1991 200 TXRP Thermostat Housing Cracked

[Doc Z....]

My thermostat housings on my 1991 Yamaha 200 TXRP have weakened and cracked for no reason. The water was pouring down the head hitting the coils making the motor skip. This happened last year to the STBD motor so I replaced the cover and never had an issue....motor has run fine. Now this year it happened to both of the covers on the Port motor. Any ideas if this is a sign of something bigger to come or just related to cheaply designed porous aluminum?

[pstephens46]

The covers I assume? Can you post pictures after they are removed? Interesting. What does the bore where the t-stat sits look like?

[Doc Z....]

The surface where the cover sits is quite flush and there is hardly any crud underneath. The thermostats look good. I don't have any pics right now.

[zenoahphobic]

Cracked or corroded? At the bolt holes or randomly across the middle?
Interesting you have now three. Yes pictures needed or more detail please.

[rodbolt17]

wow, my son was born in 1990. at age 21 he had liver cancer.
your motors are almost as old as my son. how long you want a warrenty?
replace them and go on. it happens. I replace a few parts each year.

[Doc Z....]

The are cracking where the ears meet the housing....not corroded. Rodbolt...........I don't want any warranty......just want to know if there is some bigger underlying issue...I have 10 new ones on order.

[Doc Z....]

[zenoahphobic]

Although I can't see the crack in the picture, you say not corroded.
Perhaps they need replacing after a while (considerable while), as it is the heating and cooling distorting the cover over time.
All I can suggest that you make sure the new cover sits perfectly flat at those points, maybe sitting a fine grit sandpaper(1000) upwards on the engine block and rub or bed in the cover over that paper to make sure it's surface is flat, before putting gasket on and tightening down with the bolts.

[Doc Z....]

Absolutely on the new ones I make sure both surfaces are completely flat.........I was just wondering if this is a know problem

[99yam40]

over torquing maybe

[zenoahphobic]

Absolutely on the new ones I make sure both surfaces are completely flat.........I was just wondering if this is a know problem

As suggested over torquing would pull those corners into the gasket. That design to me shows there is not enough meat around the lug. That and the grey look of the aluminium may also indicate low strength alloy (die cast look), it would not surprise many that they crack. I would not worry about other things at play, if they fail regularly you may embark on building up aluminium that will require welding and machining.
I would only consider that if they are rare (as in vintage restoration) to find or are expensive.

[rodbolt17]

stuff gets old. stuff breaks. it happens. I do replace t-stat covers occasionally.
after 20 plus years if it was a "known" problem you would know it. my 61 Studebaker had a "known" problem about the door latch wear. I found out in 1989 about it.

[pstephens46]

over torquing maybe

Too much torque would be easy on those bolts. I think my my housing shows 5.5 lb ft in the book. Hard to measure that. As Rodbolt suggested, I use a 1/4 inch ratchet and snug them back and forth.

I don't care for the design of what is shown in pic. Replace and move on. The gasket on mine is pretty forgiving in regards to reuse. And put a little grease on the threads.

[Doc Z....]

Update..............the housing are cheap metal and over time will crack due to motor vibration............got new ones in and they are clearly stronger than the old ones which I can break the ears off by hand. Problem solved.........got lots of spares now.

[99yam40]

Why did you order so many?