Rep Power: 202

What grade are the two bolts that hold the thermostat housing in place?

Story:
I replaced my 2003 LX Coupe thermostat today. I messed up with my inch-pound torque wrench and broke (into two pieces) the lower bolt holding down the housing. About 11 mm of the lower bolt are stuck in the "water passage" onto which the housing bolts. The top bolt is holding the housing in place so far (after 20 mins of driving). I am going to re-drain the radiator and take apart the housing. I hope a "nub" will remain. I will try to get my vise-grips on the nub and hopefully unscrew it. I want to have a new bolt handy, of course, to put into place. I know the bolts are 6 mm diameter, 20 mm long, and coarse thread. They are only $0.16 each at online OEM Honda parts sites, so I figure no way are they grade 8. The bolt head does not have any numbers stamped on it.

I am thinking the two bolts are SAE Grade 5 =~ Metric Grade 8.8.

I am braced for alternative approaches here and, worst case, buying a new "water passage," part 19410PLMA00 , http://www.bkhondaparts.com/honda/ci...=passage-water


Thank you for any insight you have on this matter.

 
Rep Power: 517

Grade?
Just about any grade stronger than pine would do IMO, there isn't a lot of tension/stress in holding a thermostat housing together.

Plain-Jane 8.8 will suffice.
6mm x 1.0 thread pitch, very common

I'd hope any local parts store or maybe a hardware store would have a selection of metric nuts and bolts to choose from.

You don't have to tighten the snot out of them.......you do need it tight enough to squash the rubber gasket plus just enough twist so the bolts cannot back themselves out.

 
Rep Power: 202

Thank you, ezone. That's peace of mind. I took the broken bolt head to True Value. A clerk there observed the "lip" on the broken bolt's head and opined that this suggested it was Grade 10.9. See photo. The Grade 8.8 vs. 10.9 metric bolts at True Value are indeed distinguishable by this lip. I presume the lip spreads out the force more in the higher grade bolts. I do not now if Honda's bolt manufacturer uses the same system. I went with the 10.9.


I let my Civic's engine cool, drained coolant, took apart the thermostat housing, and found about a thread of "nub" extending from the lower bolt hole. I could not get my vise grips on it. The hole is threaded all the way through, so I got a drop of PB Blaster in the back. Using diagonal pliers and needle-nose pliers, I got it turning. It was out in about 20 minutes. The new M6 bolt seemed to thread in fine. Still my inch-pound torque wrench won't click (indicating the desired torque is reached) with the lower bolt, so I think things are a little stripped in the bolt hole. I snugged it up by feel. I may re-tap soon.

I see breaking a thermostat hold-down bolt is a common problem on a lot of makes and models.

 
Rep Power: 511

if it gives you problems, helicoil it and call it a day. It's all aluminum down there, real soft, easy to strip out the holes. Retapping it won't really let you get the necessary torque on it.

 
Rep Power: 202

Thanks xRiCeBoYx. I will try the helicoil next time I do a drain and fill.