Cylinder to piston clearance?

Hello Steve,

You may have to go to the next size larger piston, what you have now would probably run okay, it would have some piston slap with its .003" excessive clearance. I think the piston clattering around like that wouldn't be conducive to a pleasant ride or long piston life.

Regards,

Albert

I agree with Albert, 7 thou is way too much and it will clatter loudly, 3-4 thou is about right. I would imagine that RGM would take the piston back and replace with a an oversize one.If you get the next size piston, before doing the rebore (making sure that the next rebore size is known) you can take it along with the barrel and ask themechanic/engineer doing the reboreto hone thenewly reboredbarrel to exactly the specified clearance. You will have a perfect match then. Best to remove the rings before though, for his convenience. You will also need them removed to set the ring gap accurately which must be done to ensure the best gas sealing and prevention of a nip up when running in.

Regards

Les H

Previously wrote:

I am restoring an OHVModel 18, 500cc,from1936.

Careful measuring shows the piston to cylinder clearance is now .007". Is this too much?

The paperwork that came with a new piston (Australian madeJP, purchased from RGM) states it should be .004"

Your opinions, please.

Regards

Steve Snoen

British Columbia

Hello how did you mesuer the piston in the bore and with a feeler gauge down one side ?well if that's the case you have to divided .007 by two witch is ,3.1/2 thou witch would be about rigth for your engine?

Anna Jeanette Dixon says:

"hello how did you mesuer the piston in the bore and with a feeler gauge down one side ?well if that's the case you have to divided .007 by two witch is ,3.1/2 thou witch would be about rigth for your engine?"

Okay, I hope everyone knows that Dixon's statement is dead wrong and ridiculous? She is stating that if a piston is seven thousandths smaller than it's bore then the actual clearance is said to be three and a half thousandths, which is absolutely incorrect. It is too bad that people coming to the official Norton website looking for answers for spending hard-earned money have a chance of destroying their engines because of crap like this.....

Previously wrote:

hello how did you mesuer the piston in the bore and with a feeler gauge down one side ?well if that's the case you have to divided .007 by two witch is ,3.1/2 thou witch would be about rigth for your engine?

????????? well it is april !!

I agree completely with Benjamin. I laughed at what Anne had advised, but as I have been puttingso much onto this site lately (wasting my time probably) I was going to wait to see just how long it would take the experts to correct it. Sadly, they did not and you have had to correct it yourself. One can only assume it would have been left as firm advise if you had not!!

This, as you point out, is is really a shame on a club that claims to be the OFFICIAL NORTON OWNERS CLUB....readers all over the globe read these posts, so to let that wrong information stand and go on uncorrectedwas pitiful.

Obviously this was posted as just a private answer and does not carry the official seal of approval, but to leave it unchallenged was wrong.

Les

Steve

I take the following from E M Franks book.

'Norton'

'Engines Section'

'Piston Clearances'

'On a machine that has seen consderable service or where a new piston is being fitted it is desirable to check the clearance between piston and cylinder.

With both components in NEW condition it should be possible on 16 H and Big 4 to insert feelers 0.007 in. thick between the piston and cylinder at the TOP of the piston skirt immediately below the bottom ring groove.

On OHV engines the correct clearance is 0.008 in.; at the bottom of the skirt there should be a clearance of 0.005 in. on all models. It is important that this check is made in a fore and aft direction at right angles to the gudgeon pin, as the piston is not round being relieved across the gudgeon pin bosses.

Where the measurment clearance is app 0.007 in in excess of the above figures an examination of the bore should be made. Pistons of 10 20 30 and 40 thou in. oversize are normally available'

Hope this helps.

Ian

Previously wrote:

Anna Jeanette Dixon says:

"hello how did you mesuer the piston in the bore and with a feeler gauge down one side ?well if that's the case you have to divided .007 by two witch is ,3.1/2 thou witch would be about rigth for your engine?"

Okay, I hope everyone knows that Dixon's statement is dead wrong and ridiculous? She is stating that if a piston is seven thousandths smaller than it's bore then the actual clearance is said to be three and a half thousandths, which is absolutely incorrect. It is too bad that people coming to the official Norton website looking for answers for spending hard-earned money have a chance of destroying their engines because of crap like this.....

hello ben I think you better read what this guy says before saying I am wrong and its crap ? if you have 7 thou at one side of the piston then , thake out the feeler gauge a you have a bear piston at this point then fit it in the bore then put in two feeler gauges and see what you get ?the other way is too fit and bore gauge on a magnetic stand then you get and good reading of the bore ? and tell me this is crap ?

your anna j dixon

Previously wrote:

Previously wrote:

Anna Jeanette Dixon says:

"hello how did you mesuer the piston in the bore and with a feeler gauge down one side ?well if that's the case you have to divided .007 by two witch is ,3.1/2 thou witch would be about rigth for your engine?"

