Piston Ring Gap

[Colin Wilks]

I am putting new split skirt pistons into my RP. 

The pistons I had inherited with the car have been condemned by Nick Turley, so they're going. The bores show very little wear, a maximum of .002" above the standard 2.2", but for other reasons I am having the block bored +.010". 

On checking the old rings, the compression ring gap is .023" and the oil ring gap is .030", which are way more than the .006" recommended.

My question is, just how critical is the ring gap, and would these excessive gaps explain the oiling plugs I have been blighted by?

[Alan]

It would indicate that either the rings are the wrong size for the bores or that there is considerable wear. Yes it is kind of critical. If you haven’t already put things in motion I’d be thinking about slipper pistons from our usual suppliers. These are in metric sizes so the rebore dimension is different but I have these on my Chummy and Saloon. I don’t have to put any oil in beteeen changes.

[Zetomagneto]

Yes, Slipper Pistons for me too, and no baffles.

[Steve Jones]

Slipper pistons every time Colin. Bore the block to the size marked on the pistons exactly (+/- 0.00) and the rings they come with are gapped to suit as supplied. Run in carefully and no oil burnt.

Steve

[Andy Bennett]

On that subject, if you have bores which are 2.234 to 2.237 (56.8mm) diameter and fundamentally good, no marks, grooves or major steps etc, and you are looking at a balance between cleaning the bore and leaving something behind for the next person to have some material left, rather than to bore out to go racing, would you go 57.25 or 57.5 piston? 57.25 feels like it would be enough and might I guess reduce risk of going through the wall?
Cheers
Andy

[Chris KC]

Ring gap should be about 6 - 8 thou measured in the bottom of the bore with the ring level. I'd suggest 0.020 is about the limit for wear & this may be a factor in your oiling issue, but there may also be other factors e.g. what about the vertical gap between rings and grooves (off the cuff I'd suggest 2 thou new, 6-8 thou worn)? Are the rings the 'oriented' type and if so are they fitted the right way up? Have you looked at your valve guides? Are you using oil baffles? Come to that, are you using the right plugs? etc.

Personally I've used split skirt pistons all my life and never had the slightest concern with them in sports / touring use, therefore not sure why others recommend slipper type; though I have no reason to doubt they are also OK. Split skirt pistons need about 0.0015 clearance to bore (again, measured at the bottom of the bore) vs 0.004 or so for solid skirts. Wear limit perhaps 6 - 10 thou (bottom - top).

For what it's worth I would never re-bore above the next available step that yields a workable bore. Without delving into your reasons Colin I would regard 0.002 oversize as a useable bore, assuming the other criteria are met.

Others of course are free to hold conflicting opinions or to correct mine if they feel I have something wrong!

[Ruairidh Dunford]

I have used split skirt pistons on hundreds and hundreds of standard engines over the last 30 years without any issues.

I have also fitted around twenty sets of slipper pistons to standard engines without any issues either

Both work equally well (in my experience) and, if run-in correctly, will burn under a pint of oil in 2000 miles.

All is good.

[Colin Wilks]

Morning all, I'd better give the background to this.

The engine had been commercially rebuilt for the previous owner about ten years ago, but he had done a tiny annual mileage after that, so the car was not properly run in and I suspected had glazed bores. Excessive oil consumption and a smokey exhaust were disappointing but not the end of the world, and I hoped that putting in some mileage and giving it some proper use might improve things, but then oil started gushing from the square headed camshaft bush pin, so action was required.

Compression was reasonable and consistent across the cylinders at around 95psi and I had gone to a slightly hotter B5HS plug.

Initially I just raised the block to get the pin out so as to thread the front bush and make good. I had the camshaft refreshed by Paul Bonewell while it was out. I then decided to lift the block in order to deglaze the bores and this exposed the pistons and con rods. I had intended not disturbing the big ends on the basis it is generally best to leave things alone if they're not broke. Number one big end felt sloppier than I liked and number two was tight, so I was grateful when Nick Turley called in to have a look and advise. Nick reckons the rings are in the wrong way up and that number one big end is not ideal, but OK. Number two has been removed so I can blue the journal and scrape the tight spots. The white metal has burnished and dragged on one side, so I think I am lucky not to have run that big end.

The block is an early one which has been relined and the standard bores were showing very little wear, a maximum of .002" above the standard 2.2" measuring each cylinder at top middle and bottom and both north/south and east/west. Unfortunately there are no standard pistons available just now other than the rather pricey JP jobs, so I decided to buy a set of the well regarded split skirt pistons at + .010" and spend the change having the block bored to suit.

The builder from ten years ago appears to have been given a box of bits and asked to build an engine with minimal expenditure. The reuse of mismatched second hand pistons, old big end bolts with flimsy split pins waving in the wind, an old and virtually threadless camshaft bush pin and a less than pristine camshaft bear witness to this. The RP is intended as a daily driver so split skirt pistons that have reputedly given 65,000 miles without incident will do for me.

I shall gap the rings to a minimum .006" and run the engine in like my Dad did, by not letting it idle or labour and getting in plenty of miles on decent length runs.

I am very much in the early stages of learning about living with a Seven and enjoying gaining experience, picking up the spanners again after a gap of nearly fifty years. I am very conscious of the old saying that "an engineer can do for five bob what any fool can do for a quid" and whilst I hope I'm not a complete fool, I am certainly not an engineer. Learning about what's OK, what can be fixed and what needs replacing is a big part of the joy of Seven ownership for me. But what I really want to do of course is drive the ruddy thing!

[Tony Betts]

sorry about the shameless ad.

but otherwise many people dont realise what we are doing.

as 7 county austins, we have now extended the range of sizes available on the slipper pistons.

in stock.

56.00mm
56.25mm
56.50mm
56.75mm
57.00mm
57.25mm
57.50mm
57.75mm
58.00mm

again sorry for the shameless ad, but this is the most complete range of sizes for the slipper pistons. since dave flake started them near 20 years ago.

thanks tony

[Ruairidh Dunford]

Excellent news Tony!