McDonald Imperial Super-Diesel

At last I got back to this project this afternoon. I have borrow a large honer and it certainly has cleaned up the rusty surface very well but after a fairly long time I still cannot get the piston with no rings to slide up the bore. The top goes in with no problems but the further I slide it the tighter it gets and will only slide in as far as the bottom of the piston skirt.
This got me thinking, yep hard to believe, that maybe the piston is out of shape and not the bore as I was thinking. I had the piston off the conrod and put it in the lathe and using a dial indicator got some out of round numbers on the OD of the piston but from top to bottom it changed a lot and was messing me up.
So I engaged the grey matter again and used my digit calipers to measure the OD as far as it could reach over the skirt measuring from the side. and did this at 3 different points, top, middle and bottom.
Not sure if my calipers reached over the complete OD of the piston but with the calipers pushed all the way on then closed the gap and locked the calipers it gave the following readings.
Top - 87.77mm
Middle - 88.21mm
Bottom - 88.40mm
This meant that the piston got wider the further down I measured it and by a total of 0.63mm and this to me would indicate that the piston and not the bore is at fault.
Happy to hear any and all comments on this tricky bastard.

Is the bore round from top to bottom with the internal calipers I see in the photo

Is the bore the same size top to bottom with the calipers

I would try to nail down one thing and prove it is correct otherwise it's like a dog chasing his tail

Paul

Standard bore size is 3.5". Give me a sec......88.9. I see wear in the bore, so the piston is not at fault. I wonder if you try the piston from the bottom (where it has to go in anyway), does it jam up at the same spot?\
A quick and dirty inside micrometer (more of a go-no go gauge) can be made from any sufficiently stout bit of wire with the ends ground to a dull point to measure the bore at any point. You could make it fit the front of the bore above the lip. This should be 3.5" if it hasn't been ridge reamed in the past. With this stick you can check your perceived tight spot.

Jarrod.

Jarrod.

The 88.4mm is the best measurment I can get using my callipers over the OD of the piston at the bottom and not the bore. I will make a wire gauge tomorrow and see WTF is going on.
At the moment I have no way of measuring the internals of the bore other than at the top of the bore at the lip and there should be zero wear there as all the issues are down a lot further.

Spend a few dollars on thsese.

www.ebay.com.au/i/183940177635?chn=ps&no...cn5dregaApdOEALw_wcB

Very useful.

You'll be surprised how often you use them.

If fitting new rings ,the ridge at the top needs to be removed,or the rings may hit it and break ....Use plenty of kerosine on the hone ,keep the bore well wet too,and work for a cross hatch pattern........piston seems to be tapered top to bottom ,which is correct ,as the top is hotter and expands more.

Thank you to everyone for the advise. There is no ridge at the top, rather what you do see is a low spot possibly from the piston knocking the bore in that area, it doesn't go all the way around.

I have been fortunate that a mate read this last night and he has kindly loaned me a set of internal gauges. The following are the measurements and some pics.

Top of bore - 3.500" exactly what it should be.
Down near the bottom of the bore - 3.4935" so this is undersize from std.
Top of piston - 3.4805"
Bottom skirt of piston - 3.4865" which is 0.007" which is 0.007" clearance from the bottom of the bore.

Just thinking about these measurements they were taken on the piston from one side only so will go out and check for other numbers next and do what was suggested, stick the piston back in until is sticks and shine a light from behind.

I have no idea of what amount of clearance is required between piston and bore, any clues?

Cast iron pistons dont need much clearance,maybe 003 for a 3 1/2 bore would be plenty.......Id just hone the bore until you get something reasonable ,surface wise ,without enlarging it ............the only safe way of testing clearances is with a long feeler strip between piston and bore ......this eliminates measurements........Feeler strip should have a bit of drag to pull from between piston and bore (no rings)...Seeing as it was running,I would expect everything to be sized OK ,no need to increase beyond cleaning rust out.

Hi John,
Yes it did run a long time back but the 2 engines arrived with only one piston in a bucket of bits so I dont know which engine it came from. These engines sat for a long time with a heap of rust and dirt in the water hopper around the cylinder and all the time putting pressure against the outside of the bore hence my thought is has been squashed in.
I did another trick today and that was put the piston in until it got firm and then put a bright light behind the piston and then look down the bore from the top. Visually I got a good clear gap at the bottom and none on the sides or top. Got the same result when I slid the piston half out of the bottom of the bore.

The extended break from working on the bugga has helped in keeping me calm so I just have to keep trying different approaches on this issue. I even thought about taking it off the transporter and running it into town to the engineering shop and get him to run his boring machine down its guts. If I did this I may have to remove those 4 head studs and that would be another bloody challenge.

Very difficult to bore out a few thou.......much easier to hone out......although the spring hone shown is just a glaze breaker ,and you really need a solid adjustable hone ......I have hones ,but nothing to hone that small ......mine is a battle to get it in a IH281 ,which is 3- 13/16 ,