engine rebuild- piston to wall clearance??
01-28-2010 | 09:17 PM
#1
Thread Starter
Senior Member
Superstock
Joined: Mar 2009
Posts: 277
From: south nj (philly area)
engine rebuild- piston to wall clearance??
Honda manual says .0008" to .0025" for piston to wall clearance?? My bike was smoking a tiny bit under acceleration and the measured piston to wall right now is .0025" My experience, at least with automotive forged pistons, says any tighter than that is a little risky. Any thoughts?
01-29-2010 | 07:33 AM
#2
COBB Member
Superstock
Joined: Jul 2009
Posts: 292
From: NorthEast Connecticut
I like to set mine at 0.0015 its middle of the road so to speak, but I know people who race like to st 'em up on the loose side to avoid catastrophic failure under race conditions, but, some folks figure in spec is in ya know what I mean? You got it torn down anyways right? Are you doing the engine work yourself or farming it out? Depending on the condition of the juggs, A quick refresh on the bores and new rings may be all it takes. Depending also on the cause of the smoking, rings usally handle the compression duties, valve seals/guides the oiling duties, also what kind of smoke was it? Black smoke, rich condition, blue smoke, oil, white smoke, coolant. Just be sure you're chasing the right gremlin. Good luck
01-29-2010 | 08:49 AM
#3
Thread Starter
Senior Member
Superstock
Joined: Mar 2009
Posts: 277
From: south nj (philly area)
definately oil smoke id believe its valve stem seals but im in the motor now and i want as tight of a clearance i can without damage...im doing the work myself, i can easilly buy a set of oversized pistons and punch the cylinders...i also know that in auto apps the forged pistons need a greater piston to wall clearance due to the expansion with heat. are honda stock pistons not forged? im not trying to do this again for a long while. i just want everything to be as right and tight as possible!!
also the smoke is extremely little barely visible inder load and none at idle or decell...
also the smoke is extremely little barely visible inder load and none at idle or decell...
Last edited by maniac2313; 01-29-2010 at 08:51 AM. Reason: forgot about smoke
01-29-2010 | 11:07 AM
#4
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SuperBike
Joined: Dec 2005
Posts: 1,689
From: South Lake Tahoe, CA, USA
It is a 3.85" bore, so .0025" clearance is not too bad. Boring and OS pistons are going to be expensive. Ring gap is not as much of an issue as skirt clearance.
Measure the bores (or have them measured). Look for roundness and taper. They will most likely be a bit out of round at the top (.002-.003"). Not much you can do about it, unless you are reboring with a torque plate installed.
Also measure the piston skirts. They are probably worn. You can probably tighten up the skirt clearance with a fresh set of pistons in the stock bores. Stock pistons are cast. No matter what people say, all aluminum pistons will expand and run tighter when the engine warms up. For some alloys, there is less diferential expansion, but still some. Stock pistons are cast. I don't know how much differential expansion there is.
But a fresh set of rings will probably freshen it up and stop the blowby. If it's valve seals, it normally puffs smoke on startup. Smoke under load is more likely rings.
Measure the bores (or have them measured). Look for roundness and taper. They will most likely be a bit out of round at the top (.002-.003"). Not much you can do about it, unless you are reboring with a torque plate installed.
Also measure the piston skirts. They are probably worn. You can probably tighten up the skirt clearance with a fresh set of pistons in the stock bores. Stock pistons are cast. No matter what people say, all aluminum pistons will expand and run tighter when the engine warms up. For some alloys, there is less diferential expansion, but still some. Stock pistons are cast. I don't know how much differential expansion there is.
But a fresh set of rings will probably freshen it up and stop the blowby. If it's valve seals, it normally puffs smoke on startup. Smoke under load is more likely rings.
01-29-2010 | 11:23 AM
#5
Fastest Color
SuperBike
Joined: Jul 2007
Posts: 2,091
From: Lithia, FL
It is a 3.85" bore, so .0025" clearance is not too bad. Boring and OS pistons are going to be expensive. Ring gap is not as much of an issue as skirt clearance.
Measure the bores (or have them measured). Look for roundness and taper. They will most likely be a bit out of round at the top (.002-.003"). Not much you can do about it, unless you are reboring with a torque plate installed.
Also measure the piston skirts. They are probably worn. You can probably tighten up the skirt clearance with a fresh set of pistons in the stock bores. Stock pistons are cast. No matter what people say, all aluminum pistons will expand and run tighter when the engine warms up. For some alloys, there is less diferential expansion, but still some. Stock pistons are cast. I don't know how much differential expansion there is.
But a fresh set of rings will probably freshen it up and stop the blowby. If it's valve seals, it normally puffs smoke on startup. Smoke under load is more likely rings.
Measure the bores (or have them measured). Look for roundness and taper. They will most likely be a bit out of round at the top (.002-.003"). Not much you can do about it, unless you are reboring with a torque plate installed.
Also measure the piston skirts. They are probably worn. You can probably tighten up the skirt clearance with a fresh set of pistons in the stock bores. Stock pistons are cast. No matter what people say, all aluminum pistons will expand and run tighter when the engine warms up. For some alloys, there is less diferential expansion, but still some. Stock pistons are cast. I don't know how much differential expansion there is.
But a fresh set of rings will probably freshen it up and stop the blowby. If it's valve seals, it normally puffs smoke on startup. Smoke under load is more likely rings.
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