I have the same issue with an SM7 -- several keys, notably the period and comma, require extra effort to make a good impression on the page. The slugs seem to be slightly farther away from the platen when the typebars are brought up to contact the ring; I can't figure why this is. (Other slugs on the same side of the keyboard do not have this problem.) No sign these slugs were damaged.

I have toyed with the idea of taking a file to the projection on the typebars that comes in contact with the ring, to lessen the gap. (Thoughts?) Probably the longterm answer, however, is to simply use backing paper all the time, to close the gap some.

Now that's fascinating. You're right, it's only the period. comma and semi-/colon keys that do this. Can't imagine where this adjustment might be, though. I figured it was just part of the general condition of the typewriter -- lots of superficial internal rust; tabulator brake inoperative; drawband was badly tangled around the mainspring; lots of stuck keys.

CoronaJoe wrote:

 I did check ring & cylinder for type slug contact with the platen per Uwe's suggestion.  Only a few "end" keys such as the  "Qq"  the cents key on the right side or the "=+" key seemed to show a very slight gap.  However they still typed good impressions, full character top to bottom.

There should be a slight gap and those end keys appear to be the only ones that are correct. I'm not sure what the specs are for the HH, but my Olympia standards are supposed to be adjusted to a 0.007 inch (0.18 mm) gap. It sounds like the ring on your HH might be worn, hence the "deep impressions" in it. Replacement is obviously not really an option, and I'm not sure you'll want to go down the road of trying to refinish/resurface it and then set up the ring/cylinder again from scratch. At the very least you might want to see if adjusting things will help to alleviate the situation somewhat.

Fleetwing wrote:

..several keys, notably the period and comma, require extra effort to make a good impression on the page. ...I have toyed with the idea of taking a file to the projection on the typebars that comes in contact with the ring, to lessen the gap. (Thoughts?)

Thoughts? Don't do it! Such a radical approach as taking a file to a typebar would destroy it. There are so many other potential factors that should be considered first, it could even be something as simple as the type bar being slightly bent. Remember, a typewriter is a precision instrument, and often small adjustments are all that's required to properly repair the machine. 
 

I agree 100% with Uwe.  Without a repair manual, there are just too many unknowns that "Hail Marys" can send one down the road of no return.

For example, in assembling the carriage back on my HH; I discovered a binding in the middle of the lower rack after the clamps were installed and adjusted.  In keeping carriage looseness at the ends to a minimum, (what they call end shake), I found it was tight towards the end of left to right travel.  Reading the Ames manual, I discovered Royal has a .002" bow in the middle to help keep the carriage free from slop throughout it's movement.  I'm speculating here, but probably it is because most carriage movement is within the center 6" of travel on a 10" or 12" carriage.  There was a simple but definite order to how and when the carriage clamps were secured.  When I followed the instructions, the carriage was nice to move with little or no excessive play at the ends.  A bow?  Who would'a thought?  Geared ball bearings for trucks?  Brilliant!!

Richard Polt's website has some really good PDF files of the Ames books a lot of repairmen back in the day used.  With my experience on old vehicles, I've found the only thing I truly know is what I don't know; so I go for the service manuals when and where ever I can buy, trade, or salvage them.

This is a tip I heard from a Portland typewriter repairman:  Most Olympia type bars are hardened stainless steel.  His experience is they do NOT bend well if at all.  More than likely, they will crack.  I've never crossed that bridge, but we were on the subject as sometimes I come across a machine with deformed bars. I do not know, my guess is it depends on the severity or if it's just an alignment at the slug.

CoronaJoe wrote:

This is a tip I heard from a Portland typewriter repairman:  Most Olympia type bars are hardened stainless steel.  His experience is they do NOT bend well if at all.  More than likely, they will crack.

That tip sounds strange to me. Is this guy a bona fide professional typewriter serviceman, or a hipster who claims to be able to fix typewriters (we have a couple of the latter group in my area)?

Type bars are made from tempered steel, and some even have ridges to give them additional strength, but that hardness is not uniform along its entire length (its softer at the slug) because of the action a type bar must perform during its type stroke. Type bars do get bent over time and through abuse (especially when slamming one into another during a jam), and I can assure you that they can also be bent back into a correct shape. I have a number of specialty tools meant to manipulate a type bar in a number of different ways and haven't yet cracked a single one. I once had to straighten a bar that inexplicably had been bent to look like a banana. Incidentally, one part of a type aligner's job in the manufacturing process was to bend type bars so that each machine had perfectly aligned type.