I picked up a coupe of mid-90's trek Mountain bikes to do some light riding with the kids, and learn bicycle repair basics. I ended up with two bikes. A Trek 800 and a Trek 820. They seem to have similar components, but the frame is noticeably thicker on the 800, is this a benefit?. Which bike would you recommend I keep? Ease of working on, availability of parts, etc. will try to upload pics later, the uploads are failing now.

pictures

By "thicker" I assume you mean the tubes are larger in diameter. That makes the frame stiffer but, if the tube's wall thickness is adjusted, it doesn't necessarily make the bike heavier. Those are two very similar bikes with the 820 being a bit "higher-line" originally and the 800 probably being a couple of years newer based on the larger diameter frame tubes. There is little to choose between them so keep the one in better condition, or keep them both. Ease of maintenance and replacement component availability are identical.

Both are decent frames to work with.
If you pick just one, pick the lighter one. It would have "better" tubing.
The 820 was a step above the 800, but different years "might" make a difference.
Look on the front of the seat tube for a sticker. You want-
CrMo vs HiTen.
Butted vs non butted.
Double butted vs single butted.

The 800 has an aluminum frame and is newer; the 820 has a steel frame, probably with 4130 CrMo in the main three frame tubes.

I'd prefer the 800 for the newer components and the aluminum frame.

However, both are small frames and might be best suited to riders 5'8" in height or shorter. How tall are you?

Look carefully at the front end of the first bike versus the second. The fork angle appears to look much steeper than the other bike's does. Inspect the fork blades and the frame tubes to make sure there's no damage. You might want to post profile photos of the fronts of each so we can look more closely.

the upper right picture shows frontal impact damage, to my eye .. they ran into something pretty hard.


FWIW the 820 model is still in their product range , now with a suspension fork..

Both are very similar as far as quality. I'd take the one that hasn't been crashed. They also appear to be two different sizes.

Generally I aim higher on rigid frame MTBs as a higher end one might cost $25 more than a lower end model. So you can find a bike that originally sold for $800 to $900 for $25 or $50 more than one that originally sold for $250.

Older rigid frame MTBs can be great bargains! The serious MTB crowd wants no part of them. They want 29ers, disc brakes, full suspension, etc. The road bike crowd doesn't want the either. That depresses prices so much that they can be great bargains for the recreational rider.

please excuse my misinformed friends....

ALL trek 800 series MTB frames are STEEL...

there are model year differences in the two bikes you purchased.

both frames use low end steel.

i am not seeing any indication of "frame/Crash damage to either frame.... the STEERING ANGLES were changed in later models.

the differences between ther two bikes will be:
1) the one with the steeper fork angle will steer a bit quicker/easier, but will be a bit less stable when riding down very steep hills...
2) the parts on the 820 will be of a slightly higher end group... this is designated by the "820" model number... all trek frames of an 8XX series within a particular year will have identical frames... the bike with the larger tubing is a newer model... if the deralleurs are the same, that just means trek chose to upgrade the later model 800 to a slightly better parts group...

the 8xx treks are third in order in the trek MTB line of STEEL MTB's... they are rugged and dependable frames that hold up well to abuse... in good working order they sell for 75 to 125 dollars US.... some sell for $150 if in nearly new condition.

Trek MTB bikes with FOUR digit model numbers have aluminum frames, THREE digit trek MTB's have STEEL frames.... 3700, 4300, 4700, 4500, etc. are aluminum framed... 220, 7xx, 8xx, 9xx have STEEL frames, period. First number is the frame type, second number designates the parts group installed at the factory....

the "thicker" tubing lends flex resistance to the frame, and not much else, other than looking more high tech, of course... tubing QUALITY would probably be identical.... the WEIGHT may indeed be lower with the "thicker" tubing... stiffer frame designs allow for reduction in tubing wall thickness... whether trek decided to thin out the tubing is a question they might have answered in the past.... weighing the bare frames would be a way to find out... not worth the effort, IMO... both frames are stout, and barring extreme abuse, they wont break.

if one of the bikes will be for you, choose the 820... give the kid the quicker/easier/newer steering 800.

OH, and do install the seat on the 820 correctly to the seat post... the clamp SHOULD be to the REAR of the bike... looks like it's now pointing forward... and set the seats LEVEL to the ground... the way they are angled now will cause pain.... and the twist type shifters on the 820 suck, IMO... upgrade to CLICK shifters like the 800 has, if you want and can afford... you will not be disappointed.