Okay, I hope everyone knows that Dixon's statement is dead wrong and ridiculous? She is stating that if a piston is seven thousandths smaller than it's bore then the actual clearance is said to be three and a half thousandths, which is absolutely incorrect. It is too bad that people coming to the official Norton website looking for answers for spending hard-earned money have a chance of destroying their engines because of crap like this.....

hello ben I think you better read what this guy says before saying I am wrong and its crap ? if you have 7 thou at one side of the piston then , thake out the feeler gauge a you have a bear piston at this point then fit it in the bore then put in two feeler gauges and see what you get ?the other way is too fit and bore gauge on a magnetic stand then you get and good reading of the bore ? and tell me this is crap ?

your anna j dixon

Hello Anna, I agree Ben has been a bit unkind in the way he has responded although I think what he stated is correct. It may be that he has not understood yourmessage which is is easily done from my experience.

I'm afraid what Ian has written is no help. here is why.

1) Pistons are always smaller at the top than at the bottom. This is to allow for the higher temperature and hence greater expansion of the top of the piston in relation to the bottom. The book suggests the measurement is taken mainly at the top (just below the bottom ring). This is why he states a largish clearance and this is normal.

2) The book goes on to say at the clearance is 5 thou at the bottom.

3) For new pistons the instructions usually give the clearance specifying a certain position, (something like 15 mm?) up from the bottom of the skirt at the front or at the back (it makes no difference)

4) The clearance specified by piston manufacturers will be the TOTAL clearance, for the JP one, it is 4 thou TOTAL.

5) Steve has measured 7 thou so withthe new piston he is already 3 thou too loose, so a waste of piston life and the extra piston rock will cause poor ring sealing anyway.

6) The better way to measure is to use 2 feeler of around half the size, so Steve should use a 3 and 4 thou feeler. This helps as the piston is held more vertical in relation to the bore and is not forced to rock over to one side. (perhaps this is where Anne has misunderstood the technique?)

7) Using two feelers will also reduce the frictional feel of the guage in use as the feeler blade will be around half the thickness and consequently will flex to the round curvature (width-wise) of the bore. Using one thick gauge can give an eroneous tight feel and the gap will be larger than the apparent finger sense tells you. So in Steves case the gap might well be 8 thou!

8) Bore gaugesand micrometers will be more accurate ultimately.

9) Steve requires a rebore and a new oversize piston set with the correct 4 thou clearance, not 7 thou.

10) Each piston expands differently, and the manufacturer provides what he considers the best clearance. The recommended clearance is NOT transferable to a different manufacturer or different style piston unless coincidentaly the same.

11) Mr Franks book cannot be used as a reference.

Regards.

Les

Previously wrote:

I'm afraid what Ian has written is no help. here is why.

1) Pistons are always smaller at the top than at the bottom. This is to allow for the higher temperature and hence greater expansion of the top of the piston in relation to the bottom. The book suggests the measurement is taken mainly at the top (just below the bottom ring). This is why he states a largish clearance and this is normal.

2) The book goes on to say at the clearance is 5 thou at the bottom.

3) For new pistons the instructions usually give the clearance specifying a certain position, (something like 15 mm?) up from the bottom of the skirt at the front or at the back (it makes no difference)

4) The clearance specified by piston manufacturers will be the TOTAL clearance, for the JP one, it is 4 thou TOTAL.

5) Steve has measured 7 thou so withthe new piston he is already 3 thou too loose, so a waste of piston life and the extra piston rock will cause poor ring sealing anyway.

6) The better way to measure is to use 2 feeler of around half the size, so Steve should use a 3 and 4 thou feeler. This helps as the piston is held more vertical in relation to the bore and is not forced to rock over to one side. (perhaps this is where Anne has misunderstood the technique?)

7) Using two feelers will also reduce the frictional feel of the guage in use as the feeler blade will be around half the thickness and consequently will flex to the round curvature (width-wise) of the bore. Using one thick gauge can give an eroneous tight feel and the gap will be larger than the apparent finger sense tells you. So in Steves case the gap might well be 8 thou!

8) Bore gaugesand micrometers will be more accurate ultimately.

9) Steve requires a rebore and a new oversize piston set with the correct 4 thou clearance, not 7 thou.

10) Each piston expands differently, and the manufacturer provides what he considers the best clearance. The recommended clearance is NOT transferable to a different manufacturer or different style piston unless coincidentaly the same.

11) Mr Franks book cannot be used as a reference.

Regards.

