Correct Alternator Pulley

[Greg Y.]

Hi, I have a 1970 LS5 w p.s. no a/c Vin 00504, built Jan 70. I need advice on which pulley to put on the alternator. I understand pulley #'s 3883978 and 3871242 are correct. Which is preferred and why? Thanks in advance!

[Joe L.]

Re: Correct Alternator Pulley

Greg-----

GM provides conflicting information on this. For PRODUCTION, GM says that pulley GM #3829193 was used. This pulley was replaced by GM #3871242 in January, 1973, although the 3871242 had been around for other applications since 1965. Both of these pullies are 3-5/8" OD and are nearly identical.

Other information suggests that the GM #3883978 was used for LS-5 application. The 3883978 was 3-3/64" OD.

So, which of the above was actually used? My guess would be that it was the 3883978. Its smaller diameter would provide for a higher alternator drive speed. The larger diameter 3829193 and 3871242 would have provided for a slower drive speed to prevent over-speeding the alternators with higher RPM big blocks (although none were actually available for the 1970 model year).