CB175 blown head gasket

Dtorpey

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Douglas, MA. USA
I recently had a valve job done to my ‘71 CB175, after installing new overbored jugs, pistons and rings. I installed a new(NOS) head gasket that lasted maybe 25 miles. Torqued per the shop manual, criss-cross pattern etc. Question: is there a right-side-up to the gasket? And is retorquing the head after heat up/cooldown helpful to seat the heat down tight? Any tips to avoid another blow-out would be appreciated
Dave
 
When you say "blown" head gasket, are you referring to oil leakage or compression loss? It's very rare to see compression loss from a Honda engine with a good head gasket, NOS or otherwise. Since the 175 engine has o-rings on the rear outer studs for the oil flow to the head, I don't believe there's a right side up for the head gasket as I think it's symmetrical. Seems you should be able to flip it left to right, but of course not front to back due to the differences in the cam chain tunnel opening.
 
I have to be devil's advocate and ask "Are you certain the gasket areas of cylinder deck and head are not damaged AND that ALL the previous gasket was removed?"
Unless both were clean, bare aluminum (no discolored gray spots), those "gray spots" ARE left-over previous gasket and can allow failure......
 
There was compression loss for sure( see pic) . Also, both the bottom of the head and the top of the jugs were lapped on fine Emory paper on a heavy sheet of glass before assembly. So both were clean and reasonably flat. 01EB4E16-8D55-44C8-B7D7-CF3EE8D04261.jpeg
 
That's a rare sight for a Honda twin, I've only seen it a couple of times previously. My first thought is the cylinder studs and the nuts on them, if the nuts bottomed on the studs and didn't reach proper torque I can see how that could happen. What torque value did you use?
 
Tightened snug first, then to 10 ft-lb then to 15 ft-lb, per the criss cross sequence in the factory shop manual. I don’t think the nuts were bottomed out on the studs honestly. I guess I can just try again with a new gasket and see what happens.
 
Time for a machinist straight edge and feeler gauges. Not a cheap tool or easy to find a good one, avoid Harbor, Amazon, etc.
The gasket blew on both ends which tells me either the surfaces are concaved or the torque was uneven on the ends.
 
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