[mounthopewalt]

The thermostat housing bolt on my 1990 Reatta is 11 mm.
On the 1988 Reattas the bolt size is 10 mm.
Did Buick change the size of the bolt? Or did someone install an oversized bolt on my Reatta?

[63viking]

Did you try a 3/8 wrench on it?

[mounthopewalt]

3/8 is too small. 7/16 is a touch loose. 11mm fits perfectly.

I'm trying to determine the correct OEM size for the 1990.

[Barney Eaton]

If a 11mm wrench fits, something is wrong.

GM made a habit of using nuts, bolts, etc that used 10, 13, & 15 mm wrenches on the common fastners..... I don't know if there were common smaller and larger sizes

[Richard D]

My 90 coupe is also a # 11.

[Drake]

Just checked my 89, definately 11mm.

[mounthopewalt]

I'll pass on the 50/50, 11mm is the final answer.

Thanks for the response folks.

[padgett]

add 18 mm - brake calipers and serpentine belt idler & 36 mm - front axle nut.
Have seen two different sizes on the crank balancer nut: 15/16" and 1 1/8" AFAIR