Les

JUST TO MAKE IT CLEAR: when using two feeler gauges, the feelers are positioned either side of the piston. Ie: one at the front and the other at the rear of the piston. Les

All

I simply quoted from EM Franks. (A.M.I. Mech. E) 1956 Ed.

I would disagree it cannot be relied upon for guidance.

I have not recommended any course of specific action but I would agree a rebore is best in any case.

Perhaps we should hear from the poster. He should be better informed from the experience at least.

He's been silent since.

Ian (Eng. Tech. M.I.R.T.E., M.S.O.E., M.A.E)

Previously wrote:

Previously wrote:

I'm afraid what Ian has written is no help. here is why.

1) Pistons are always smaller at the top than at the bottom. This is to allow for the higher temperature and hence greater expansion of the top of the piston in relation to the bottom. The book suggests the measurement is taken mainly at the top (just below the bottom ring). This is why he states a largish clearance and this is normal.

2) The book goes on to say at the clearance is 5 thou at the bottom.

3) For new pistons the instructions usually give the clearance specifying a certain position, (something like 15 mm?) up from the bottom of the skirt at the front or at the back (it makes no difference)

4) The clearance specified by piston manufacturers will be the TOTAL clearance, for the JP one, it is 4 thou TOTAL.

5) Steve has measured 7 thou so withthe new piston he is already 3 thou too loose, so a waste of piston life and the extra piston rock will cause poor ring sealing anyway.

6) The better way to measure is to use 2 feeler of around half the size, so Steve should use a 3 and 4 thou feeler. This helps as the piston is held more vertical in relation to the bore and is not forced to rock over to one side. (perhaps this is where Anne has misunderstood the technique?)

7) Using two feelers will also reduce the frictional feel of the guage in use as the feeler blade will be around half the thickness and consequently will flex to the round curvature (width-wise) of the bore. Using one thick gauge can give an eroneous tight feel and the gap will be larger than the apparent finger sense tells you. So in Steves case the gap might well be 8 thou!

8) Bore gaugesand micrometers will be more accurate ultimately.

9) Steve requires a rebore and a new oversize piston set with the correct 4 thou clearance, not 7 thou.

10) Each piston expands differently, and the manufacturer provides what he considers the best clearance. The recommended clearance is NOT transferable to a different manufacturer or different style piston unless coincidentaly the same.

11) Mr Franks book cannot be used as a reference.

Regards.

Les

JUST TO MAKE IT CLEAR: when using two feeler gauges, the feelers are positioned either side of the piston. Ie: one at the front and the other at the rear of the piston. Les: ??

Hello Les,, thats is what I ment?? but you guys have no imagination ? the other way too mesure is by using the piston ring by starting at the top too get your first reading then get your piston with no ring on it push down about a inch and take out the piston and take your next reading and keep going down the bore in the same way too the bottom of the bore then you will have different readings as you have gone down the bore then you will know where the wear is in the bore ?

Previously wrote:

Thank you to all who replied to my question about piston to cylinder clearance.

As expected, different people have different opinions. That is healthy.

Just to expand a little about the initial post: The Norton (Model 18, 1936) waspurchased a year ago in rideable condition. Duringpast summer it became clear: oil-consumption was excessive (a quart every 300 miles). Ok fine, I will dismantle the bike this winter and go through everything.

When I measured the cylinder it was very round and hadinsignificant taper. Honing marks were clearly visible from last time the engine was apart. Previous owner (wonderful man, age 74 and still riding!)barely used the bike over a seven year period.

Measuring the piston (atbottom ofskirt, right angle to wrist pin) it was .007"smaller diameter than thecylinder. The pistonhad very little wear. The cylinder had at one time been sleeved and is nowback to standard size, 79mm.

Measurements were taken with an electronic digital caliper. I have spent the last 30+ yearsin the machinist trade, so hopefully Ishould know how to measure by now

My thought was: .007" is way to much? My 1973 Commando was given an over-bore some years ago, the cylinders were honed.0045" bigger than the pistons.It seems to work very well.

Clearly theModel 18had covered very few miles since last timethe engine was apart. I desided to buy an oversize (+.020)piston from RGM. In the papers that came with the piston (Australian made JP)it stated the clearance should be .004".

ThenI started wringing my hands: Do I really have to do a re-bore just for the sake of .003"?

So that was the background/reason for the question.

Anyway, enough of this. I am going for a ride. It clears my head. Always does

Regards

Steve Snoen

British Columbia

Ride safe Steve.

We must never depart from the basic fundamentals and you have them it seems.

We can alter and deviate, too fine a deviation and problems will arise, it's all about compromise, keep it simple and reliability will be the reward.

Norton endorsed the book by Franks, it had too he used their data!

He lectured to students of engineering, (like myself) books are for the guidance of fools and the observance of wise men.

Ian