Valve Setting on a Leak Compound

This time we are looking at how to fix the problems with the Leak’s valve gear design. This is now part of our Leak Hints and Tips section on the known errors or problems with Leak design……

As we get to the end of January 2022, the tuning/rebuild/shopping of Befur’s engine continues, once this is complete we can get on with the list of hull and fittings-related work. But it seems we should at least explain what work the last few months have contained…

Refurbishment & Fettling

While we have the engine on the bench, we have taken the time to give it a good inspection and address a number of wear and poor manufacture issues, and a redesign of the pump drives.

This involved a lot of “minor” work, quite a lot of which was documented in earlier posts. But the latest bits have been:

  • Bushing the weighshaft bearings, these had accumulated quite a lot of wear, so new phosphor-bronze bushes were made an fitted.
  • Re-fitting weighshaft drop arms, to get them truly square and secure. This meant fitting new and larger taper-pins. The weighshaft clearly carries a lot of dynamic loads, and one might argue that the Leak’s design in this area is a little “light”.
  • Remaking the pump drive “jackshaft” due to wear from the out-rigger bearing spinning.
  • Remaking the LP and HP valves – (see next section)…

Decision to remake the valves

As noted in this earlier post, that we had realised from measurement that the valve events on the engine were far from ideal.

picture of new valve parts

The refurbished/replaced valve gear parts

Initially we decided that a new HP (piston) valve spool/bobbin was needed. This because its length needed to be modified to get better valve timing, and also because it’s fit was not good enough, and there was a lot of leaking. (I’ll comment on the methods we used to improve this later in this post).

The 2nd realisation was that the LP valve events were equally poor (probably worse), and this demanded that we make a new LP valve too. Fortunately (for once) the original kit of castings contained a spare LP valve casting for some unknown reason (phew!).

Leak Valve Timing Errors

It has been noted by other builders designers (see Autumn 2021 SBA Funnel) and here  that there are a number of problems with the Leak Compound design, and these included errors/problems in the valve gear design.

diagram of valve timing

Proposed Valve Timing for refit

John Maltby, a good friend and qualified engineer, from the SBA helped me analyse the timing events we were seeing and applied his knowhow to generate new dimensions for the LP and HP valve. He produced the adjacent timing diagram by means of deep steam-engineering wizardry, that I do not understand one iota!
(remember click for a full sized picture)….

He also provided the following commentary, which explains the problems:

To this end I have had a look at Leak’s drawings:- both the HP and the LP are shown with 0.188″ lap top & bottom, this is despite the eccentric throws are different (HP 0.438, LP 0.5)  I have drawn out the HP diagram using these figures and it is crap,  there is no angle of advance shown on the drawings (it maybe in the book?) but to fit in with these laps this angle would be about 23 deg. and the cut off would be very late although the ports would have full opening. I have drawn a version how I would have done it, with laps around 0.24 as per the “falcon” engine, the actual laps being  0.234 top, 0.255 bottom, this gives approx. cut offs of 65% bottom stm and 73% top stm, not too bad although the ports are  not fully open but I have not found this a problem at the moderate speeds we use. Incidentally the laps are different due to angularity and of course there are many inaccuracies due to the link design. Also, these are based on a zero lead situation which with crossed rods on your engine would produce a negative lead when linked up. You can of course turn the eccentric to put a bit of lead on but this would reduce the cut off slightly.

He then provided some words which allowed me to make new HP and LP valves, and allow for the miss-sized LP ports (see previous discussion of  drawing error):

Here is my recommendation for your engine:-

HP valve. top lap 0.196″ bottom lap 0.219″ the angle of advance will work out at about 28 degrees plus lead so could be achieved with a stepped key.
The cut offs would be about 81% top and 74% bottom, rather late but ok (this is without lead, I would advance the eccentrics to give a lead of about 0.015″ so this would reduce the %age cut off a bit). Piston valve lead can be checked by measuring the liner port edge to the valve case top surface before assembly then using a depth gauge to measure to the top of the valve having pre calculated what it should be. lead is of course measured at top & bottom dead centre.
steam port max opening about 0.24 top & 0.22 bottom. not bad.
LP VALVE. top lap 0.227″ bottom lap 0.196″ angle of advance about 25 degrees plus lead.
The cut offs would be about 84% top and 80% bottom . lead as above.
Port opening about 0.272 top, 0.300 bottom these are improved due to your wider ports.

