This is from my post on the thread Air Filter on Diesel back in summer of 08.
I looked up the information on my K&N filter. FYI I have a 1983 4-108 with the heat exchanger
as part of the exaust manifold but I imagine that this combination would work on other 4-108's also since the manual shows the same type of dome air cleaner on other 4-108's (measure twice cut once). Sorry to say I could not find my original sales reciept and the numbers on the unit (FO22A6) don't seem to mean anything to the search engine
at the K&N site. I'm pretty sure it is an RC-650 however as it looks the same and has the correct dimensions. I attached the filter to the engine
with a piece of exaust tubing 4" long 2 1/4" ID 2 3/8" OD. The centerline of my engine vent hose connection hole is drilled 1 3/4" from the bottom of the tube (best fit up for my layout). I cut slits around the bottom of the pipe with a hack saw to match the original dome filter then hose clamped it in place. I treaded a hose adapter into the hole I drilled and sealed it with some fuel
(from NAPA I think). As K&N says I streched it slightly to fit during installation
on the pipe and since the one I bought has an offset I turned it to be over the top of the engine then tightened the hose clamps. If other's have a different layout more space, less space etc. the K&N site has a nice search engine for looking up filters by dimension and type (round, oval, etc.). I did this same modification on a 30 HP Universal diesel on my last boat
12 years ago and found it easy to match a K&N filter to the dimensions (had to use a parts
book back then). Hope this helps.