I make the urethane ones sold on ebay and elsewhere. If the spacers are red I probably made them. I use a high quality urethane elastomer to make my spacers. This plastic is not the cheapest thing I can use but is the best quality. They DO NOT sag, compress, or squish like other urethane spacers have been known to do in the past. Other companies, Daystar, use to soft a compound and thus the smashing effect. I have ran them for 6 years and no deflection. They do not squeak or make the ride sloppy like stated.
Don't get me wrong the Delrin spacers look very nice and probably are. The guy who makes them actually bought a set of mine and copied them. I know from experience that Delrin will crack on impact in cold weather. We have used Delrin, Nylatron, UHMW, cast exopies, etc, in a severe industrial setting and have found that this type of urethane is the best material possible to use. I work with plastics every day and have for 16 years now. I know what works and does not. Delrin is very durable and may hold up but in my experience the urethane just works better. Besides, I make the urethane ones, LOL.
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The choice is a personal one but I have sold THOUSANDS of kits with no complaints. If my parts were junk in any way I would hear about it, and you would have too. My .02
Contact me directly and I will save you some money versus the ebay listing. I also make a rear diff spacer I can include in the kit. hopekers(at)charter.net