experience I had a few years back with the custom Ross pistons I tried using in a 401. They too ultimately needed about .009 clearance to work but rattled like crazy.
So what the **** is the secret to the oem pistons and where do I go from here guys? If these engines weren't so cool and unique I'd be giving up already and I've rebuilt over a hundred engines in my years mechanicing from Briggs and Stratton to Cats, Cummins and Detroits. :confused:

Stephen Michnick
Nelson, BC Canada

March 9th, 2013, 03:35 PM
Wow man, what a journey you've had. The only thing I can think of is this: (and I have no idea if this is possible or not on the BBv6) but is there a chance that the pistons are installed wrong? For example, I know that on my Olds 5.7 diesel, the pistons have offset wrist pins by about .015". They did this to help rattling at idle. You can't see it with the naked eye, but it is there and it is possible to install them wrong.

Did GMC do anything like this? Have you measured the wrist pin to piston wall?

By the way, I love that country around Nelson. I used to live in Victoria and once took a road trip over the Crows Nest hwy to Medicine Hat. Just beautiful!

March 10th, 2013, 06:37 PM

Rick Rusconi here. I remember emailing you several times when you were working on the 401. I too was working on a 401. I think we have/had similar problems. When I built my 401, I bought a set of pistons that had no markings at all to tell their brand. I too was concerned with clearance, as I did not want to play the "in & out" engine game. So, I did as you are doing by heating a stock (original) piston and one of my replacement set in an oven and comparing expansion. I found that my replacement set grew by .003 more than the stock set. So, I had the machine shop increase the bore by .003.
It is now 6+ years later & about 35,000 miles and I have had no problems with the 401. I have no "rattle" when cold. So, I think you are on the right track, by comparing expansion in an oven. If you are looking for engine parts, I will pass on a possible contact. This is the person I bought all engine parts from to build my 401. He had or got everything I needed, I mean everything. Now, the last time I contacted him was 2007, so he may not be in business now. Note: in 2006 I went to his shop to pick up the parts I ordered. He had at least a dozen sets of 478 pistons. I do not remember sizes, or brand names. But they were there.
Good Luck

Frank Correia, PO Box 753, Pleasant Grove, Calif. 95668, (916) 655-3185
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March 12th, 2013, 12:21 AM
Mechanica Pedrosa has been mentioned here before.

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