Piston Ring gaps?

[molten]

Hello

Just doing a bit of pre-assembly engine checks. I have put the No1 piston ring (new rings & pistons) an inch into the cylinders with depth guage and used feeler gauges to measure gaps.
Although all measurements are within spec (.015 - .055) the range is btween .019 - .025.

What i am trying to do is acheive a gap close to smallest tolerance on each ring, by swapping top rings to suit cylinders.

Is top ring same spec as 2nd ring? If it is I can look for optimum ring gap on each ring in each cylinder. But would this small variation described above make any real difference anyway?

Thanks
Scott

[Nick Jones]

Whether top and second rings are the same depends on the ring maker.  They were on the County 2.5 pistons I've recently done - Grant rings IIRC.

I tried one ring in all 6 bores which showed that there was very little difference between the bores, then checked all the rings in no.1 (could of been any of them, 1 was nearest), which showed that the rings were all pretty similar too and about mid tolerance apart from one oddball one that wouldn't even go in the bore until filed a bit - which just goes to show it's wise to check them all!

If you are determined to get them all spot on minimum clearance then you'll need to get a set for the next overbore up and file them down.
Waste of effort IMO.

Nick

[molten]

cheers Nick

This is only a road going car, so I donth think I will see any noticable benefits by trying to get as close to bottom tolerance as I can. So I happen to agree. I just wanted to see what others thought.

It would seem that all the rings may well just go in randomly as these too seem to be around mid tolerance.

Cheers

Scott

[drofgum]

Scott,
It is not worth the effort trying to get the ring gaps at the minimum. Very little leakage passes through the gap. The minimum gap is chosen to prevent the ends of the ring from butting under any forseeable conditions. If somehow the ends do butt the damage won't be minor. If you selectively fit it is probably better to aim for even gaps rather than smaller gaps.
                                                                                    All the best,
                                                                                    Paul

[JensH]

I did a rebuild on my mk 3 spit engine last year, with county pistons +20. Quality was good as for pistons and rings; same weight (less than +/- 0,5grams) and same ring gaps.

Your time is better used in balancing all conrods etc.

[smithy]

Quoted from drofgum

Scott,
.... If somehow the ends do butt the damage won't be minor.

Amen, Paul. I once had a tiny section from a piston ring shear travel right along the crankshaft - the damage was major.

[sparky_spit]

Rings supplied with AE pistons have different rings for top and middle.  The middle rings have a taper to aid oil scraping and the top side of this ring is identified by the printed marking.

[jaybee]

It's common for replacement top rings to be 'barrel' shaped, i.e. not square, but with a convex outer contact surface. The reason is to prevent the ring 'hanging up' or catching on the cylinder wall when fitting new rings to a worn cylinder (i.e. not rebored, but still within usable limits). In this case the top rings will be clearly marked, even though they are easily distinguishable from the 2nd ring.
Ring gap isn't critical, so long as the gap isn't smaller than the recommended minimum. You must square up the ring in the bore when measuring end-gap, or the measurement won't be accurate. To ensure the ring is square, fit the ring to the top of the bore and use a piston with no rings fitted to push the ring down the bore.
Also check the ring side clearance in the groove of the piston - it's equally important that this is correct.
When fitting the rings to the piston, position them on the piston so that the gaps are 120 degrees apart. Don't fit them with the gaps all in a line on one side of the piston.
Finally, use a glaze-buster on the bores before rebuilding. This slightly roughens the surface (removes the worn glazed finish) and allows the new rings to bed in correctly. Failure to do this usually results in poor compression and high oil usage.
As a footnote, when your engine is up and running, don't mollycuddle it! Driving at very low RPM and light throttle is not good for running in a new engine. Use it reasonably freely, but not at excessive RPM. This will allow new components to bed in reasonably quickly and give a longer engine life. Change oil AND filter at around 300 miles, reset carbs and ignition, and you can use it more-or-less as normal, but resist prolonged full-throttle and high-RPM use for around a thousand miles.

[Brit car fan]

Before I got my block rebored from standard to +20, I checked the compression ring gaps in the unworn area at the top of the bore - even with negligible peripheral wear the gaps ranged from 38 to 40+ thou. The engine had suffered from blow-by pressurising the crank case. So IMO be conscious of making the gaps too big. The bores weren't bad except for a step at the bottom of No 4 indicating a twisted rod. If the ring gap data is not available from the manufacturers, the Leyland manual gives 12 to 22 thou, and rule of thumb gives 4 thou for every inch of bore, so 12 thou in the case of a Spitfire 1500, which I would treat as a minimum. My engine refurbisher suggested about 16 thou which seems about right. The N.O.S. TRW pistons I've bought come with rings gapped at 10 thou which seems a little tight. In the absence of any manufacturer's ring data I'm opening these up to 16 thou.
It is imperative to get the ends clean and square - I would not recommend hand filing.

As a point of interest:
I've made an attachment for my lathe for precision gapping of piston rings. The angle of the cut is controlled by the compound slide and the actual cut is taken on the rotary swivel of the compound slide so that the end angle is always in line with the centre of the ring. I always keep one end untouched for reference. The dial test indicator shows exactly how much has been removed. The grinding wheel is mounted on an arbor between centres. It's very accurate, currently increasing gaps from 10 thou (as supplied) to 16 thou.
http://i1057.photobucket.com/albums/t386/Vulp53/P2052850_zpsuwqsw96u.jpg
http://i1057.photobucket.com/albums/t386/Vulp53/P2052848_zpsgnyx8ay6.jpg

[Brit car fan]

Sorry all, the two links above appear to be dead so have included them again





Dave

[RedRooster]

Quoted from drofgum

Scott,
The minimum gap is chosen to prevent the ends of the ring from butting under any forseeable conditions. If somehow the ends do butt the damage won't be minor.
                                                                                    Paul

What will happen is as the ring expands the ends will touch & with no where else to expand too they will buckle up & fracture your piston, bits will probably fall off wreck your bore & who knows what else in the engine.
Common problem with NOS engines that haven't been built properly as the rings run hotter.
RR