Thermostat modification drill hole size

Want to know how to get more out of your Beretta? Or have a mod you would like to share?
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Thermostat modification drill hole size

Postby yellow3800 » Tue Feb 24, 2015 8:20 pm

Regarding taking a drillbit and allowing the thermostat to bleed coolant to effectively reduce its cycles, I have found that a 1/16" a good size.

How do I know? Ive been doing this for years and last summer, my 89gt got gummed up in deteriorating block seal [been running 50k miles with a 2" vertical crack in the front drv side cylinder WALL]. The block seal stuff also clogs the radiator cap, preventing flow from reservoir during cold-down... look for small fiber stuff if you use this stuff too. just rinses out with kitchen sink water...

anyway, I drilled my 89gt thermostat with a 1/8" bit since previous experience of 1/16" had been great. More better desired right? well, the 1/8" is WAY TOO BIG. 45min at idle before the fan kicks on is way too long. I drive 30min before passing the 1/4 mark on the coolant gauge when it normally rests at the half mark.

For those of you interested in this mod, drill 1/16" if the thermostat doesn't come with the bleed hole in the first place.

This is for a 2.8 V6 with manual trans [no auto cooling lines with manual] in my 89gt.

89gt <---subject car
lemon 3800 <---2.0 I4 radiator still in effect with trans fluid! no thermo mod, no problems

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Re: Thermostat modification drill hole size

Postby ifixalot » Thu Feb 26, 2015 3:08 pm

Good to know, I drilled one to 1/8' to let air get out
on the 1990 v6 auto.
Made the engine take way too long to warm up.
You wouldn't think it would make much difference but it does.

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Re: Thermostat modification drill hole size

Postby Rettax3 » Fri Feb 27, 2015 2:14 am

Thanks for the info. I've been thinking about this lately too. Have you tried orienting the bleed hole in any particular way? It seems sensible that the hole should be towards the top for air to more easily find its' way through, but maybe it doesn't matter.

1989 Super Charged 3800 Series II (First)Six-Speed Conversion GTU
1990 Turbo 3.4 5-Speed T-Type
1990 4.0L 4-Cam 32-Valve V-8 5-Speed Indy GTi (Project)
1990 Stock(!) 3.1 MPFI Auto Indy
1995 3.4 DOHC Turbo 5-Speed Z-26(Project)

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