I thought I had checked these parts two years ago - but it seems that I didn't....
My +20" piston has been in the wars. But the barrel hasn't. As the piston scoring is on the skirt, my feeling is that I can ignore it, on the basis that the rings are doing the sealing work. Or is that faulty logic?
The piston skirt clearance front to back in the bottom of the bore is about .006". This is greater than Rupert recommends on a fresh rebore tolerance. The question is, given the unmarked state of the bore, it this extra clearance a game stopper?
Next question relates to the rings. The engine hasn't been run since new rings were apparently fitted in 2016. The compression ring end gaps are .026" - so close to Rupert's .030" maximum. Which seems odd. And the oil ring end gap is the best part of 1/8". Is that normal?
My inclination is to get a new set of +20 rings to see if the end gaps improve, and if so, bolt it together.
BSA service sheet 701 will give you exactly the clearances you desire.
piston skirt to bore is max .0055 --- are you inserting the feeler blade into and then wrapping round the piston sideways in the bore - not straight up into the gap between the skirt and bore.
the ring end gaps are anywhere between .009" and .013" max
the piston could do with some fine 1200 cloth around it , there are some deep scores there.
the dilemma is to go straight to a rebore or try some known quantity new +.020" rings( how much are they?? £35??) down the bore in 3 or 4 places up and down the bore (use a naked piston to push the ring down squarely in the bore and then measure the end gap) and gain an average of your readings to see if their end gaps are within or without tolerance. if you can get readings of .009 and .013 then you are lucky , tidy up the piston and away you go.
but if your average up and down gaps are outside then it is rebore time --- but you say that these are new rings 2 years ago, so maybe you have just proved to yourself that the bore is outside max end gaps
Clean the piston, definitely new rings and give the bore a light hone so it holds a bit more oil.
Any wear ridge at the top of the bore?
Quote from: rhyatt on 17 Dec, 2025, 15:46 are you inserting the feeler blade into and then wrapping round the piston sideways in the bore - not straight up into the gap between the skirt and bore.
I've got the piston near the bottom of the bore, and am pushing the feeler gauge straight up the front face.
I'm not really understanding your description of an alternative measuring method. Does it require loose feeler gauge blades? Or do you mean I should go at it at a jaunty (e.g. 45 degree) angle?
There isn't the slightest wear ridge at the top of the bore. (There is a change in colour - but no ridge.) There is also signs that some wet and dry 'honing' was done back in 2016.
I'm not convinced about polishing the scores out of the piston. How will scores in the piston influence things? Obviously scores in the barrel will mean oil can get past the rings. But I can't see a problem with scores in the piston. To my way of thinking, the scores in the aluminium piston aren't going to scratch the steel bores. (I'm loathe to do anything that makes the piston a looser fit in the bore.)
ok - measuring the piston to bore
my feelers gauges are about 1/2 " wide give or take
with the top of the cylinder on the bench and the bottom of the bore looking up at you - stuff some rag into the bore , enough to hold the piston at the bottom(to stop it descending into the bore to give you 2 free hands - with the piston in place at the bottom of the bore , use 1 finger to push the piston to the front of the bore and slide your feeler into the back - slide the feeler blade as near to horizontal around the piston and slide it around the piston into the piston/bore gap and see how thick a gauge you can get in until it wont allow entry.
reason - with a 1/2" wide feeler blade, the blade going straight up into the gap will not bend/curve across its 1/2 " width and you will get a false piston gap reading
polishing the piston - always nice to clean things up and get things tidy - yes the rings do the sealing of the compression and oil but any polishing is not going to make that much difference frankly , but don't do this until you have established whether or not is needs a rebore.
the honing / deglazing mentioned above is beneficial if you just need new rings to help them settle into each other , but a rebore will do that anyway assuming you need one
Quote from: rhyatt on 17 Dec, 2025, 15:46 but you say that these are new rings 2 years ago, so maybe you have just proved to yourself that the bore is outside max end gaps
There were lots of receipts with the 'unfinished project' when I got it - including a receipt for new piston rings. But I have a growing suspicion he might have had two C15 projects on the go... I have learnt over the last two years to trust nothing that was apparently done before. So I would not be surprised to find that the 2016 owner took a file to the ends of brand new piston rings and opened the gaps up way beyond what was needed. I have no explanation for the massive gap in the oil ring - other than over-enthusiastic filing.
in that case, buy a new set of +020" rings then you have a known quantity of size when it comes to measuring the end gaps - they are about £35 i think
looking at the picture in your post 4 , looks like x hatching deglazing has been done sometime
the rings that you are measuring in your pictures - are they the rings on the piston or are they your "supposedly" new rings??
BSA service sheet gives more piston to bore details --- .002 to .003" at the skirt
The ring in the picture is the 'supposedly new' 2016 oil ring - with nearly 1/8" gap. I took it off the piston.
I've ordered a set of new +20 rings. Hopefully that will give me some solid ground.
I'll try your way of measuring the piston gap tomorrow.
The data from the service sheet appears to me to be manufacturing / build tolerances, rather than service limits. Ah ha! I've just looked in my A50 workshop manual. It has the table of specifications / tolerances at the front. But in the main body, engine section, cylinder bores, it states '...the amount of wear can therefore be considered as the difference between the base measurement and the point of maximum dimension shown by the dial. If wear exceeds .005 in. (.127mm) at the top (rotation) then a rebore with new piston is indicated.' I'll see what the new rings show, and if there is still doubt I'll take a trip to my local engineer to see what his dial gauge shows.
I bet there is a mathematical way of measuring the difference in the ring gap between the ring at the top and then the bottom, to work out if the diameter cnges more than .005 in., but I'll think about that tomorrow.
Thanks to you both for input! (I like to throw ideas around before committing to a course of action...)
According to my maths .127mm of wear (diameter) would produce a 0.4mm difference in the ring gap (circumference). So I compared the ring gap at the bottom of the bore with near the top, and it was .15mm difference - so loads of life left in it. I also did some careful measurements of the top and bottom fore and aft diameter with my digital calipers, which came up with a .08mm difference. Which was again less than the .127mm limit recommendation. And as previously stated, there is no wear ridge. So whilst accepting the limitations of the measuring systems used, I have convinced myself that the piston and bore have useful life left in them.
As long as you have ring end gaps in 3 or 4 places from the top down to the bottom of the bore somewhere between .009 and 013" , you're fine.
Using the piston to push a ring down the bore , starting from roughly 1/2" down from the top of the bore , measure end gap there and then push the ring down with the piston and inch or two and measure end gap again and then down again and measure.
If the 3 readings are close , then the bore is not barrel shaped or worn abnormally and , in your case , must not exceed .013" end gap
As a very rough guide end gaps should be about .003" for every inch bore diameter.
So a 3 inch bore should give you .009" end gap.
Are you sure the rings are the +20 rings and not standard size..? Using piston rings is not the best method of measuring the bore size. The best method is to use an internal micrometer, and to also use a micrometer for the piston. That way you can be sure that the rebore was to +20 and not to +30 thou.
As there is no wear groove at the top of the cylinder, you have done less than 2000 miles on it, So you have a further 50,000 miles to go.