Long time lurker, now I have some info to give back to the forum.
I have a wholesale account with a parts distributor, and yesterday I ordered a set of Goodyear heater hoses for my '98 GTP. When I went to install them this morning, I found that both hoses had 5/8" inlets and outlets. On the car, both engine fittings are 5/8", but the heater core has the usual 5/8" upper and 3/4" lower.
I went to Goodyear's website and used the online part number lookup to verify the part number so I could yell at my supplier, but they had sold me exactly what Goodyear calls for on the site. Turns out Goodyear's catalog is wrong.
I then went to Gates belts and hoses online and used their catalog - they showed the correct heater hose parts, and even listed the inlet and outlet sizes in the catalog. Armed with the correct gates part numbers (19109 = 5/8" x 5/8" and 19110 = 5/8" x 3/4"), I called my supplier back and they were able to cross-reference to the correct Goodyear part number.
I've just finished installing the hoses, and I can verify that the following Goodyear part numbers are correct:
64115 (5/8" x 3/4", or lower heater core fitting to engine)
64119 (5/8" x 5/8", or upper heater core fitting to engine)
The Goodyear catalog calls for 63115 instead of 64115 - that will get you a straight hose with a 90* bend on each end, and 5/8" outlets on each end. It won't work very well for a 3800 in a GTP.
If you'd like to do your own online parts research for Goodyear or Gates, the websites are below:
Goodyear: Goodyear Engineered Product Online Parts Catalog
Gates: NaviGates
