We have a 70 W-31 with a 12 bolt cast iron rearend with a code of TO stamped in the axel housing. I have read where that is correct for a 391 gear. But, I have learned I can't believe what I read. So, is it correct? Also, we bought a W27 rearend with SX on it. What's that stand for. One more. This other W27 rear from Wisconsin has what looks to be a SZ on it. The S looks pretty good, but the Z is very hard to read. What's it stand for? Thanks in advance.
Wow, you are one lucky guy to have those parts! According to my "By the Numbers" book from supercarsunlimited the codes are as follows: TO 1967-70 3.91 Posi, 1971-72 3.73 posi W27 rears SV 3.23 Posi SX 3.42 Posi SZ 3.91 Posi If you decide to clean house and throw one of these out throw it my way!!! John
Thanks John. Glad you showed me the difference in the TO code. I wondered if it changed ratios in 1971. Also, glad to see a SZ on the other. I was having a hard time reading the Z. They had a horrible stamp when they punched it years back. I hope we don't get in the situation where we don't have to clean house. It took too long to find these.
2 letter codes are reused in different years and meant different things. In 1971, Olds stopped using their own design 12 bolt rear end and began manufacturing and using the corporate 10.5 design. The two letter codes were used to build the cars originally, not identify them later. Actual part number stamps are about the only positive id even though we didn't use them in the assembly plant. The broadcast "build sheets" referred to these codes in the mfg. process. The assembly lines were stripped of non carryover parts as the last car went down the line for the model year. Carryover parts were moved to the new point of use location (if relocated due to a process or line layout change.) As long asa the old parts wer removed from the plant, the same 2 letter code was reused on new year parts.
