Roundness measuring

[Old SchoolParticipant @oldschool]

[Old SchoolParticipant @oldschool]

i am making new pistons for my 2.5cc tether car engine, the liners are brass with a chrome plated bore. As they have been used and the original is worn out I would like to check that the are round. Does anyone in the midlands have the test kit to do it and would they check 6 liners.

[Chris Evans 6Participant @chrisevans6]

You may well just get away with turning a/some plug gauges, then a combination of "Feel" and engineers blue should get you to where you need to be.

[Old SchoolParticipant @oldschool]

Chris thanks for the suggestion, I am after a number so I can pick the best ones to use and see what the limits of out of round I can use. The Stelling tether car engine is a high performance engine it does around 40,000 rpm when flat out the piston life is measured in minutes.

[vintagengineerParticipant @vintagengineer]

Are the pistons round or oval like full size pistons?

[Old SchoolParticipant @oldschool]

The piston we use are round, no piston rings.

[EmgeeParticipant @emgee]

Can you measure the piston OD at the piston crown and use the largest to trial fit a liner ? you may be lucky if you have 6 sets.

What size bore do you have ? and what percent silicon content is the piston ?

Emgee

[Old SchoolParticipant @oldschool]

The bore is nominally 15mm the silicon content is 25% from memory Dispal 226. I have made the pistons they are oversize at the moment and will be machined to fit the liners. I just want to check the used liners are still round after being used I have not been able to do it but fit/feel. The piston when finished will have a 0.75 degree taper starting about 2.5 MM. below the piston top. Oliver

 

Edited By Old School on 14/03/2017 22:20:16

[EmgeeParticipant @emgee]

Hi Oliver, you will get some idea of roundness by measuring piston diameter at the contact ring (bright ring), if the cylinder has a decent size top flange any ovalling will be kept to a minimum above the exhaust port which is where you are mainly concerned about. It may take considerable time and trial fitting using a piston lap to achieve the desired fit but I think if you try 1 piston then see how even width the bright ring comes with running, there may not be any ovalling of the cylinders, or not enough to effect performance.

Emgee

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Old School on 14/03/2017 22:17:33:

The bore is nominally 15mm ….

.

[waiting to be ridiculed for my naiveté]

I think that a 'precision ball' might be a suitable test device.

MichaelG.

.

See here: http://www.precisionballs.com/ball_gage_technical.php

Edited By Michael Gilligan on 15/03/2017 07:04:40

[Old SchoolParticipant @oldschool]

Emgee, thanks for your reply I am making pistons to fit the liners and as a last resort will lap it in but prefer to turn them. The thing is some go faster than others and I am trying to determine why measuring and having a value is key to this process. As you know the difference between a worn out piston and a new one is not easy to measure with a micrometer.

 

Edited By Old School on 15/03/2017 08:55:14

[Old SchoolParticipant @oldschool]

MichaelG, never seen those before and look interesting but I am after a value for roundness the liners have a tapered bore about 0.003" over the length and I only want to check the liner above the exhaust port. The engines are two strokes so it's an area of about 8 to 10mm. Oliver

[Nick HulmeParticipant @nickhulme30114]

I'm just off M1 J33, depending on where you are you could drop in and I'll run my Diatest bore gauging kit through them for you, drop me a PM,

– Nick

[Anonymous]

Measuring roundness is non-trivial. In essence the work needs to be rotated around a fixed axis and any variations in radius measured. The issue is that the TIR of the bearings on which the work is rotated need to be significantly better than the out of roundness values to be measured. Taylor Hobson do a range of products for measuring roundness:

**LINK**

Measuring the diameter at various points only tells you that the diameter is constant, or otherwise. It says nothing about the roundness of the part.

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Andrew Johnston on 15/03/2017 09:33:33:

Measuring roundness is non-trivial. …

Measuring the diameter at various points only tells you that the diameter is constant, or otherwise. It says nothing about the roundness of the part.

.

Quite so, Andrew … That's why I was hoping that the test balls would be useful

Oversize, Undersize, and a 'Goldilocks' [just right] would make a nice set … combine with a strong light source, and I would have thought a very reasonable assessment could be made.

MichaelG.

[Nick HulmeParticipant @nickhulme30114]

Posted by Andrew Johnston on 15/03/2017 09:33:33:

The issue is that the TIR of the bearings on which the work is rotated need to be significantly better than the out of roundness values to be measured.

They're liners so unless measured in-situ or a fairly loose fit any out of roundness measured may well be meaningless anyway, however it's measured

– Nick

[JasonBModerator @jasonb]

I think the ball method could get quite expensive, if you search that link for 15mm balls you get about 40 differentsizes in the range between 14.989##### and 15.009#### with some of them costing over $50. So you would have to know the exact spec of the liner.

I see they also do go-no-go balls that have flats on them that can be rotated in a hole to feel a tight spot but again you would need to invest in quite a few as it is only a nats difference between a good engine and a bad one.And would only work on an oval not a tri lobes cylinder.

 

Jim Allen who builds some very powerful engines cylindrically grinds his chromed liners by holding only by the lip, I assume they are not tight enough in the cylinder to affect this. He posts on MEM forum. 7HP out of 0.9cu.in can't be bad.

Edited By JasonB on 15/03/2017 10:33:47

Edited By JasonB on 15/03/2017 10:36:48

[Nick HulmeParticipant @nickhulme30114]

A standard micrometer and a V-anvil micrometer could be used to map the roundness of the OD, add a pipe wall micrometer or a mic with a ball anvil and by adding wall thickness readings you can map the roundness of the bore, and at least you end up with useful kit for other jobs

– Nick

[Old SchoolParticipant @oldschool]

Thanks for all the responses food for thought, Nick Hulme last suggestion seems within my capabilities.

I have followed Jim allens what he has achieved in his home workshop is outstanding model engineering building these racing engines.

Tether cars and there engines are designed to do a flying 500metres as fast as possible piston life is measured in minutes. The modern 10cc (0.60cu in) engines are developing around 8bhp at 32,000 rpm a liner piston will do about 9 sprint runs before it needs replacing. Being able to determine if the liner is reusable by fitting a new piston is a significant cost saving and an interesting engineering challenge.

Oliver

[Neil WyattModerator @neilwyatt]

My understanding is that ABC engines have pistons that are an interference fit with the chromed liners at teh toip oif the bore. Lapping to piston to the bore might give too loose a fit.

Neil.

[Old SchoolParticipant @oldschool]

Neil you are correct the amount of interference is controlled by measuring the liner lift. This is done with the piston and bore oil free as you turn the engine over as it nears the top the stroke you measure how much the liner lifts out of the crankcase.

[Maurice Cox 1Participant @mauricecox1]

To check for roundness, rest the cylindrical item in (on?) A V block, and rotate it with a d.t.i. Resting on the top surface. I believe that to be thorough, both 60 degree and 90 degree V blocks should be used. This will tell you if the object is round or not. I don't don't that the amount out of roundness could be calculated from the readings, but not by me I'm afraid.

[Maurice Cox 1Participant @mauricecox1]

That should read" don't doubt". Sorry.

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