JP3 piston rings

[londonron]

I have never taken a piston out before. Tonight i took out all three. As well as some great help from John in stafford and martin I am also now a fully apprenticed you tube mechanic so was thinking the gaps in the piston rings a bit large. More tape measure than feeler gauge. Am hoping this gap explains my lack of compression and therefore bad starting.

 

So can anyone tell me what the gap should be ? and should they rattle about so much and be so easy to take off

[The Bagdad Boatman (waits)]

Is it the rings that are worn or the cylinder bore?

[Tony Dunkley]

I have never taken a piston out before. Tonight i took out all three. As well as some great help from John in stafford and martin I am also now a fully apprenticed you tube mechanic so was thinking the gaps in the piston rings a bit large. More tape measure than feeler gauge. Am hoping this gap explains my lack of compression and therefore bad starting.

 

So can anyone tell me what the gap should be ? and should they rattle about so much and be so easy to take off

It sounds to me as though you're describing and measuring the ring gaps with the rings fitted to a piston that's been removed from it's bore . . . . the gap dimension is measured with feelers with each of the rings separated from the piston sitting in the unworn portion at the top of the cylinder bore.

[koukouvagia]

I don't know about JPs, but Kelvins seem remarkably tolerant to large ring gaps.

 

 

Admittedly my Kelvin was getting a bit reluctant to start and the reason was obvious - a two inch gap! All the other rings had about a quarter of an inch gap (measured with the ring inside the cylinder).

Once the rings and pistons were replaced the difference was amazing. It starts first touch of the button, every time in all weathers. No more faffing around with petrol start and magneto.

 

 

[Bee]

Where did the other bit of ring go?

[MtB]

I'm inclined to think it was missing when the piston was fitted in the first place.

 

MtB

[rrt2]

As well as measuring the ring gap at the top of the unworm part of the bore you should also do the same with the ring right at the bottom of the bore.

General rule of thumb for unworn rings is a gap of 004/1000 per inch of diameter of the bore. In the case of a 3 inch bore that will be a minimum gap of 12/1000 in an unworn area. If you look on a search engine for diesel piston ring gaps you will find out more.

[londonron]

so as i understand it i take the ring off and put it back in the liner / bore. and measure the gap. makes sense and will do that this weekend. Now i understand about putting it in the bottom as an unused part of the bore however i also think whats the point of that if thats not where the ring is working. So if anyone fancies giving it a go im all ears.

 

Second question. Once the piston is out how loose should the rings be. They really were rattling about. and you can spin them around with no effort and i only need my fingers with no effort at all to get the top 2 rings off. As its new to me i dont know whats loose and whats tight. Someone mentioned checking the gap on the piston grove and the ring if they were rattling about as well as the gap once installed.

 

All advice gratefully received.

[rrt2]

Where the ring is working is not really relevant, when new bores are the same diameter all the way from top to bottom.

Measure the ring gap at the top of the bore above the wear line and then right at the bottom.

 

Vertical movement of the ring in the groove in the piston should be not very much cannot give you are figure - you don't want the ring sliding up and down in the ring groove.

Be very carefull with rings they are cast iron and snap very easily.

They should be a reasonbly easy to spread to get them out of the ring groove.

[Timleech]

The top ring in a JP piston is of a wedge section, sitting in a tapered groove, so it's not easy to get a true picture of ring/groove wear.

I think the JP book does give figures for acceptable side clearance, though.

 

 

Tim

[Bee]

I Don't know about JP's but the piston in the picture seems to have the top ring groove machined into a cast iron insert. Posted this really just to emphasise, as Barry has said, be careful with the rings, cast iron snaps easily.

[by'eck]

The top ring in a JP piston is of a wedge section, sitting in a tapered groove, so it's not easy to get a true picture of ring/groove wear.

I think the JP book does give figures for acceptable side clearance, though.

 

 

Tim

 

Prompted me to look at the manual. Ring gap 12-16 thou measured at bottom of bore.

[martyn 1]

Take the rings off and check them in the bore to the measurements that Richard has mentioned above, but then also get out an internal micrometer and check the bore diameter at various points between top and bottom across the bore (side to side in the engine) and inline with the gudgeon pin. If the bore is any larger that 4.504" at any point then new liners I'm afraid and also check for any patches where the chrome lining has come off (if original liners are fitted) again if this is present new liners will be needed.

 

Finally check the clearance between the ring and the groove they sit in. to slack and you will either need an over width ring set or new pistons. anything over 0.008" on the compression and 0.014" on the oil scraper rings is in need of rectification.

[londonron]

what i really like about all this is i understood it. Somehow its all sinking in. All helpful advice so thanks a bunch. Martin thanks for such a detailed reply. fingers crossed etc but also i really do want to find a problem. As i need to have it starting properly

[londonron]

seems buying an internal micrometer isnt as easy as i thought. Would you guys buy.