I have also attached pictures of the new valve drawings, I can provide these as CAD drawings if you PM me…

HP Valve and Liner

cad drawing of HP valve

Dimensions of new HP valve based on fitted liner and J Maltby design

New LP Valve from J Maltby drawings

CAD drawing of LP valve

New LP Valve drawings as per J Maltby drawings


Lapping the HP Valve

Following suggestions from John M, we decided that the new HP valve bobbin would be made without rings (as the rings result in indeterminate port events), we would make it with “water grooves” and aim for 0.001″ bore clearance. This was better than I can usually achieve! So, I took a new approach of lapping….

As seen here, we commenced by re-boring the liner on the mill… I then decided to follow “The Book’s” defined approach by lapping the bore to get the required fit of the valve.

This meant turning a gauge, with a coarse thread, loading this with fine valve grinding compound and then using this to lap the bored liner to size. The detailed steps were:

  • Boring Liner,
  • Initial honing of liner with electric drill homing tool, to get off tool marks.
  • Lapping with gauge & grinding compound by hand
  • Wash clean in paraffin, check fit and repeat above lapping for days!
  • Final lapping in lathe (see video)
  • Clean and final assembly
  • This in total took about 3 days work….!! But now it does not leak!


Making the LP Valve

There is not much to say about the LP valve manufacture (to the new drawings), except that a trial assembly showed considerable blow-by (continual blowing thru exhaust).

I guessed that this was caused by leaks around the piston ring used to seal the balance part of the valve to the main part (remember this is a balanced valve where the exhaust is through the valve chest cover).

It would seem that there is little movement between the two parts, so I decided to fit a silicon o-ring in addition to the cast iron piston ring. This was an immediate fix! The original drawings suggest an o-ring, but my castings came with piston rings for this joint, so I fitted them – a mistake.

Valve Setting & Results

The setting was quite easy using this approach, I recommend it….

Step 1

The first step is to set the “Leads” equal at TDC and BDC for each cylinder. To be clear this means that with the Piston  at Top Dead Centre and Bottom Dead Centre the inlet ports are open by an equal amount – the actual amount does not matter at this point!!!

To get this equality of opening you move the valve on the valve rod (HP) with shims under it, or lengthen/shorten the valve rod for the LP.

Some care is needed in determining where TDC and BDC is, as the valve is moving very fast at this point of the stroke, so a few degrees of crank movement makes for large errors in measurment of valve position.

For the LP you can see/measure how far the ports are open when aiming for top/bottom opening equality, so this is easy.

But, for the HP you cannot see the ports, so you need to do a little preparatory work to determine the position of the valve when the inlet is opening.

I used the “blowing” technique again.

A low pressure airline is connected to the inlet, and then the valve is moved by hand until this air can be heard entering the cylinder… then measure how far it is from the top of the valve chest to the top of the valve with a digital calliper. (I did this on the bench, you could do it on the engine, but you will need to disconnect the valve spindle and have some mechanism to accurately move the valve).  Do this for both ends (TDC and BDC), and you now know where the port opening positions are – write down your answers! 🙂 .

Armed with these two depths you can then subtract the required “Lead” measurements (0.015″ for this engine) from the top measurment and add it to the bottom measurement and now you know the two positions the valve needs to be in to get desired Lead!!!

Step 2

Now we know the Leads on each end of the valve are equal, we need to get them to the right values. This is done by advancing or retarding the eccentrics to get the Lead you want.

Stepped key for LP Valve

On my engine the eccentrics are keyed to the crankshaft, but there is also an Allen screw in the eccentric to allow it to be “temporarily” locked in place.

By removing the key, and loosening the screw and setting the engine (accurately) at TDC you can manually adjust the eccentric position to get the desired lead. Once done check that you have the right value at BDC too, and then repeat the whole kerfuffle in Astern on the other eccentric!

We needed to make stepped keys to correct the eccentric positioning (see pic).

I will post a full report on the timing the new gear provides, once we have finished setting and measuring this… but I can sell you the HP end has much better “definition” now we have removed the leaks, and there is no more blow-by on the LP end.

We seem to have more sensible cut-offs, but I am not yet convinced that we have the “final” settings in place! …and then there will be a pause while we finish the boats re-fit and get her back into the water for sea trials!



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