That is the feed for the heater core.
However, I don't suggest getting an "OEM Replacement" part - it's thin-gage mild steel. Fortunately, the threads are standard - 3/8" NPT.
1) Get a 3/8"x whatever brass pipe nipple at your local hardware store. 4-6" should serve. Cut off one end just beyond the threads and debur. Screw into water pump housing, apply some RTV black on the outside of the cut end, and slip the hose on (1/2" hose fits neatly over 3/8" pipe.) Allow at least 1-1/2" of overlap, put one clamp at least 1/4" from the bitter end of the hose, and the other should fit fully on the pipe with 1/4" between clamp bands.
2) I don't recall the length, but you can use a longer nipple (8-10"?) an elbow, and a shorter nipple (cut the threads off of one end) and eliminate the right-angle moulded hose - which saves you a few bucks on service. Use brass. Why brass? It won't corrode anywhere near as badly on you.
3) Not for the faint of heart, but you can go "whole hawg" and eliminate most of the heater hoses from the outset - I did this. I know it can be done on RENIX, and it can probably be done on 1991-up rigs as well.
NB: I added a hose bibb on a T fitting (for backflushing) and a shutoff valve (to force reverse flow when backflushing.) You need do neither - but at least the hose bibb isn't a bad idea. It allows me to screw my garden hose directly to the thing, I have a 3/4" hose bibb gender changer attached to a shutoff valve in my toolbox for this use.
Sorry about the big pic, but it should get the design across. I don't have any notes on measurement, and I do plan to revise them anyhow.
Construction is 1/2" copper line and fittings, sweat-soldered together. 1/2" copper line also fits neatly into 5/8" heater hose, although the two ends with 3/4" hose will fit better onto 1/2" copper tube unions instead of the line proper. Same rules for attaching the hose stubs (for vibration) - allow at least 1-1/2" overfit, 1/4" between the bitter (cut) end of hose and the clamp band, 1/4" between clamp bands, and full seating of both clamps over metal tubing. Apply RTV black on outside of tubing before installing hose (a small bead will serve - it helps the seal, and keeps the hose from getting stuck on too badly.)
The extra caution in sealing is done because the copper tube (or brass pipe stubs) are not beaded to accept a hose, and the hose therefore needs a little extra help to seal.
Good luck! I did the setup pictured in something like a half-hour, but I'm used to working with copper and heat. Most people who are figuring it out on the fly would take 60-90 minutes, I think.
Make sure to debur all cut ends before assembling, particularly where rubber hose goes over metal line!
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