Martin

Mr Dunfords question in pointed.

We made the bores on my traction engine which were 80tho barrelled, both round and parallel using a 3 armed lorry honing tool and much time and measiring. Took a set of new stones mind.

3tho should be 10mins work

Martin , I am with the others as regards to rebore and new piston if you want to be sure your rebuild will be ok first time. However if you are like me and do not take your time and effort into account go ahead the most you can loose is the time of doing it all again together with the cost of rebore and new Pistons together with a few new gaskets.
As to your bore measurements over size normally go +2. +4.+6. Try measuring more than one bore and measure top bottom and centre of the bores. Also measure different diameters as worn bores tend to go oval in shape.
Following that assemble your block and piston ( with rings) together with rods to the crank and crankcase well lubricate bearings,big ends and bores and see how it turns. It should turn very freely as there is no compression.

John Mason

Location: Nottinghamshire

Martin

What we dont know is what size the +60 pistons are in reality

Just been reading an old book here.

It says cast ali pistons need 1.5 to 2 tho clearance per inch diameter. As a general rule.

Lets say 2. This suggests 5 tho clearance needed give or take max.

You can easily measure this with feeler gauge. Measure top. Middle bottom.

5 is a nice easy number.

4 is in the range. 3 is too tight.

Courting controversy and with the greatest of respect to the other contributors on this thread I say why not. It appears that Martin is well aware of the limitations when going down this route, and as anyone who has been around Austin sevens for any length of time will know nice loose clearances make for a quick engine. I have on more than one occasion gone down this route on a budget engine rebuild with success. So IF the bores are round and parallel why not, sure the life of the engine will be reduced but as long as you accept this whats the problem. I disagree that it will reduce performance in fact I suggest it may actually improve performance, my quickest race engine ran with quite large clearances, and those who know me will appreciate just how quick that engine was.

Location: NZ

Ruairidh - it's a basic, new 3-arm honing tool. I've run it through just enough to clean off some very light surface rusting.

John, as I mentioned above, this is meant to be a cheap stopgap so I can spend some real money on the RP's original engine. I simply can't afford a full job on two engines.

I've measured all four bores top and bottom, side-to-side and fore-and-aft using a vernier caliper. 13 out of 16 measurements were 2.257", the remaining three, all on the rear cylinder, are 2.260" to 2.261". I've checked for lips and barelling with a back-illuminated steel rule and the bores appear to be straight.

I have standard, +020", +040" and +060" pistons to hand. The +060" are a good fit and slide comfortably with light resistance when fitted with rings.

Given that the valves and seats are clean and in perfect condition, I'm pretty confident that I have a reconditioned nominally +060" block with close to zero miles on it.

I'm not worried about potential lack of compression; it's whether the apparently slightly undersize bores will cause trouble, but Hedd's response probably addresses that.

Location: Herefordshire, with an "E" not a "T".

I would hone it a little more and suggest it will be fine for your stated purpose.

Like Mr Trump, Ruairidh, I'll hire and fire experts until they give me the answer I wanted in the first place!

Location: Herefordshire, with an "E" not a "T".

Like Mr Trump, Martin, you missed out loads of facts when you asked the opening question!

Since I'm not an expert I'm in no danger of being hired or fired. The Austin 7 engine is a simple robust device which is why so many have survived despite being worked on by people like me. I learned everything from the Austin Seven Companion book which is full of advice for the impecunious or mean. Have a go, it's all good fun, and it will probably work.

Location: Isle of Wight

Hi Martin,

Have you bought a honing tool, or a deglaze bar.

A £30 deglaze bar from machine mart. Will not be man enough to hone.

It would probably brake the three grinding stones of there glued basses before you can take 3 thou out. And would probably take six weeks to do it.

A deglaze will help the rings bed in. But if you have 3 thou variation I doubt the bores were done with an experienced hand. So would expect there to be plenty of gaps for the rings to run over.

Although for what you need this won't really matter.

Location: Huncote on the pig

Tony,

It didnt take 6 weeks to take 80 tho out of the bore of my traction engine. That is 8 inches diameter.

I should know, I was paying by the hour.

A vernier is not accurate enough of checking bore diameters and I expect the sizes you have taken may be misleading, you would be safer checking clearances as Hedd suggested

Location: NZ

Hi hedd,

Thought you sed you used a honing tool.

A 3 finger deglaze bar from machine mart is not a honing tool.

It would take weeks to take out 3 thou,

Tony.

Location: Huncote on the pig

So the standard size for a Cylinder bore is 2.2" not 56mm, if its the latter they are probably .050 oversize. I'm sure this is common knowledge in these parts, but just checking...

