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This is a how-to guide about how to modify a HyperTech Power Stat #1007 (170 degree) thermostat so it will fit the 3800 engine used in the Buick Reatta. The HyperTech Power Stat #1007 thermostat was intended to be used in the 1984-85 Buick Grand National 3800 engine that has a larger diameter thermostat. The working parts of the Hypertech are the correct size to fit the Reatta 3800 engine.

AACA Reatta forum member drtidmore was the first to do this modification and install the HyperTech Power Stat #1007 in his engine. The following instructions and photos show how I modified the HyperTech thermostat to fit my 3800 engine based on drtidmore's previous work.

Most Reatta owners won't be interested in using the HyperTech Power Stat #1007 thermostat and I'm not recommending it. I'm providing these instructions for those who want to keep their engine cooler than normal.

In the photo below you can see the difference in size between a standard 3800 thermostat and the HyperTech Power Stat #1007 thermostat. It is possible to reduce the flange diameter of the HyperTech Power Stat #1007 so it will fit into the 3800 engine by carefully grinding away the flange.

HyperTech Power Stat #1007

When I laid the parts out for the photo above I noticed the Water Outlet o-ring (that fits in the groove) was about the same size as the standard Reatta thermostat and it measured to be only slightly larger in diameter (+0.068"). I found that the o-ring can be used as a good template to mark the HyperTech thermostat for grinding off the excess metal so it will fit the engine in the Reatta. All you have to do is lay it on the backside of the HyperTech. The HyperTech already has tabs sticking up that the o-ring fits snugly against to keep the o-ring centered.

O-ring centered on the Hypertech

Once you have the o-ring centered on the HyperTech you can use a magic marker to trace around the o-ring which will leave a nice black line to use as a guide when grinding the HyperTech down to size. Be sure you make your mark on the outside diameter of the o-ring and don't allow the tip of your marker to go under the o-ring which would result in a smaller diameter mark. If you carefully trace the o-ring you can then grind away the black mark you made leaving the HyperTech almost perfectly round with about 1/16", left to remove.  (I couldn't hold the o-ring and take the photo at the same time so the o-ring isn't centered in the photo.)

Marking the Hypertech

Once I ground off the last bit of metal to get the Hypertech the correct size (44 mm), I used a Felpro 35126 Thermostat Seal to install the thermostat. I went to AutoZone and picked out the seal for my '88 model Reatta that fit the Hypertech the best. With the Felpro thermostat seal I was able to install the modified HyperTech thermostat without problems.

You may need a different thermostat seal. It is my understanding that a number of different seals and/or o-rings were used for different model years in the 3800 engines. Be sure you get the correct thermostat seal for your engine.

Felpro thermostat seal

Thanks to drtidmore, member of the AACA Reatta forum, for providing information used in this how-to guide.

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