The 99 and older Northstar aluminum engine blocks were cast differently than the 00 and later Northstars, the older ones are more porous. From what I understand the stop leak "coolant supplement tablets" dissolve in the coolant and stay in suspension until a small leak forms. Then the stop leak comes in contact with air, hardens up and plugs the leak. These tiny leaks tend to develop around the head gaskets. This is part of the reason why Northstars are known for blowing head gaskets. There are two factors that come into play with these Northstar'* when it comes too head gaskets. One is coolant maintenance, changing the coolant to keep the anti-corrosion properties at an expectable level. The second is the use of the stop leak, in fact there is a sticker under the hood of my 98' ETC that says "failure to use the tablets will result in serious engine damage". One other point of interest about the tablets/powder, this product is all organic. I believe it is made of ginger root and almond shells. A wealth of information on this subject can be found at http://www.cadillacforums.com/

I am not saying it'* bad, lets get that right out front.

The bars leak and stuff like that is fabled to coat everything, possibly restricting some cooling capacity.

I am the first person to recommend the powder version as it has saved my A** before and kept working like a champ for years after.

78 Mercury Grand Marquis 460ci 4 hours from home and the heater core starts fogging the windows in a massive rain storm. We stopped...got the powder Alumseal and within 5 minutes were cleaning the windows and rolling at the speed limit again.

http://www.barsproducts.com/coolf1.html

Additional info from their website on how it works. My brother just put in the liquid in his Montana van, and it stopped the leak. I will likely be doing that to my Bonny soon.

The only stop leak product I would ever use is the organic Bar'* Leaks tablets or powder. I would strongly discourage use of the metallic products, they are known for clogging radiators and heater cores. The organic product requires air to allow the product to harden.

When I cleaned out the coolant surge tank on my 98' ETC, I found a sludge type residue from the tablets in the tank. It was not hard at all, and I see no way that it could clog a pressurized system.

For over a year my 98' ETC has been running with 6 tablets in the cooling system. About 9 months ago my 95' SE had the dreaded upper intake failure. I put 4 tablets in the Bonnie, after replacing the intake. Both cars have experienced no problems. I also have an 88' Continental, last summer I did a flush and fill on that beast. I put 6 tablets in that one too, and it has no cooling systems issues.