Aftermarket Radiator Questions- Cap with Deeper Neck

[adf5565]

I purchased/installed an aluminum Liland Global 850AA3R radiator from rock auto last year for my K30 with the 6.2 diesel. I noticed yesterday that the depth of the filler neck on the radiator cap is more than the OEM radiator. I believe OEM is .75", I haven't measured this yet to be exact but it is probably closer to 1".

First question, does anyone have a source for radiator caps that have the deeper neck? I noticed a small leak this morning from the cap, I removed/reinstalled it as maybe it wasn't on quite right but I'd like to obtain another cap in the event the seal goes bad. I called Liland Global but according to them even though the cap is listed as a part in the catalog, they don't sell it separately from the radiator....

I was also attempting to pressure test the cooling system but as the radiator has the deeper fill neck, a normal pressure tester does not fit/seal correctly. Any thoughts if there is a particular pressure tester or adapter i can obtain that would work? I could try to sandwich a thicker piece of rubber between the pressure tester and the inside of the neck but I'm not sure how well that would work.