1985(ish) Mongoose ATB - might be a slow burner!

[Lhatch4]

That looks good. I like the headset choice too.

 

[pwebb]

I think headset wise the goto was Shimano 600 in early 80s, and or tandem stuff, there weren't any MTB specific groupsets in those days so people went with available and durable. Items. I can remember my 1984 bike snapping solid axles on a regular basis, so much so that I carried a spare on longer rides.
The original takagi xt crankset and pre biopace chainrings also wore out very quickly in muddy conditions as did the bottom bracket and pedals, but headset lasted a lot longer. I eventually swapped out for deerhead, but that disintegrated even faster. I think the vintage Dura ace is a good option if you can get it to fit ok. I must admit that having fitted more recent and robust components, wheels et al, I would not try to fit period early 80s items any more as they are both heavy and frail. I have recently built up some similar age muddy fox bikes with vintage dia compe and suntour parts and those seem to be stronger and restored ok, but I got shot of the solid axles asap. Btw I don't think billy bonkers would look right, wrong colour sidewall and garish modern lettering. Some period Tioga or panaracer would be more fitting imho. This is a nice looking frameset and will make a very fine build whatever you go for. Watching this one with interest

 

[gtturbo]

Just stumbled across this on ebay if you need a cheap stem to get you going. If you scroll down it says for 21.1 forks.

https://www.ebay.co.uk/itm/32660030...HNDEST&hash=item4c0ae5195e:g:kEwAAeSwLD1oM3ym

 

[DesertRyder]

I had a chrome Mongoose ATB like yours. You mention the biplane fork. I took mine off and replaced it with a Trek DS2 fork. I ended up using the original chrome biplane fork as a floor jack handle...lol

 

[Uncle Grumpy]

One issue I had is that the headtube is a weird diameter, approx 32.4mm, so a standard 1” wouldn’t fit.
I made up some shims in order to take up the gapping and get this old school dura-ace headset to fit.

32.6mm is a BMX head tube standard, which makes sense given Mongoose's roots in BMX. Look for a headset in 32.7mm cup diameter to give the interference fit. Lot's of decent headsets for BMX still being made in that size, or track down something period correct.

Oooh, this one! Damn, the red is out of stock, that would have looked sweet...
https://alansbmx.com/products/tange-seiki-mx320-headset-chrome

The shims are a tidy solution though, so problem solved.