Tips on reinstalling pistons
- Thread starter wesnor
- Start date
AlphaDogChoppers said:
Not any more. back in the days of Hepolite pistons in Triumphs, we used to add a lot of clearance to allow for how much they would grow. On a CB160 motor we set the piston to bore at stock clearances and that's relatively tight.
Tight engines seal better and make more power. Same with 2 strokes. the trick is to know how tight is right.
AlphaDogChoppers
Science is true whether you believe in it or not.
teazer said:
I like to build as tight as the parts will tolerate. I had a cylinder honed for an oversize piston recently, and it was supposed to be 0.0004" clearance. Fucking old machine shop guy honed it to about 0.002" because he thought that was more correct. Fucker will never get any business from me again. The ring gaps were at 0.018" and the service limit for ring gap on that engine is 0.020" with nominal being 0.006"-0.014".
4 tenths is a little tight by any standards. 1.5 though is still tight. His 2 wasn't too far off.
Rings for older TZ race motors are supposed to be about ten thou and new - out of the box they often exceed the wear limit. It's not the end of the world, but it is annoying.
Rings for older TZ race motors are supposed to be about ten thou and new - out of the box they often exceed the wear limit. It's not the end of the world, but it is annoying.
AlphaDogChoppers
Science is true whether you believe in it or not.
Factory spec on this Honda 250 scooter, (water cooled,) was 0.0004"-0.001.6" as the nominal clearance.
It's a low performance water cooled engine. I trust that Honda knows what the clearance is supposed to be.
It's a low performance water cooled engine. I trust that Honda knows what the clearance is supposed to be.
rockcitycafe
I make things.
imo there's no reason to run an engine at .0004", not only is it pretty nearly impossible to hold to less than .0001" in a crush fit cylinder sleeve, at least without some really specialized equipment, and likely a grinding, not cutting operation, but it is not going to aid in performance, sealing happens at the rings, not the skirt.
crazypj
Split personality, I fake being smart
Holding a 0.0001" tolerance is pretty easy if you don't rush. (one then thousandth of inch)
I used to teach people how to do it in a couple of hours.
The 'trick' is knowing what to do if cylinder is tapered or out of round after boring (or even worse, tapered and out of round)
As cylinders heat up while being machined, slight taper is pretty normal
Major problem with real tight piston/cylinder clearance is the possibility of 'cold seizure' where piston expands faster than cylinder (ADC knows it's common with wannabee's on HD's, look Ma, revving it to warm up faster)
I run my 378 motors with 0.0016~0.0017" clearance.
I used to teach people how to do it in a couple of hours.
The 'trick' is knowing what to do if cylinder is tapered or out of round after boring (or even worse, tapered and out of round)
As cylinders heat up while being machined, slight taper is pretty normal
Major problem with real tight piston/cylinder clearance is the possibility of 'cold seizure' where piston expands faster than cylinder (ADC knows it's common with wannabee's on HD's, look Ma, revving it to warm up faster)
I run my 378 motors with 0.0016~0.0017" clearance.
rockcitycafe
I make things.
crazypj said:
giving a machinist a clearance of .0004" would usually be interpreted as less than +-.00005" deviation though... that's the kind of thing where you put your hand on the block for 10 seconds and watch the dial move
AlphaDogChoppers
Science is true whether you believe in it or not.
This particular engine is not a sleeved cylinder. It's one-piece cast iron, water cooled.
crazypj
Split personality, I fake being smart
rockcitycafe said:
I've had 'machinist's' laugh when I told them what the were going to do, then be totally shocked when some green kid actually does it by following instructions.
Standard clearance is checked at 68 deg F, but, as you point out. it varies with temp
The top of the cylinder does heat up first and expand more, that's were the heat is produced
Honing to size at 210 C gives some major taper when cylinder is 'cold', water cooled blocks are much easier to keep at temp as you can pump water/oil through them
After doing a couple of blocks, I have a fair idea of a 'standard' taper for hot block, air cooled is more than water cooled as temp variation top/bottom is higher
It's still something of a guess as you can't accurately measure temp inside cyl so outside temps have to do
It's he reason cylinder fins are larger at top than bottom and spigot is down inside crankcase, trying to minimise thermal variation
Cant give away 'ALL' my secrets ;D
