MONSTER 40mm Top Cap 2003+ — important: inner chamfer required before assembly!
A production batch of our 40mm Top Cap passed QC but carries a sharp inner edge that must be chamfered before installing the O-ring. Assembling without this step may damage the O-ring. Please excuse any inconvenience.
Dr-Zocchi 
This advisory applies to the current production batch of the Dr-Zocchi 40mm Top Cap. Please read before assembly.
What happened
During final inspection of the current batch, a sharp inner edge at the O-ring seating bore was identified. This edge should have been chamfered during machining — a small 45° break that allows the O-ring to slide over the bore entry without catching. It was not machined to spec (check drawing), and it was not caught at our standard QC inspection.
If you press the O-ring in without addressing this edge, the O-ring lip may be cut or torn on entry. A damaged O-ring will not seal correctly — and you may not notice until the fork leaks under load.
How to fix it — before you assemble
- Inspect the inner bore edge visually, unfortunately you can't get a finger in to feel it. If you see a non chamfered edge you need to add one.
- If you see a non-chamfered edge: use a hand deburring tool or a 90° countersink bit. Two or three light passes by hand around the bore entry is enough. The tool I used to do the chamfers is a PFERD Kegelsenker C90° HICOAT HC-FEP 6,3mm DIN 335 C90°, Art.-No. 25202141 having a 5mm shaft. Just a short press using an accu driller is enough.
- After deburring, wipe clean with air pressure and with a lint-free cloth. Unfortunately it can only be visually inspected due to its exposed position. The chamfer should look smooth all the way around with no catch.
- Tip: To check the correct O-ring size matching the Top Cap's hole, place the new oiled O-ring over the cartridge head and insert it from the top into the top cap. You should feel some resistance — this means the O-ring is about to be compressed during sliding in to properly seal inside the top cap. If it moves in too easily, the O-ring is too small and will not seal. [ToDo: picture to add, O-ring size]
- Lightly oil the O-ring before installation and press in squarely.
- Sorry to say, but if you are not familiar with using such tools and this simple processes, ask a mechanic to do it. It is important to use a sharp tool and drill the chamfer vertically to have it plain and smooth.
What we have done
The production drawing has been updated with the correct chamfer dimension and tolerance. All future batches will have the chamfer machined in from the start. This batch will not be re-stocked once sold through.
If your O-ring was already damaged
This is simple to fix DIY. If you can do a proper service your fork and handle tools, you can fix it yourself or if you are unsure, ask a mechnic to do. If not, send the cap in — we'll confirm whether your top cap bore needs any further attention and add a new O-ring.
