Binding pistons

[namloop]

Hi to all
I am rebuilding a longstroke and have the bottom end and crankcases together.
Con rods with new bushes are installed and all rotates without issue.
New bearing and bushes throughout.
Bore is +.020 and new pistons and rings fitted.

Pistons with rings were pressed into bore and each moved without binding within the bore.

I then lifted the barrel with the pistons over the con rods and slipped the gudgeon pins and retaining clips in.

Rotating the crank, the barrel lowered itself to the studs that were in the crankcase halves - one or two needed a tap to align to the base of the barrel and I finger tightened the nuts.

The barrel is aligned flat with the base but now I find the engine has "seized" and the pistons will not move up or down with the cylinders.

Any thoughts where the issue may lie?

Any advice appreciated.

Regards
Stephen
Gold Coast Australia

[Brian]

Hello Stephen,
                   did you check the end gaps on the rings, they all need about .010".

[namloop]

Will do, once I can get the barrel back off.
They were NOS rings and Pistons from Draganfly and both stamped as such.

Stephen

[Brian]

Its quite common for rings to be supplied that are the incorrect ones.

When you get it apart tip the barrels upside down and use the piston to push each ring into the bore about  a inch or so and then measure the gaps with a feeler gauge.

[iansoady]

Yes, but he says the pistons moved in the bore before fitting the barrels.

Has it been rebored? The bores may be off-centre or not parallel.

You could try taking one piston out at a time and see if you still have the problem with just the other fitted.

[BSA_54A10]

If they are jambing at the bottom of the stroke then the pistons are too long or they are not rotating around the little end enough.

If they are jambing near the top then as previously mentioned have a close look at the bore.

Partially lift the barrels off the pistons ans run some bearing blue around the bottom of the skirts.
Run the crank back wards & forewards a few times to the tight spot then remove the barels.
With a bit of luck the smears of blue on the barrels will give you a good idea of what is going wrong.
An old trick is to put a length of dowel through both little ends at TDC about 1 to 2 foot long.
This will verify that the crank is running true

[unclerob]

Hi Stephen, although I haven't worked on a bsa engine for many years I do rebuild other engines as my day job and a couple of thoughts occur to me....from what you've written I think you fitted the gudgeon pins and circlips with the pistons already partially in the bores....did they slide in the rest of the way alright after you'd fitted them? My other thought is when you say seized...how hard have you tried to turn the crank? I may well be wrong here and happy to be corrected by those who know more but I wouldn't expect at first to be able to turn the crank by hand on a newly rebuild engine....? Cheers, Rob

[warmshed]

Did you lubricate the bore? I know there is a following of assembling the pistons in the bore whilst dry to speed running in.  As yours are stuck then a little thin oil in the bore can only help. You will need to undo the cylinder studs and try to raise the barrel to the position where they had previously been free to move.

Circlip pinging out and jambing the piston is areal horror. but normally it would not happen until a few turns at least, I hope that this has not happened on you engine, it means new piston at the very least!

The barrel being bored out of line, well anything is possible but hopefully unlikely.

If all fails then a piece of wood on top of the pistons, light taps and see if you can get them to move, though hold the barrel so you direct the force through the piston/barrel not the rods. An extra pair of hands helps.
Try ice packs on the piston crowns to reduce the temperature of the pistons, As its hot where you are the pistons could well expand and bind in the heat if the clearance is not right.

Good luck and report back once you get them out.   You will then need to check end gaps of the rings and piston/bore clearance before you can proceed and find what's wrong.   Dave

[namloop]

Progress report.
Barrels are off. No damage to bore - yes I did use oil liberally.
Next I will remove rings from pistons and check their clearance per Brian's instructions.
Many thanks to all for your insight to date.

Stephen

[warmshed]

Don't forget the piston skirt clearance measurements.

[namloop]

Warmshed - thanks for the response.
Can you remind me of this process and the tolerance?

Stephen

[muskrat]

G'day Step, you will need to take the pistons off the rods. Turn the barrels upside down, put some rag in the bores so when you insert the pistons into the bores the bottom of the piston skirt is level with the bottom of the barrel skirt. With your fingers clamp the front of the skirts together and use feelers to measure the clearance at the back of the skirt. A tip is the feelers will insert easier at the side (gudgeon pin side) and can be slid around to the back. Clearance depends on the piston. Split skirt type can go as low as 1 thou, std Heps 3 thou, forged 5+ thou. Rings are 12-15 thou.
Cheers

[namloop]

I have found the problem.
There are two bolts in the upper half of the crankcase that hold the halves together,
Evidentally the bolt I used was too long and as the barrels were knocked down, the end of the bolt has dimpled the casting causing a dent into the line. Picture is attached.
It radiates about 1/2inch from base and is no higher than the semi-circle cutout at the base of the cyclinder.
Once piston rings pass this point the piston moves freely.
I did also check the ring gap and they were 10mm so I am thinking as the pistons were in the base area so I could connect the rods, they were basically 'pinned' by the pressure of the bolt on the cylinder.
I have changed the crankcase bolts and will await any replies to my post here before re-assembly.
Best regards
Stephen

[muskrat]

Mmmmm, it's put a decent crack in the barrel. They have been re-sleeved so you have two thin bits and you know what happens after a crack starts, sooner or later it grows.
 I don't know much about the long strokes but wonder if that part of the barrel skirt could be ground off to stop any growth of the crack.
That must have been a typo 10mm ring gap  .
Cheers

[namloop]

Correction 10thou - you can see the sun through 10mm!
Will take your advice about the crack under consideration.
Stephen