Russell and all: My recollection of stop leak products is that they do seal small leaks but also plug up heater cores, radiators and control valves. The radiator shops love it as it tends to triple their business. There may be better stuff now, but I doubt it. There are very few easy fixes in this world. This isn't one of them. Stop leak is only appropriate for an emergency repair job when there are no better options with the realization that permanent repairs and system flushing should be done ASAP. Rich Barber Brentwood, CA 1955 Chrysler C-300 We routinely use Bar's Leak Cooling System Supplement (J-120?) that does not unintentionally plug things up. We use AC Delco Cooling System Seal Tabs in 4100, 4500 and 4900 engined Cadillacs as it is a must use block sealer item specified by GM as otherwise the things will leak. I do not think that GM would spec something that would plug up their cooling system parts and that has been our experience with the stuff for about 20 years now. Will it stop a leaking heater core? Hmmmm - - -Well, my opinion is as stated by Mr. Barber except if some additive is to be tried, what I have found to not cause additional damage are the above products. Note that it will make extended life Dex Cool turn a (harmless) brown color that can offend and concern some folks. AC Delco #10-108 (GM #12378255) seal tabs are: Ginger Almond shell Turmeric - CAS #458-37-7 Sal soda MSDS disclosure from Bar's Leak shows same contents. I think you will find reference to such a home grown mixture of ginger root and friends mentioned in such automotive manuals as Dyke's published back in the '20s and that it will pop up in Ford 'T' literature also. It worked on the Model 'T' and it works for us on modern stuff also. Definately nothing high tech. Warren Anderson Sedona,AZ ------------------------ Yahoo! Groups Sponsor --------------------~--> Yahoo! Autos. Everything you need to know about buying or selling a car. FREE Quotes, 360° Tours, Research, Blue Book, Compare Vehicles, Buy Used http://us.click.yahoo.com/kEZsdA/bwnGAA/YiGOAA/8LmulB/TM --------------------------------------------------------------------~-> To send a message to this group, send an email to: Chrysler300@xxxxxxxxxxxxxxx For list server instructions, go to http://www.chrysler300club.com/yahoolist/inst.htm Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Chrysler300/ <*> To unsubscribe from this group, send an email to: Chrysler300-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/