Piston ring end gap

[Welderpaul]

Perkins P3.144 engine....

Couple of questions - Firstly; the ring end gap for the top and second compresion rings should be between 0.254-0.381mm and 0.2286-0.3302mm respectively according to the Perkins handbook. The new rings i have just tried in at the bottom of the bore give me an end gap of 0.54mm. They are the existing liners, but when i measure the bore it is still within the limits given by Perkins - very little wear on the bores shown by a minimal ridge i have reamed off the tops of the liners.
Out of interest i tried the old ring back in - the end gap was in the order of 2.5mm!!! The engine ran ok, and didnt seem to struggle in any way.
They are ebay-bought Dexta rings. Can i obtain longer (?) rings and file them to give me the correct end gap? I have the proper ring filer... Or will the new rings be ok at 0.54mm end gap given the massive gap the originals had.

Second question, and might be a difficult one if you're not familiar with the engine but - I know the pistons are stamped 1,2,3 to correspond with the bores, 1 at the front, 2 middle, 3 back of engine. I know the con rods go in with the number stamp go to the camshaft side of the engine, but which way round do the pistons go on the ends of the con rods? Does the number on the top of the piston face the front of the engine?
The big end con rod cap is also stamped with a number, does this face the number stamped on the con rod? Would seem to make sense but nothing on here or in the book to confirm.

The thing was so mucked up when i took it apart i could even see any of the numbers!

 

[rtbcomp]

Are they stamped "front" under the carbon?

 

[wheresmejumper]

shouldnt the rings be gapped around an inch from the top of the bore?

 

[Welderpaul]

shouldnt the rings be gapped around an inch from the top of the bore?

The rings need to be gapped at the bottom of the bore where any wear is minimal. If you gap them at the top there is a risk of them jamming and bucking/breaking when the stroke gets to the bottom of the bore.

 

[wheresmejumper]

jesus ya,its the opposite of a piston

 

[Forestryengineeringservic]

On the pistons is there any shapes/ combustion chamber that is off set to corgis pond with either the valves or injectors in the head.

 

[normspanners]

When measuring the ring you should push the ring down into the bore with a piston to make sure it is square in the bore. The cyls may have been heavily honed in the past but i think the gap you have will be OK, the standard is usually 004"- 005" per inch of bore, dont forget the rings will expand considerably as they heat up, just make sure when fitted the ends of the rings do NOT line up they must be staggered around the piston to stop the flame front shooting directly down the gap. The gudgeon pin hole in the piston will probable be over to one side of the piston which should go to the thrust side of the engine, this will only be a small amount maybe 1-2 mm.
Also if they are used pistons the thrust side will have more wear marks on the piston skirt, than the other side.
The big end caps will have been bolted together with its respective rod then machined/ bored, the machining marks will line up and there should be no step where the two halves clamp together.

 

[normspanners]

piston rings are made to a nominal size and the gaps can only be measured with accuracy in the correct ring gauge.

if big end caps will fit either way, it won't matter which way you install them. I imagine you will be fitting new big end shell bearings anyway.

I am sorry but this is totally WRONG DO NOT TAKE THIS ADVISE!

 

[normspanners]

A ring gauge is only a perfectly circular machined tool with a step in it for the ring to sit against that mimics a new bore dia. it just assists checking and gaping rings to make it easier than fitting the ring in the bore each time. But if used you are only gaping the ring to the actual bore gauge, not the actual cyl.
With a worn bore you are better testing in the actual cyl.
Not wishing to insult but you should not be advising people if you are not familiar with building engines, just by quoting from a manual.

 

[AndyStobbs]

I don't think you'll have any trouble with that end gap. Sometimes the manual and the metal just don't seem to tally up. If your rings are pattern parts then fit ups are often not perfect. Big end caps and rods are best kept together, as norm says. Which way are the numbers orientated?

 

[Welderpaul]

the standard is usually 004"- 005" per inch of bore

The bore is 3.502". So a 'standard' gap would be say .005" x 3.502" = 0.0175" which is 0.4447mm. I measured the ring end gap in a worn cylinder as 0.54mm.
The end gap probably is a bit big, but miles under the 2.5mm or so that the original ones were. Think its ok to use???

I've spent a lot of time measuring up various engine components before putting them back. Wear, suprisingly, is within tolerance in most cases. What makes me wonder about the validity of the upper and lower tolerance margins is when i measure certain shafts and they are larger than the upper tolerance limits - they are original parts so have either been used when they have been too big a diameter or the book is wrong. They have hardly grown in diameter as they have worn!

Thanks for your help so far fellas.

 

[normspanners]

Paul
As stated you would be surprised how much the rings grow when heated when the dia- circumference ratio is 3.142 and the last thing you want is the ends butting, some competition engine use much larger larger gaps in all the tolerances which you would think they would do the opposite but this is to accommodate the extra heat.
Please spend time making SURE the big end caps and main caps are for their respective rods and the correct way round they should have been marked when stripped, other wise you will wreck the new shells.

 

[wristpin]

A ring gauge is only a perfectly circular machined tool with a step in it for the ring to sit against that mimics a new bore dia. it just assists checking and gaping rings to make it easier than fitting the ring in the bore each time. But if used you are only gaping the ring to the actual bore gauge, not the actual cyl.
With a worn bore you are better testing in the actual cyl.
Not wishing to insult but you should not be advising people if you are not familiar with building engines, just by quoting from a manual.

x10!
Good engineering / engine rebuilding practice is to return all mating parts into the same relationship from where they came (!!) whether they be pistons, big end caps, main bearings, push rods, valves or camshaft caps etc.
A piece of wood with nails and one with holes to keep everything in order is as essential and any special tools!

 

[rtbcomp]

Paul
........
Please spend time making SURE the big end caps and main caps are for their respective rods and the correct way round they should have been marked when stripped, other wise you will wreck the new shells.

+1 The big-end bores are machined with the caps bolted up, they must always be fitted the same way round.

From what I've seen on engines the con-rod positions (eg, 1,2,3,4) are stamped on the cap and the rod, and the numbers must line up with each other.

As a matter of interest, which Perkins engine is it?

 

[normspanners]

PS the anti rotational 'tags' on the bearings (old engines used to all have tags) are usually on the same side ie to either side of one B/E bolt.
and if any arrows on the piston crown usually point forward to number one but on some engines they are denoted by somekind of marking on the thrust side of the piston.
splits in piston skirts are usually on the none thrust side.
Piston rings have normally a mark on the top/ uppermost face , and usually HAVE to go in their respective grooves if ridge buster rings they may also have a step on the top outer edge to save hitting the wear ridge.

 

[Welderpaul]

Thanks all, Rog, its a P3.144 engine.

I'll put the rings in as they are. See what happens.

 

[rtbcomp]

I don't know if this applies to that engine, but on the 4203 you can get "Chromard" liners and chromium steel rings, and you shouldn't use the two together, just a thought if you're getting new rings.