LP Valve Chest: Part 3c

  1. Cover Stud Error: Colin Sims (another SBA Leak builder) reminded me of this one – The LP Valve Chest Cover is attached to the Chest by 12 studs; and the chest is attached to the cylinder by 8 (EIGHT)…. the two at the top and bottom cannot be attached as the studs clash with the ports. So as I wrote at the time “It may be that my Technical Drawing A-level from the ’70s was to a different standard but only 8 of the 12 holes for the valve chest cover mounting holes should be drilled through. The 8 down each side do carry through to the block, but the 4 at the top and the bottom, should be blind – as they “clash” with the port pockets.“…. Moreover this also means that the stepped studs shown in the drawings should only be used on the sides (the top and bottom are plain studs as the larger ones would (do) clash with the stuffing box and tail rod)… Lastly in the BOOK (at least) only the four right hand holes are shown threaded – in fact the left hand ones need to be too (to take the aforementioned stepped-studs).
  2. The LP Valve Lap Confusion/Error: The ports as shown in the BOOK (sectioned drawing of valve chest on page 10), have an undimensioned dimension indicating where the “LAP” is/can be measured? There is no indication of what this should be…….Later: I found The lap and lead dimensions are given at the end of the Book in the valve setting section, Lap is given as 3/16″ and the Lead as 1/32″ (Para1 page 35). Correction (2021) This is repeated on the HP valve drawings, and having reviewed the drawings with John Maltby (a professional Chief Engineer) I now understand that these dimensions are meant to apply to HP and LP gear, but for reasons below they don’t!!: I now realise these dimensions probably only refer to the HP cylinder. So I am still not sure of the implications of the LP port height issue noted below.  
  3. Port Height Dimension Error: This is dimensioned in two places as two different sizes! I have only just realised this. I noted that the ports on my valve chest were 3/8″ high, and that this did not match the 5/16″ dimension shown on page 10 (of the BOOK) I thought this was just a blunder on my part. But tonight as I am writing this page, I realised I cut them this size because they are shown as 3/8″ high (on page 7 of the BOOK), I a drawing showing the rear of the LP Chest. The implication is perhaps they are meant to be tapered, although it is not shown on the drawing, I think it’s just another error! This will mess with the LAP, I guess I am going to find out when I time and steam the engine!December 2021 update: I finally realised where the problem arises – the drawings show the dimensions correctly (ports in LP Valve Chest shown as 5/16″ high), but there was a “scrap drawing” included in the “Model Engineer” articles, which was copied into “The Book”  on Page 7 labelled “LP Chest Back Face” which incorrectly dimensions these ports as 3/8″ – this has dire consequences for the valve timing, which is more fully discussed here
    Moreover, Mr Maltby’s analysis suggests that the LAP needs to be about 1/4″ to get reasonable valve events…. this is covered in the above article…

2 thoughts on “LP Valve Chest: Part 3c

  1. Pingback: Valve Setting Corrections | SY Befur

  2. Pingback: Engine Rebuild – Valve timing analysis | SY Befur

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