Perkins 4.108 - thread standard & size for heater/calorifier head tappings?


New member
15 Apr 2009
Two hosetails are screwed into the cylinder head tapping on my Perkins 4.108M.

They appear to be 1/2" BSPT (they fit a 1/2" BSP female thread). However, they are a poor fit to the head tapping; they only screw about half way (3-4 threads) into the tapping before jamming (I've cleaned the male & female threads), and a lot of thread dope was used to achieve a seal.

I have a corroded iron hosetail which needs replacing, and I'd like to use the correct size fitting.

The engine workshop manual states that
All threads used on 4.99, 4.107 and 4.108 Marine engines, except on proprietary equipment are Unified Series and American Pipe Series.

I took American Pipe Series to mean NPT, so I purchased a 1/2" NPT brass hosetail. This feels like a slightly better fit than the old BSP one - it screws about half-way into the tapping by hand, and gentle application of a spanner will get it about half a turn further.

On closer inspection, the head tapping looks like a parallel thread (no visible taper to the eye), and its ID is about 18.2 mm (give or take 0.1 mm).

Does anyone have personal experience of what size the head tappings are? Are the tappings actually 3/8"?

Thanks & regards
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