Location: Fremantle Australia

While it might not stand accounting scrutiny of the time spent, cobbling up an engine from discarded bits can be very satisfying when successful and is in the Seven spirit of old. If things rust through or go bang from other causes the loss is less. And if your own labour not a big deal to repeat.

Vernier callipers tricky; the slightest pressure leads to errors. Difficult to assess cyl state without mikes, and then using separate internal and external mikes can accumulate errors. Bore size and wear can be established with lengths of nail, wire etc fitted to length or some simple telescoping gadget and measured with the same external device as used for pistons. The exact point of maximum wear can take some searching. Measuring change of ring gap gives an indication, although taper will be more than 1/3 the change. If an engine has genuinely parallel bores, seems a pity not to utilise. Mikes now very cheap, esp 2nd hand inch.

There are various reasons bore may not be a standard oversize. Split skirt pistons typically need and have at least .003 clearance across gudgeon and .0015 across faces. The skirt needs only collapse .0015 as prone to do and may enter .003 under bore. The diameter at gudgeon may be greater than skirts. Must not operate with small clearance across and near gudgeon axis. I don’t think Austin ever fitted split pistons and do not know what typical life of split pistons is; solid ones are everlasting. Inspect closely for worn grooves and cracks at the ends of the split. Significant collapse suggests high mileage.

Bores can be accurately expanded with a rigid hone but few have these. It is not good practice but if already parallel with care could expand bores .003 with a ring resurfacer hone, which you apparently have.(Not a ball hone!) These are used after rebores to finish and remove nearly this. A range of stones are available. An elaborate cleaning with soap and water after any honing is recommended.

My limited experience has been that with the original Austin wide rings satisfactory operation and oil consumption is obtainable with considerable wear, but I found low oil consumption elusive with modern narrow ring split pistons even after rebore. Using one piece oil rings.

I don’t know what piston clearance and bore taper is considered the tolerable max for ordinary Sevens. For cars in general .007 taper was often quoted as rebore limit, but now much less. I had an engine with bores .003 barrel shape, pistons generous clearance, and consumption was notably high.

Clearance across gudgeon diameter can be increased by hand. (Some aftermarket solid pistons were round and needed this treatment to match the original Austin ones)

The one oversize cyl is a puzzle. Reminds of a mechanic we knew who used to do rebores on owners premises using a coarse rigid hone. Said he went for afternoon tea, came out, and did a cylinder a second time! Perhaps that is why the block has been set aside.

Location: Auckland, NZ

If you must do it with the 3 stone springy job then do it lightly, fit the pistons and rings and push each piston 1" down the bore. Fill each bore to the brim with paraffin and see how quickly it leaks down.

Call it what you like. It was a 3 finger tool. But a commercial vehicle one, not a car one.

It took a bit of time. But nothing like the time you suggest. Done in situ with a large electric drill drive, but operated by a man.

Thanks Hedd. I've only had the chance to check one bore so far, but on that I can get a 0.0015" feeler in on both sides of the piston all the way down. 0.002" won't fit at any point, so that appears to confirm my suspicion that the bores are a little tight.

I'll have a gentle go at it with the three-armed thing (hone or not?) and will see what happens.

Location: Herefordshire, with an "E" not a "T".

Now there's an interesting bit of confusion to add!

I work in Imperial, so I'd cheerfully assumed that the original bore was 2.2" - it had never occurred to me that usually-quoted 56mm isn't an exact conversion.

So, can anyone confirm with absolute authority that a +060" rebore should finish at 2.260"?

Location: Herefordshire, with an "E" not a "T".

2.200" converts to 55.88mm

56mm converts to 2.2045" only 0.0045" ( 41/2 thou.) oversize from standard.

Ian Mc.

Location: Shropshire

I had always understood it to be 2.2 bore. 3 inch stroke.

This works out to be 747.48CC assuming pie to be 3.1415.

If you do it with 56mm and 76mm you get 748.73CC

That seems to confirm the imperial dimensions.

Martin,It seems a lot of interest in your project and although I agree with some of the opinions as to "doing it correctly" I understand your initial request for an economic rebuild option. There is obviously some confusion as to the measurements of both bore and piston diameters and their compatibility,there is really only one definitive answer and that is to get both measures professionally. I just googled Precision engineers in your locality and there would seem to be quite a selection. I'm sure one or two would measure the dimensions for you at a reasonable cost. Camcraft in Bishops Frome may be a good start. Once you have good compression, do you have a crank and rods good enough to take the load? Trouble is when the ball starts rolling nobody can say where it will stop. Best of luck.

Location: Piddle Valley

Thanks Peter.

I think that the excitement caused by bores and pistons is enough for one decade!

The full horror of what I'm doing with crank and conrods will remain a secret between me and my engineer's blue!

Location: Herefordshire, with an "E" not a "T".