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You would get me into this, Bob! The early pumps fitted to the FE 35 (before s/n 65685) had a smaller piston (less than 0.91", I don't know how much less!). The later pumps fitted with .91" pistons had p/n 184 472 M91 in the FE 35 UK parts book. ( I've also seen a designation "Mark 1" for the pumps with the 0.91" piston.) The Vapormatic catalog shows p/n 184 472 M93 for the later (0.91" piston) pumps. Presumably this has a strainer. The US MF parts books show p/n 184 473 M91 (note - 1 digit higher - 473 rather than 472 - than the UK numbers!) without strainer or piston rings, M92 with strainer, without piston rings (prior to 230456 gas, 231716 diesel), and M93 - with strainer and piston rings - for later models. A caution on serial numbers - UK serial numbers may not be the same as US. The pump parts list in the FE35 UK manual has identical part numbers for backing plates, cam blocks, cam shaft and most other internal components for both the smaller pistons and the later 0.91" pistons. (These same part numbers are found in the US parts books.) It is reasonable to assume from this that the smaller pistons in the very early units may be replaced with the larger pistons and valve chambers. I have not done it, so can't say for certain! The later 0.91" pistons with rings will fit the valve chambers in the early models with 0.91" pistons. In summary, the valve chambers and 0.91" pistons with rings can be installed on all 0.91" pumps; it appears they will also replace the smaller pistons on the very early pumps. 2 further notes:
When overhauling the pump, replace the control valve at the same time. There are three precision fit washers on the unit, and a small amount of wear on them substantially increases the three-point leak-down rate. There are valve chamber overhaul kits available. We have had no success with these units. With the availability of (very much cheaper) aftermarket valve chamber assemblies, it is best to replace the chambers with new when overhauling.
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