Piston Sizing

[Hedd_Jones]

I have an interesting conundrum.

Father left his RP to my daughter when he passed. Iv'e been driving it round since as a family car for the last 4 or 5 years but I knew from day 1 it is sick. 

At some point after putting a phoenix crank in it, (after breaking the Austin crank), he ran it with no coolant, it got hot and nipped (siezed) briefly. He ran it for a year or so afterwards. Then it came here. 

It starts badly from cold, spits oil passed the spark plug washers and fouls the plugs. It did leak oil into the coolant until I changed heads, but this problem has now reoccured. But it goes like stink and never misses a beat.

I've had the head off and the bores look unmarked and not particularly glazed. My conclusion is the rings have lost their tension through the overheating/siezing. I did fancy retempering them for a giggle (low cost) for a minuite, however...

It has recently developed a fair leak between the block and case, and if this block is coming off, its not going back on!. Its a 1927 screw in core plug job which has suffered 2 broken cranks to my knowledge, and is cracked in the centre stud like most.

Father must have known the top end was not good as amongst the 3 or 4 ton of spares was a fully overhauled block. Bored. Studded etc etc. All greased up ready to build up.  Plan will be to use this on the existing bottom end which seems 100%.

My problem is that I do not know what pistons were intended for this block 

I am currently in isolation as daughter has caught the plague, but once I have freedom I shall get my mate Ken with his internal mic to confirm bore specs. 

Is there any reference material anywhere which correlates bore to piston type/supplier?. Clearly I would like to source the most suitable. A days or so of work and I should have a good engine again.

[JonE]

excellent question. You could start by putting all the metric Slipper type measurements from David C or Tony Betts' websites (aren't they bore as marked?) into imperial form and populating a spreadsheet/(list on here) with their operating tolerances, to add/fill in the others.
Presumably you may come up with a couple of options, and the onus will be on something where you don't want to remove or hone away any more material.

[Chris KC]

Hedd, there are others here doubtless better qualified than I to answer, but the rules of thumb I have always followed are that piston to bore clearance on new parts should be 1.5 thou for split skirts and 4 thou for plain skirts; measured with a feeler gauge at the bottom of the bore. 
I have a note of 6 thou here as the notional scrap limit (probably refers to split skirt types), and 10 thou when measured 3/4" from the top of the bore.
You would probably need an actual piston rather than specifications to check the above. 
Only a partial reply I grant you but possibly of some help.

As a parting thought, it takes a long time to get an engine running quite so sweetly again, and most of them leak or have the cracks you describe... If it's starting to become troublesome or unreliable then yes, time to do something.

[Tony Betts]

hi hedd,

if metric bores should be more accurate.

if done in inches, your problem will be its more down to he who bores it? for the allowance of expansion. rather than the type of piston used.

tony.

[Hedd_Jones]

Tony

Knowing who will have likely bored it, It'll be imperial.

I'll clearly have to mic it, and once I know that made a decision as to which way to go.

Not having done this sort of thing before (in my other hobby where cylinders are irreplaceable we take minimum bore to clean, then make the pistons, rings etc to suit) Just been measuring various unused piston sets in stock. Via vernier not mic. And have therefore believe I have learnt that pistons are undersize vs nominal bore plus the oversize. Some +40 Heptolites are 'broadly' 2.225 . Some +30 Wellworthy 'broadly' 2.218. Incidentally I did a course check on he vernier my measuring the bore of the GE cup engine (standard pistons) and the vernier is in the right ballpark.  So Bore 'should' be 2.2 + whatever oversize. ID?. We shall see.

Chris

Starting the car is troublesome, better now with a recent battery, but when the old one went off the boil is was a worry as it wont go on the handle. 

Basically the compression is well down, and I was always taught oil leaking passed the spark plugs was bad news on an Austin. I've also had to go up a heat range on the plugs to keep them from fouling to quick, which makes the starting worse. Its a H/C head.

Sooner or later the oil in the water will become water in the oil, and I don't want it buggering up what is a pretty fresh bottom end.

I have identified the oil leak that I have been chasing is block to case, so its got to come off. The way I see it the block is either glazed, or the rings have lost their spring (or both). There is a flatness issue between the block and head somewhere, possibly also block to case. I know this block/case combinationhas suffered 2 broken cranks, so who knows what I might find when I take the block off. I have a block to do a transplant, I have plenty of cases to do the bottom end if need be. But this block is not going back on if its coming off.

Car is booked in to Audlem at the end of the month so it will be going there with wife at the helm. At some point beyond that its coming to bits.

[Chris KC]

Fair enough Hedd, you reach a point where it is simply knackered. I have long since given up trying to stop leaks between block and crankcase though.

[John Cornforth]

Hi Hedd

"oil leaking passed the spark plugs" - I may be missing something, but aren't spark plugs and their (crushable) washers supposed to be a gas tight seal ? The only time I have had trouble in this area was when the mating surface in the bottom of the spark plug well wasn't quite clean/flat enough.

[Howard Wright]

Hi All

I’ve had oil in the spark plug well recently on my RK.  Solved by fitting new crush washers on the plugs. Green spark plug sell them for pennies!

Cheers

Howard

[Hedd_Jones]

They shouldnt leak, your quite right, but they often do.

Remember that excessive oil above the piston rings usually means something is wrong. Well, on an IC engine it does.

Coupled with crusty and fouled plugs....it means.......

[Bob Culver]

The nominal size is usually that of the bore. The piston is undesize to give appropriate clearance. Used fully split skirt peistons often collapsed to give much the same clearance as solid ones.