1958 Positraction Differential

[Richard H.]

How does one differentiate a positraction differential for an early 1958 Corvette from any other GM vehicle of the same vintage year using a positraction unit if the only information available is the casting number and date and the large letter P? Thanks. Richard

[Roy B.]

Re: 1958 Positraction Differential "none" *NM*

[Ted S.]

Roy, how about thin P verses thick P....

any truth to vettes only used thin P castings?

[Roy B.]

Re: Roy, how about thin P verses thick P....

That's something I don't know, but I have one original owner friend with a early 57 that I've seen the pumpkin that is a Posi, with no P casted in the unit.But I believe that Corvette and car had the P. Thin or thick I cant say .

[Jim D.]

Re: 1958 Positraction Differential

Corvette housings had a thin raised "P" cast into the housing, pass. cars had a wider flat "P".

[Ted S.]

Re: 1958 Positraction Differential

Jim, I thought the thin verses thick P was due to the foundry the casting was made in. So cars could have thin or thick P castings verses Corvettes only had thin P castings. Correct?

[Jim D.]

Re: 1958 Positraction Differential

You are correct. Corvette only had the thin P, pass. could have either.

[Justin B.]

Re: 1958 Positraction Differential

I was told (don't know if it is true or not)that another visable way was the presence of the three machined holes on the end of both of the axles which are visable with the hubcaps off? Also was told that the "P" didn't come till late 1957?

[Gary C.]

Re: 1958 Positraction Differential

'57 Posi's - Jan and maybe Dec through March had a small "X". P was added in the Mar timeframe. Recall my posi axles have 2 holes in end. Not sure about the thin vs. thick P casting for Corvettes. Gary....

[John H.]

Re: 1958 Positraction Differential

"Thick P" castings were made at Buffalo, and only went into passenger cars (they also had 2-digit year characters in the casting date). "Thin P" castings were poured at Saginaw and machined/assembled at Detroit Gear & Axle, and were used in both Corvettes and passenger cars (and had a single-digit year character in the casting date). Gear & Axle was the sole source for Corvette differentials.

[Mike M.]

Re: 1958 Positraction Differential

john: yer amazing. never heard or picked up on the two sources of posi pig cases.its a sad day when we don't learn something new about the c-1 and c-2 vettes. sad days becoming more frequent for me as the knowledge of their builds becomes un-earthed. today not a sad day. thanks for the enlightment. mike