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Are the liners worn or the pistons, or both? - Engine/Ancilliaries - The Lotus Forums Jump to content
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Paul Coleman

Are the liners worn or the pistons, or both?

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I'm in the process of trying to determine what needs doing to my S1 engine...

On the advice of somebody I took the top ring off one of my pistons and placed it in its respective bore 50mm down. I then used feeler guages to measure the gap between the piston ring which came out at about 0.8mm. According to the manual it should be no more than 0.508mm which means something is out. It could mean the liner has worn or it could mean the piston rings have worn or it could be both - how do I tell which it is?

I don't have an internal micrometer to measure the inside bore diameter.

Thanks, Paul.

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Paul - It sounds to me that you are actually measuring the clearance of the top ring land - not the piston/liner clearance. So there may be no problem. However, I agree with Robin on the other post, you could do with somebody familiar with this type of engine to help you out.

cheers Steve

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Hi Steve

I don't disagree but I don't know anybody who knows these engines. I'd rather do bodywork or electrics in preference to mechanics any day of the week!

With regard to the piston ring I read in the manual the figure for piston ring gap (top) and assumed that meant the size of the gap in the top piston ring when it was inside the bore. Have I misunderstood what it refers to?

Thanks, Paul.

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Ahh I see what you mean - I thought you were referring to the piston in the liner! OK - How much does the gap vary if you try at different points in the liner? If the measurment is consistent, then the ring is worn (or was always gapped too large. If the measurement varies, then the liner is also worn...

cheers Steve

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D'oh! Yes of course it's obvious when you tell me :unsure: I shall try it tomorrow.

What do you mean it may have been gapped too large - is this something that's set? I just assumed you bought a set of piston rings and bunged 'em on!! Here I go again showing my ignorance!!

Thanks, Paul.

Edited by Paul Coleman

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Even with new rings, you have to check their gap on each cylinder in which they're going into. Every engine is different, therefore the ring manufacturer typically takes this into account when manufacturing and sizes the rings slightly large (smaller gap when installed). It's up to the builder to file the ends of the rings to match the exact specification for the cylinder they're working on. It's likely that your rings were either over filed, or the liners have worn. Generally however, rings will wear on the leading and trailing edges of the ring resulting in a cross section that looks more rounded on the ends (top and bottom of the ring) rather than squared when new. Wear doesn't typically occur at the gap making the gap larger, which leads me to believe other factors may be at work in this case.

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I put each of the top rings into their respective liner and measured the gap with the ring at various heights in the liner. I measured the ring gap with the ring 25mm down from the top of the liner and then at 50mm, 75mm and 100mm. These are the results I got but I'm not exactly sure what they're telling me?

Bore #1 (nearest the timing belt) - total variation = 0.15mm (0.006")

25mm down gap = 1.0mm

50mm down gap = 0.85mm

75mm down gap = 0.85mm

100mm down gap = 0.9mm

Bore #2 - total variation = 0.05mm (0.002")

25mm down gap = 0.85mm

50mm down gap = 0.8mm

75mm down gap = 0.8mm

100mm down gap = 0.85mm

Bore #3 - total variation = 0.1mm (0.004")

25mm down gap = 1.0mm

50mm down gap = 0.9mm

75mm down gap = 0.9mm

100mm down gap = 0.9mm

Bore #4 - total variation = 0.1mm (0.004")

25mm down gap = 0.95mm

50mm down gap = 0.85mm

75mm down gap = 0.85mm

100mm down gap = 0.85mm

I can't see a figure in the manual for the permissible wear of the liner - can anybody comment on the above as to whether they are acceptable or not? Similarly for the gaps according to the manual they're wildly out but again I'm wondering what the effect of this will be?

Thanks, Paul.

Edited by Paul Coleman

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Because you seem to have little or no variation in bore diameter (although gap at a pint where the ring doesn't normally sweep would be best) I'd be tempted to say your liners are fine (not perfect, but serviceable at least for a few years), but your rings probably need to be replaced to get a decent compression reading.

Paul.

1) What do you want from the engine between now and when you have it re-built professionally?

2) How long is the re-build likely to be away in miles?

If no 1 is something like "the car to run and drive OK, power not bad but hasn't got to be as new", and 2 is something like "10,000 to 20,000 miles maximum", then you could probably get away with re-using the rings.

If you want performance as per new engine and it could be 50,000 miles before re-build then you need to replace them.

Somewhere between the two options, is where you have to make a decision which one to go with.

Pretty much the same for shells (other post).

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Hi Andy

To answer the questions...

I would like the engine to last a few years (ideally 3) before I have it rebuilt fully and spend the money. During that time I doubt I'd do 5,000 miles. As long as it starts and runs okay I'd be happy, I'm not going to race it or anything like that. Of course part of that 3 years is me still working on the car to finish restoring it and that could take 2 years at the rate I'm going!!

I'm sooooo tempted to just button it all back up...

Cheers, Paul.

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Yes I think I'm going to. Somebody (not on this site) mentioned it might be worth getting something called a glaze buster and running it down the inside of the bores. I shall have to clamp the liners first though I think. Does anybody have any experience of these things?

Thanks, Paul.

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Paul

For reference, the piston ring gap clearances etc for the later 910 and 920 engines are, cant imagine the early 907 will be that much different.

Top and second 0.26 - 0.59mm (0.01 - 0.023in)

Oil control 0.16 - 0.54mm (0.006 - 0.021in)

Liner dia grade A 95.255 - 95.269mm (3.7502 - 3.7507in)

grade B 95.267 - 95.281mm (3.7507 - 3.7512in)

Liner diameters are measured across the thrust axis at 70mm from the top.

Piston diameter measured at 90° to the pin axis and 10mm (0.4in) up from the skirt lower edge is

Grade A 95.205 - 95.219mm (3.7482 - 3.7488in)

Grade B 95.217 - 95.231mm (3.7487 - 3.7493in)

There is a bit more wear than I would expect but it is not catastrophic. As Andy says what are your expectations from the engine. If it gives adequate performance and doesnt drink oil then carefully put it back together.

With regard to the glaze buster, it is generally like a big toilet brush with abrasive stones on the end of each bristle which cuts new honing grooves into the liner. Be careful not to over do the glaze busting as this tool will remove material from the bore and increase the clearances further and dont allow the drill to rotate without moving in up and down the bore. Before you start you will see honing marks in the bore. They cris cross alonge the length of the bore and are generally at 45°. You should aim to match this if possible. These honing marks help to lubricate the bore as small amounts of oil are carried in the honing grooves as the piston travels up and down. No honing marks are bad and you must avoid creating horizontal honing marks, hence the need to move the glaze buster up and down as in rotates. You may find that you will have to allow the engine to bed in again after glaze busting and for a time may experience slightly higher oil consumption until in does.

Cheers

Ralph

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Hi Ralph

Thanks for the info

It sounds a bit tricky if you've never done it before, so I'm going to take it to a local machine shop (who are checking the head for me) and ask them to hone it on their machine. That way they will have the necessary liner clamps which I don't have and don't really want to buy or try to make.

Cheers, Paul.

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