Sorting the HP Valve problems

HP valve Bobbin ready for measurment

The next stage in the refurbishment and sorting of the valve timing issues is to pay some attention to the HP Valve problems.

As noted previously, the HP valve timing measurements revealed that the HP piston valve seems to suffer from a lot of leakage, so we stripped the engine to inspect, measure and refurbish this valve.

My suspicion was that the leakage might have been partially caused by leaks between the liner and valve chest, and leakage between the liner and valve bobbin.

Inspection and measurment of HP valve

On inspection a few things became apparent, my analysis helped by John Maltby from the SBA (a far more experienced engineer).

  1. The liner’s fit into the valve chest would be almost impossible to check, without making a new liner and fitting it in the hope things might improve – not a small amount of work, with some risk in attempting to push out the existing liner. So, I have decided to “hope for the best” and stick with the liner as installed.
  2. While the bore of the liner was tapered, it was not as bad as I feared/remembered. It appeared to have a taper from top to bottom of around 1.5 thousandths of an inch.
  3. The bobbin was also tapered by 2.25thou. As this taper was in the reverse direction to the liner bore, so we had installed this inverted to make the best of a bad job! Thus we had an “average” bore to bobbin clearance of around 4 thousandths of an inch. I had hoped that the 6 cast iron rings fitted to the bobbin would have made this acceptable. However, measuring the ring gaps in the bore showed that these were too large (~0.6mm from memory). One ring got broken in the examination, but I had a spare, and fitting this showed that the same ring gap was present, so I guess they were designed for a slightly smaller bore.
  4. Given the larger bore/liner clearance it became clear that the moment the valve opens is to some extend uncertain. This is because as there is the moment the ring clears the port, and the moment the bobbin clears the port. This and the ring gaps reinforced the decision that the leakage was mostly down to the liner/bobbin fit.
  5. Lastly, measuring the overall length of the bobbin showed it was about 5-thousands too short and when inspected in the bore this suggested that we had an effective negative exhaust lap of around 10thousands ….so the bobbin’s overall length needs to go up by at least this amount.
  6. From John’s analysis we also need to increase the admission LAP from 3/16″ to a top lap of 0.196″ and a bottom lap 0.219″. This requires the centre admission dimension of the bobbin to be reduced by about 40thou, and I’m clean out of  “putting back on” tools for the lathe! So, a new bobbin is in order 🙂
  7. Given the above analysis, John’s suggestion that we make and install a new bobbin without rings but with “water grooves” and a bore clearance of less than 1 thousandth of an inch. This would not appear feasible, with the level of taper currently in the bore.

So, the conclusion was to re-bore the liner to reduce the taper, and make a new water-groove bobbin to the required fit and dimensions….

So, first job is the re-bore. Having checked that the cylinder end caps were concentric (so the valve rod should be in the middle of the bore, we mounted the valve chest on the mill, made a longer boring bar and sharpened a new carbide-tipped tool in the Quorn cutter-grinder.

Two 5-thou cuts were taken (very slowly) and next we will hone the bore to get rid of the last 8-tenths of a thou of taper we were left with (where did that come from!!!) – actually I think this might have been my measurement error, but the lapping to come will answer that question….

Here’s a video: (you can just hear the tool running over the top set of exhaust ports….)

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