 

http://www.ebay.co.uk/itm/DRAPER-EXPERT-02753-50-160mm-BORE-GUAGE-SET-LIST-PRICE-150-50-HUGE-SAVING-/271730205161?pt=LH_DefaultDomain_3&hash=item3f4461e1e9

 

http://www.ebay.co.uk/itm/Draper-52416-Expert-6-Piece-Telescopic-Gauge-Set-/190343086141?

 

or something else? My own thoughts are buy the first one as if the dial moves as you wander about inside the bore i can test more places. But thats only true if i hold it level. Help as want to buy it soon as poss.

[Timleech]

seems buying an internal micrometer isnt as easy as i thought. Would you guys buy.

 

http://www.ebay.co.uk/itm/DRAPER-EXPERT-02753-50-160mm-BORE-GUAGE-SET-LIST-PRICE-150-50-HUGE-SAVING-/271730205161?pt=LH_DefaultDomain_3&hash=item3f4461e1e9

 

http://www.ebay.co.uk/itm/Draper-52416-Expert-6-Piece-Telescopic-Gauge-Set-/190343086141?

 

or something else? My own thoughts are buy the first one as if the dial moves as you wander about inside the bore i can test more places. But thats only true if i hold it level. Help as want to buy it soon as poss.

 

Neither of those is an internal micrometer.

The first is strictly speaking a Bore Comparator, it shows differences but not an actual measurement. They're intended to be 'set' with a Ring Gauge, but you can 'set' them with an ordinary external micrometer or even digital calipers, it's a fiddle to do, and then 'compare' that with the readings for the cylinder bore.

The second are spring bore gauges, again you need an external mic to 'read' them, there's a definite knack to using them and I find they're trickier to use on the bigger sizes but you can with practice measure to a thou or less.

 

Tim

[jonesthenuke]

seems buying an internal micrometer isnt as easy as i thought. Would you guys buy.

 

http://www.ebay.co.uk/itm/DRAPER-EXPERT-02753-50-160mm-BORE-GUAGE-SET-LIST-PRICE-150-50-HUGE-SAVING-/271730205161?pt=LH_DefaultDomain_3&hash=item3f4461e1e9

 

http://www.ebay.co.uk/itm/Draper-52416-Expert-6-Piece-Telescopic-Gauge-Set-/190343086141?

 

or something else? My own thoughts are buy the first one as if the dial moves as you wander about inside the bore i can test more places. But thats only true if i hold it level. Help as want to buy it soon as poss.

You need something like this,

 

http://www.ebay.co.uk/itm/Moore-wright-internal-micrometer-set-/291353544695?pt=UK_Measuring_Tools_Levels&hash=item43d60647f7

 

This one does 2" upwards, a bit fiddly to use but will do the job to required accuracy. Is there anyone nearby you can borrow one from?

[David Mack]

If you've already measured the ring gap at the unworn top of the bore, can't you just push the ring down a bit (keeping it square) and measure the increased ring gap, and divide by pi to get the additional bore diamter?

[rrt2]

Direct reading bore gauges are made by Bowers, before looking at the price purchase some smelling salts and its also best to be sitting down.

[jake_crew]

If you need "special" rings made, talk to www.ringspacers.com (Otto engine works) in the USA.

 

I broke several rings getting them unstuck and out of their grooves on our engine which is very similar to Kooka's Kelvin.

 

They were able to supply quickly (possbly from stock), for less than $10 each.

 

Also some of the original rings were mated with thin (~ 3thou thick) spring steel spacers presumably to take up the slack in a wide groove. Whether this was a manufacturing techniquue in the 1930s, or a subsequnet bodge we'll never know.

[londonron]

thanks guys, Was shocked (maybe a strong word) to find the local boat yard dont have one and use vernier gauges at the top and then just stick them in,

maybe thats why they have a "may still smoke a bit" disclaimer on the recon engines.

 

have made an offer on the ones you found jones the nuke.

[londonron]

my youtube engineering course continues. https://www.youtube.com/watch?v=gjN0fGfUzl0 Would this not work for me then ?

[martyn 1]

You need something like this,

 

http://www.ebay.co.uk/itm/Moore-wright-internal-micrometer-set-/291353544695?pt=UK_Measuring_Tools_Levels&hash=item43d60647f7

 

This one does 2" upwards, a bit fiddly to use but will do the job to required accuracy. Is there anyone nearby you can borrow one from?

 

What you need to do the job properly is what was posted by jonesthenuke. As Tim has mentioned you can get away with using the bore gauge in your first ebay link but you will need to zero this against a ring gauge or appropriate external mic.

 

How did your rings measure up against the ring grooves in the pistons?

[jonesthenuke]

There is now a cheaper internal mike on eBay.

[londonron]

waiting for the bits to be delivered to check the bores. Then going to get in there and check it all in one go. Shall report back when i have news. Work and study have got in the way (plus a certain someone is demanding to actully see some of me)