<VV> 66 Monza 110 PG engine - sounds like big problems

[Jedd Hebrink]

Many thanks to all on VV that have given me so much helpful advice while I
rebuilt my daughter's 66 Monza 110 PG.  I thought I was finally finished,
after the usual roller coaster of excitement, followed by the disappointment
of discovering a new problem, followed by relief that the problem wasn't
really so great and you all offered easy instructions to fix it.
 
Here is the latest;  Last weekend, after following the recommended break in
procedures and several satisfactory local test drives (probably had about an
hour and half total driving time racked up), I set out to drive the car to
my daughter's place about an hour away.  All was well as I clipped along at
55 mph for about 45 minutes.  After pulling away from the first stop sign on
my route (45 minutes into the trip) I heard a noticeable knock and noted
significant loss of power.  I pulled into a parking lot and shut it off.
Checked the oil - it was at the full level.   Towed the car back to my shop.
 
Last night, I adjusted the valve train, and I am quite certain it is not
valve clatter.  I fear something more serious; perhaps a rod or crank
bearing.  It is difficult to describe the noise in words, but I am quite
sure it is originating from within the crankcase.  It is not noticeable at
idle, but becomes apparent as I increase rpms.  I have not tried to drive it
since the weekend, but am pretty sure it would increase under load.
 
Any ideas what I may be looking at?  Also, any chance I can repair this
problem without removing the engine and transmission form the car again?
What diagnostic steps would you recommend I take to to narrow the problem?
 
Thanks for being patient with me.  I have worked on many cars over the
years, but I am still a newbie when it comes to Corvairs.
 
Jedd (trying to keep my chin up) Hebrink
Austin, Texas area

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