Thermostat Adjustment

by Robert S. Hoover

"How does one adjust a thermostat properly?"

Adjusting your thermostat is pretty simple. But first you have to get at it. To do that, you'll have to remove the rear section of the lower tin-ware on the passenger-side of the engine.

Once you have access to the thermostat and its bracket, remove the bolt securing the thermostat bellows to the bracket. The engine should be cool and the thermostat bellows fully closed when you do this.

With the bellows free in the bracket, LOOSEN the nut holding the bracket to the stud that projects from the side of the crankcase. You want to be able to slide the bracket up & down but the nut must be firm enough to hold the bracket in position when you let go. Now check your flaps to make sure they are FULLY OPEN. The procedure here varies according to the year you have. The basic idea is that when the rod attached to the bellows is pushed UP the flaps will be pushed OPEN, so one way to check is to simply push the rod up as far as it will go. It should stay there, thanks to the spring attached to the connecting rod linking the two pairs of flaps.

Fully up -- fully open -- is the HOT position.

Notice how the bracket completely surrounds the bellows? What you want to do is cause the upper part of the bracket to just touch the upper part of the bellows. In practice, the bracket serves to prevent the bellows from expanding too far, which can cause the bellows to crack.

When you have the bracket properly positioned, tighten down the nut.

Now comes a bit of fumbling. Reach up, grasp the bellows and pull it DOWN, rotating it as needed to cause the flat-sided boss on the bottom of the bellows to mate with the hole in the bottom of the bracket. The flat-sided boss prevents the bellows from rotating, which would cause it to unscrew itself from the actuating rod.

If the bellows can't be pulled down far enough to mate with the bracket, you can back-off a few turns from the rod. But insure you have at least six full threads of engagement (more is better).

Insert a short 8x1.25mm bolt with a suitable FLAT washer, plus a warpy washer, into the threaded hole on the bottom of the bellows and tighten it down while holding the bellows firmly aligned in the flat-sided hole. The flat washer must be large enough to span the boss on the base of the bellows and contact the bracket, otherwise the thing will simply spring back up.

That's all there is to it. The first time you do it, take as long as it takes. Once you've done it a few times, it takes only a couple of minutes to set the adjustment.

When pulling the engine for maintenance that involves removal of the blower housing, it's usually most convenient to remove the thermostat from its bracket and to unscrew it from its rod as part of dropping the engine, when the vehicle is hoisted up and there is room to get at the underside. For the same reason, it makes good sense to hold-off re-installing the thermostat and lower tin-ware until you replace the engine in the vehicle.

The short bolt and large-diameter flat washer used to secure the thermostat to its bracket are somewhat unique. It's a good idea to keep them with the bellows.

-Bob Hoover

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