65-L76 Pulleys

[Mike M.]

I have 2 Crankshaft pulleys with GM stamp and numbers on them. I was wondering if someone could tell me what they are used for or which is correct for my engine. This is a no air or power steering, it has the idler pulley. These are both 2 groove pulleys the numbers on each are 3858533 grooves are aprox 1/2in wide & 3744043-BA grooves are aprox. 3/8 in wide, both are 6 3/4in. diameter. Also is this the correct part number for the water pump pulley Gm#3871242.
Thanks Mike

[John H.]

Re: 65-L76 Pulleys

Mike -

I don't have a '65 TM&JG, but the '65 A.I.M. sheet for L-76 with the idler pulley shows the crank pulley as #3858533 and the water pump pulley as #3770245.

[John H.]

Re: 65-L76 Pulleys

Mike -

I don't have a '65 TM&JG, but the '65 A.I.M. sheet for L-76 with the idler pulley shows the crank pulley as #3858533 and the water pump pulley as #3770245.

[Joe L.]

Re: 65-L76 Pulleys

Mike-----

The GM #3858533 is the correct balancer pulley for your application. This is a 2 groove pulley of 6-3/4" OD. The pulley which you have that has the number 3744043 is actually pulley GM #3747479. This pulley was used for most 63-66 base engines. It is a 2 groove pulley of 6-5/8" OD, so it's just a bit smaller in OD than the 3858533. The GM #3858533 pulley is still available from GM.

The correct water pump pulley for your application is the GM #3770245 that John mentioned and the pulley should be stamped with that number. It is a 2 groove pulley of 7-1/8" OD. At the moment, I don't know what application that the GM #3871242 pulley is for. However, I don't think that it's a Corvette application. The GM #3770245 pulley is still available from GM.

[Joe L.]

Re: 65-L76 Pulleys

Mike-----

The GM #3858533 is the correct balancer pulley for your application. This is a 2 groove pulley of 6-3/4" OD. The pulley which you have that has the number 3744043 is actually pulley GM #3747479. This pulley was used for most 63-66 base engines. It is a 2 groove pulley of 6-5/8" OD, so it's just a bit smaller in OD than the 3858533. The GM #3858533 pulley is still available from GM.

The correct water pump pulley for your application is the GM #3770245 that John mentioned and the pulley should be stamped with that number. It is a 2 groove pulley of 7-1/8" OD. At the moment, I don't know what application that the GM #3871242 pulley is for. However, I don't think that it's a Corvette application. The GM #3770245 pulley is still available from GM.

[Joe L.]

Re: 65-L76 Pulleys

Mike-----

GM #3871242 is not a water pump pulley, at all; it's an alternator pulley. Also, it's the correct one for your application. It has a 3-5/8" OD.

[Joe L.]

Re: 65-L76 Pulleys

Mike-----

GM #3871242 is not a water pump pulley, at all; it's an alternator pulley. Also, it's the correct one for your application. It has a 3-5/8" OD.

[Mike M.]

Re: 65-L76 Belts

Joe, Now that I have the correct pulleys the belts I have seem to be too thin.They ride down in the groove and appear to be riding on the bottom. The belts are from Quanta and the GM numbers I was told that were correct for my engine are GM# 3861946 Generator and GM# 3871258 idler pulley. The AIM shows a number of GM# 3871257 generator and the 3871258 as above for the idler pulley.
Did they sell me the wrong belt for the generator. Also is the 3871257 the correct belt for my engine.Thanks to Joe & Jack for your help.

[Mike M.]

Re: 65-L76 Belts

Joe, Now that I have the correct pulleys the belts I have seem to be too thin.They ride down in the groove and appear to be riding on the bottom. The belts are from Quanta and the GM numbers I was told that were correct for my engine are GM# 3861946 Generator and GM# 3871258 idler pulley. The AIM shows a number of GM# 3871257 generator and the 3871258 as above for the idler pulley.
Did they sell me the wrong belt for the generator. Also is the 3871257 the correct belt for my engine.Thanks to Joe & Jack for your help.

[Mike M.]

Thanks John *NM*

[Mike M.]

Thanks John *NM*

[Joe L.]

Re: 65-L76 Belts

Mike-----

The belts that Quanta sold you are correct. The GM #3871257 MAY have been used in PRODUCTION, but I think that the 3861946 was either also used or was actually used exclusively for the alternator application. The belts differ only slightly in length. The 3861946 is 54.0" long and the 3871257 is 54-3/4".

The reason that the belts appear to ride lower in the pulley grooves is that these are 3/8" wide belts. In the "old days", most GM V-belts were of the 3/8" wide variety. Later, GM replaced most of the 3/8" wide belts with 7/16" wide belts for SERVICE and for PRODUCTION after about 1973. Most aftermarket manufacturers did, too. So, the belts that you're used to seeing and likely the ones that your car was equipped with prior to your installation of reproduction belts were of the 7/16" wide variety. Either width belt will function perfectly with your original pullies. The 3/8" original belts were of "wrapped" construction; the 7/16" belts are of "cut" construction.

From a FUNCTIONAL perspective, which width belt is better? The 7/16" width, of course. GM didn't change to the 7/16" width belts for PRODUCTION because the 3/8" worked better. The 7/16" "cut" belts are stronger and more durable than the 3/8" "wrapped" type.

[Joe L.]

Re: 65-L76 Belts

Mike-----

The belts that Quanta sold you are correct. The GM #3871257 MAY have been used in PRODUCTION, but I think that the 3861946 was either also used or was actually used exclusively for the alternator application. The belts differ only slightly in length. The 3861946 is 54.0" long and the 3871257 is 54-3/4".

The reason that the belts appear to ride lower in the pulley grooves is that these are 3/8" wide belts. In the "old days", most GM V-belts were of the 3/8" wide variety. Later, GM replaced most of the 3/8" wide belts with 7/16" wide belts for SERVICE and for PRODUCTION after about 1973. Most aftermarket manufacturers did, too. So, the belts that you're used to seeing and likely the ones that your car was equipped with prior to your installation of reproduction belts were of the 7/16" wide variety. Either width belt will function perfectly with your original pullies. The 3/8" original belts were of "wrapped" construction; the 7/16" belts are of "cut" construction.

From a FUNCTIONAL perspective, which width belt is better? The 7/16" width, of course. GM didn't change to the 7/16" width belts for PRODUCTION because the 3/8" worked better. The 7/16" "cut" belts are stronger and more durable than the 3/8" "wrapped" type.