Engine type

[Tbuff23]

On a new subject, I knoW this has been beat to death but my speedometer is not working when I disconnect the cable from the transmission it works with a drill up to 20 or 30 mph but bounces all over. I changed the cable but still it does not work I took out the gauge I cleaned and lubed it with the drill it would reach 10 mph it still will not work in the car. I changed the cable to no avail you can hear the cable spinning as I drive down the road. Any suggestions. Do I need a new Speedo.
thanks in advance

[Tbuff23]

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BamaNomad,
it is a 185 not a 186. The only other #’s are GM 48
and J12 8 attached are some photos of the #’s on the heads.
And engine I hope this helps. Thanks