Cylinder oval in section by 0.06mm. Significant?

[weary]

I have managed to bore a 50mm (2 inch approx) diameter steam locomotive cylinder 0.06mm (2.5 thou') larger diameter in the horizontal axis than in the vertical axis.

Is this variation significant? Should I just ignore it in 'real life'?

I intend to fit 'Clupet' piston rings. The cylinder and piston are cast iron.



Regards,

Phil.

[keith6233]

H! Phil

Have you checked if the bore is parallel to the bolting face and doe's it have to be spot on 50mm if so i would leave it ,do you know the reason for the ovality.

[weary]

Hi Keith,

Thanks for the response.
Bore is parallel to the bolting face.
Bore doesn't have to be exactly 50mm diameter. I will make pistons to fit, though I anticipate those being round in cross-section!
I have bored two cylinders, first seems to be perfectly circular in section. The second cylinder bore has this very slight ovality, I suspect due to bad clamping which I didn't spot at the time of setting-up for boring.

I will be more than happy to follow your advice and leave it as is.

Regards,
Phil

[albert]

Hello,
I would bore it out and fit a liner.
Do not forget the rings will be perfect so blow past will take place.

ALBERT

[rogsteam1959]

If it would be me I would bore it to the next diameter possible to get a perfect cylinder.

Michael

[chris vine]

if it is only 2 odd thou oval, can you just re-machine to clean it up and make it round. IE take out 2 thou from the small diameter. If you are using clupet rings, I think they will be quite happy opened up that amount and will not leak, because of their no-gap design.

It seems a shame to leave it oval because once the ring(s) have worn in, they will be a very poor fit if taken out and put back in a different rotational position.

Chris.

[arch1947]

Hi Phil, You might want to think about getting the cyliners honed by a professional such as an automotive engine re-conditioner. I found one of my cylinders had a slight taper caused, I think, by a small hard spot in the casting. The advice from this forum was to get them professionally honed rather try it myself. The result was two cylinders exactly the same diameter, round and parallel. I had the same thing done to the valve sleeves and they are outstanding.
Cheers,
Arch

[rogsteam1959]

I don't think it's possible to get a round hole out of an oval with honing.

[weary]

Gentlemen,

First of all, thank you for your responses and insights.

You have convinced me to go for a rebore, initially just skimming the minimum off to return the bore to circular in section. It looks to me like the resultant very slightly reduced wall-pressure of the rings will have no significant effect in my usage. In any event I can always make some (plain) rings to fit.

Arch, I read your thread referring to the professional honing, and that will be my plan B. I have done a bit of googling and it looks to be cheaper than I anticipated. If I was 'rational' it would definitely be the way to go, but, being an 'irrational model engineer' I like to try and 'have a go' myself. Sometimes with mixed results!

Rogsteam, one of the Engine Services places I have found seems to be able to return oval bores to round, per them by 'honing'. Don't know how they achieve this.... and am hoping that I do not have to find out!! I suspect they use some kind of initial boring operation.

I guess that in my head I knew the solution was a rebore possibly combined with a liner, but in my heart I was hoping that the crowd advice would say 'no problem, just go with it as it is'. The curse of relatively cheap accurate tooling and measuring instruments!

Thanks again, will be in workshop later ......

Phil

[vulcanbomber]

Jun 18, 2015 8:13:23 GMT rogsteam1959 said:

I don't think it's possible to get a round hole out of an oval with honing.

It very much is but it does take quite a bit of material removal to achieve it, at least to the hover it is... I can't remember how many multiples of the error it takes however.

[chris vine]

Hi Phil,

I think you could probably fix it by lapping, rather than honing. It wouldn't take long to make a hardwood lap (maybe aluminium), the solid nature of a lap compared to a hone, would restore roundness and keep the bore parallel. I don't think 2 thou would take long to remove.... Also there is no clamping force involved as you can hold it by hand while you work it on the lap.

Chris.

[GWR 101]

Hi just to say spent many years in industry honing steel tubes as used in various applications such as hydraulic pit props, and we found that using honing heads with mechanical (screw) stone expansion methods bore roundness was improved. However hydraulic stone expansion systems tended to follow the ovality, not sure what is currently used in the cylinder reboring industry but I am sure they will advise. Regards Paul

[isc]

Chris, you beat me to it, a wooden lap is easily made. Turn it to size, split it length wise, put a wood screw in the end of the split(screw it in to expand the lap), make the lap a bit over twice the length of the cylinder. Hold the lap in the 3 jaw chuck, cover the lathe bed, I use newspaper. The only compound I have is very fine, can't remember the grade, but it takes quite a while.
It would be worth lapping both cylinders, this will ensure that they are round. Then if you want to break the glazed finish hone it with a cylinder hone. Excluding the honing, this is how I treat the cylinders of my hot air engines, these run cast iron pistons in cast iron cylinders with no rings. isc

[weary]

Thank-you for your continued suggestions.

To update you:

I managed to bore the cylinder out to the correct circular cross-section and with honing it is 0.1mm larger diameter to 50.1mm.
This will allow the option of using commercially available piston rings should I wish to do so.

It was a rather time consuming job. The actual machining was very quick, but the setting-up took a couple of hours. As all other dimensions were good I was very concerned about creating more problems.

I have no experience of lapping cylinders, and I guess that overall it may be a quicker operation with far less risk of creating further problems. However my personal unfamiliarity meant that it was a further option so far as I was concerned. In the hands of an experienced machinist (I am self-taught) lapping is possibly the best solution for such a small ovality. I do not have either the knowledge nor experience to judge.

The reason that I didn't update the thread previously is that it appeared to have drifted 'off-topic' into a discussion of honing oval bores into round section, and I didn't want to take the thread 'off-the-new-topic' by reverting to the original subject. But as 'isc' has brought the tread back 'on-topic' .......... !!

Thank-you again to all contributors.

Regards,

Phil.

[ejparrott]

I've spent 10 years doing nothing but turning and vertical boring. I consider myself an experienced machinist.

I would never use these methods of lapping, I would send out to honing.