View Full Version : 478 pistons

March 9th, 2013, 08:46 AM
Steve, OEM pistons would be the answer, I just got my 478M fired up less than 3 weeks ago and it's got OEM pistons. The pistons were manufactured by "Zollner" for GMC and had "Clear O Matic" cast next to the piston pin opening. Clear O Matic stood for the cast in steel expansion band in the piston, there were two of these bands on each piston. Many aftermarket didn't have the bands or just had one. Checking out a TRW set that I have there's only one expansion band that's integral with the top piston ring slot. The OEM piston has a band there as well as a band below the oil ring, basically it's in the skirt and that's the problem area if theres no band in there. I guess I was lucky that my machinist located me a set of OEM pistons but that was quite a few years ago, they're not going to be an easy find. I would start searching, someone got to have them. Less than a year ago I came across this set of TRW 478 pistons on ebay, if I ever use them I will heat them out to see how much they expand. Anyway what to look for in OEM pistons is they were marketed by both GMC Truck as well as Zollner. The GMC version will have the GMC logo cast next to the piston pin opening with a part number over to the parallell side of the opening. On the other side will be Clear O Matic. The Zollner version will just have Clear O Matic, whichever the two you may come across just know they are the same.

March 10th, 2013, 10:36 PM
Thanks for the info Pete, I have a few sets of various V-6 pistons that I have aquired over the years, so I had a look and one of the sets is for a 351 .030 and they are Zollners, so I checked and sure enough they say clear-o-matic and a magnet confirms the steel band near the top of the skirt, none of the other sets I have show any steel in the skirt or pin boss area.
Kind of makes me wonder what kind of or if any testing went into the pistons these other manufaturers were producing in the day.

Stephen Michnick
Nelson, BC Canada

March 11th, 2013, 04:32 AM
The only other pistons I've seen with two expansion bands are Badger brand pistons, got a set of these for 351E I have. It does make you wonder what other piston manufacturers were up to as far as GMC V6 pistons are concerned. Piston scuffing has been an issue with the GMC V6 and aftermarket pistons but I do feel I can trust the Badger pistons.