Originally Posted by Duragrouch

Seatpost shim: Says it's for Dahon, 33.9mm inside diameter same as seatpost, good. If they have different lengths, get the longer one, the longer the better, this spreads the bending load over a longer length of the seat tube, and that's good. If no frame cracks currently, I think if you replace the plastic with aluminum, you will be fine. (I wrote up a very detailed engineering analysis of the crack failure, sent to Dahon with recommendation to send all owners an aluminum shim to replace plastic, they blew me off, and no frame replacement either. I promptly sent the report to the USA Consumer Product Safety Commission.) I wish they made shims in stainless steel, stiffer and stronger than the aluminum, to better reinforce that area. Note that is slot on front of seat tube, loaded in tension in bending, due to seat loads with aft-tilted seatpost. Metal fatigue happens in tension. Later Dahons, starting with the aluminum frames and then the steel frames, relocated the slot to the *back* of the seat tube (like most bikes), so loaded in compression under bending, should be stronger.

Regarding glue as they mentioned, my shim has never tried to slide out when adjusting the seatpost up; The seatpost clamp is smaller ID on top, it holds the shim in place. (And yes, insert the shim in the seat tube *before* putting the clamp back in place on top.) However I'm not a "frequent-folder", so if you need glue, use a spot of glue, something rubbery. In fact, I put a thin coat of "anti-seize" (silver, for aluminum) in between the shim and the steel frame, to discourage dissimilar metal corrosion. Be careful to NOT get any on the inside diameter of the shim, as that may make the post slip, and it's very messy as well, like grease, but with fine metal powder in it. Don't get it on your clothes.

Adjusting the headset with the large allen bolt: I think you can hold the stem in a "halfway-folded" position to get access to the bolt, if not, you can undo the folding mechanism at the threaded adjuster I think. Remember that big center bolt will not adjust anything until you loosen the clamp bolt on the side, and be extremely gentle in tightening that big bolt, it is easy to brinell the headset, just finger snug, use the allen wrench stood on end with the short bend as a handle. After adjustment, tighten the clamp bolt.

Cracked plastic ring: I can't recall if that is just a spacer (to compensate for different length steering tubes on different forks, as mentioned?!), or is the top of the headset. If cheap to replace, replace.