Compression

[lewis onions]

Just tested the engine in the TT Replica using a Snap-on compression tester. The readings are a little surprising, although I have not tried thus exercise before. It reads 27 psi right hand and 25 psi left hand and both readings quickly leak down to zero. Am I wrong in thinking that this is a little low?

[BRIAN MARSHALL]

VERY low Lewis, and it should be well over 100psi. Did you test with the throttle taped fully open ? As the pressure leaks away quickly it suggests poor piston ring fits. Repeat the test after squirting some oil into both plug holes, and with the throttle fully open.

[Kevin N Bayliss]

My Dads Two Speed had 50 psi left and 70 psi right, I didn’t even bother checking with oil as obviously theres a problem. On barrel removal the rings had broken and also had a large gap (or bits missing). My 600 engine which has slightly worn bores but not enough to warrant reboring has compression over 100 psi and most importantly both cylinders are to within a few psi. When I did the honing and fitted new rings I removed the original Scott ring end stops and fitted taper pins and ground the rings to suit the pins, if I remember right it was three thou per inch of bore for the ring gap. Comparing the before mod and after mod ring gap it was surprising how much difference there was. Kev

[attachment=0:3440iey5]IMG_0023.jpeg[/attachment:3440iey5]

[lewis onions]

Much rotating with rollers produced two starts of the engine which appeared to run quite well when warmed up. However even then, expecting that the piston rings might get free after some running , there was simply no way of starting the engine on the kick-starter.

So – off came the barrels to disclose the problem(s). The gudgeon pins were absolutely solid in the pistons and the little ends so that they could only be removed, even after the application of much heat, with a hammer and drift. The barrels are worn to a taper and the pistons are +60 thou with skirt to bore clearance at TDC .014″. Strangely there are two piston rings (one above the other) in the lower groove. All the internals suggest that the whole thing has been sat for a long time without moving.

Time for a re-think. I am considering having the barrels sleeved and using new pistons ..but I am looking in to the stores in the garage first !

[chris wastell]

How about reboring to take Silk pistons?

[lewis onions]

An update on the current frustrating issues with this engine.

The barrels were re-bored to +0.080 and new pistons and rings fitted. When reassembled there was a noticeable escape of air from around the cranks when an airline was attached via the carburettor inlet so the engine was disassembled again and the cranks removed. This disclosed a number of problems: the crank had been put together with a woodruff key which was too long for the key-way which meant that the cranks were held apart; 0.098 of shims had been added; the glands were dished and so were not sealing in the cups; and the gland springs were both left-handed. Reground glands were blued and lapped in before reassembly with a shortened woodruff key, new left and right hand springs all with 0.012 end float.

The engine was refitted in the frame and ignition timed, checked and re-checked. The BTH Mag-generator was producing large fat blue sparks. The Binks carburettor has been rebuilt and the float level set as prescribed. So all was ready….

Or seemingly not. The engine stubbornly refuses to fire or run by kick stater or on rollers. The plugs quickly become warm and there is a faint smell of burning petrol but nothing more.

After many frustrating attempts using varying starting procedures – drenching the pistons with oil, liberally dousing everything in petroil, squirting neat petrol into the carburettor or down the spark plug holes there was still no sign of life whatsoever.

To make things worse, reverting to the Snap On compression tester the engine still only seems to be able to struggle up to 50 psi compression (at least we’ve doubled the original readings!) and observing the pistons on the up stroke through the transfer ports suggests that air is leaking back past the rings

Does anyone have any ideas before I push the thing into the canal and have done with it?

[BRIAN MARSHALL]

Do you know what the piston ring gaps are ? Have you checked that the conrods are long-stroke and not short-stroke ?

[Barrie]

On a newly rebuilt engine I would expect the compression to be on the low side, quite normal. when the engine starts the rings will lap themselves
Recheck your timing , did you fit new seal and gaskets to the bases of the cylinder block?

[lewis onions]

Thanks Brian and Barrie – the barrels are off (again) and it appears that the rods are long stroke (5.813 inches). The ring gaps are variable between 0.110 and 0.121. I guess that this might be the problem. Back to the drawing board !

[BRIAN MARSHALL]

Do those gaps take the ring stop pegs into account ?

[Barrie]

@lewis onions wrote:

Thanks Brian and Barrie – the barrels are off (again) and it appears that the rods are long stroke (5.813 inches). The ring gaps are variable between 0.110 and 0.121. I guess that this might be the problem. Back to the drawing board !

So did you put the rings down the bores then meaured the Gap?
I take it that the quoted measurement is in inches.
The quoted rule of thumb for these engines is the end gap of1.5% of the bore diameter ,(technicalities)
so you have bored out to 75mm which is 73mm plus 0.080 thou, so your end gap should be 1.12 mm ,which is 0.044 thou ” plus the width of the pin ,if it comes out flush with the cylinder, or if the piston has an internal or side notch discount the pin width.

The book of Scott states 0.010 thou in excess of the ring stop

[Roadster]

Lewis, In your early post on this thread you said that the engine ran, after a fashion, with the old pistons which had seized rings and onlygave around 25psi compression pressure. With your new pistons the 120 thou ring gap does not seem particularly excessive if you allow for the peg. I remember some research carried out quite a few years ago when new rings were fitted to a Ford(?) engine and the gaps increased in stages up to, I think, 60 thou before there was any noticeable effect on performance! I don’t think that your ring gaps would prevent it starting.
Were the plugs getting wet with fuel when you were trying to start it? If not, the liner base seals or the transfer port seals could be an issue. I think it is a matter of re checking everything yet again until you find what has been missed.
If you would like me to take the bike down to the canal for you just ask, you don’t even need to come with me!
Rod

[Kevin N Bayliss]

Lewis, fit the barrel without the pistons but use all the base gaskets and make sure you have a gap between the barrel and crankcase, (no bolts fitted) I have had several where the barrel will sit flush to the case without any bolts fitted which means the cork / “o” rings are not compressed when the bolts are tightened, seals not compressed = no crankcase pressure = no start. Kev

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