Hitchcocks Forum

Hi, rebuilding a twin engine, this is fitted with a single start worm drive. My understanding is that early twins had single start then moved to two start and reverting to single start on interceptor with larger pump. My confusion is that all manuals across the years refer to the same 1/6th pump speed. The two start doubles the speed so does that make it 1/3 speed or is it 1/6th and the single 1/12? Vince

No,if the way the manuals suggest to you that they were all 1/6 then they are wrong or you are reading it wrongly.The early Twins and that includes some Meteors if the Parts book online on this site suggest the single start worm was used.And it is easy to confirm 12/1 by turning over the engine and watching the pump rotation with a cover off.

Hi, thanks for that, precisely what I surmised. How will that effect pressure relief valves?

My understanding is the relief will only open if there's a blockage or possibly silly operation such as overreving a cold
engine,otherwise they will never open.
Bullets will shear the oil pump shaft if the rocker galleries block up.
If you fit new parts i'd relax with the Twin,i'm not sure when the 500 relief opens as i never saw +40 psi on my '51 .
And that was cold.Gauge removed since then.

Hi, are the one and two start interchangeable or are the spindles different. Vince. Ps this engine may have mismatched parts!

No,you have to match them as the Helix angle is different by about 2 degrees,and eyeballing
them is nigh on impossible,if you work out a simple tool to differentiate the spindles please post!

Hi, thanks for you input, luckily we haven't started the engine as when fitting the timing cover it didnt feel right. Two start it is . Vince.