new leakdown results
- Thread starter comnoz
- Start date
- Status
- Joined
- Oct 19, 2005
- Messages
- 18,978
The main feature and reason for low crank case pressure in almost all other engines *expect* old Brit Iron designs is to increase-aid ring sealing not to prevent oil leaks - so something else in Jim's oil and breathing system may be causing trouble but not because it sucks pressure down below ambient.
lcrken
VIP MEMBER
- Joined
- Mar 15, 2009
- Messages
- 5,063
This has been an interesting thread, both from the standpoint of cylinder wear vs piston design and for the discussion or leakdown, or differential pressure, testing.
I'm not an expert on the leakdown testing issue, but do have some experience with the technique, so I'll add what I can to the discussion. I've been using a tester that I bought from an airplane supply house several decades ago. I don't know what the orifice is, but I suspect it is .040". I think that was pretty standard back in the day. I've done a fair number of measurements with it on Commando engines, both race and street, and my experience has been pretty much like Jim's, although I haven't done anything like the number of measurements he probably has. I pretty much only use it for troubleshooting if an engine isn't performing properly.
It appears that leakdown testing is kind of like dyno testing, in that you can't necessarily compare measurements unless both are done with the same instruments and procedures. Still, my experience is that a good race engine will show about 5% when fresh, and I don't worry about it until it gets past 10%. That may seem like a lot, and it probably is if it's from the valves. It it's the rings, and the bike still runs good, I'm assuming that the ring seal is better with combustion pressure behind the ring.
Keep the results coming Jim. The whole piston design subject is fascinating, short skirt/long skirt, high pin/low pin, best alloy, ring material/coating, piston coatings, ring type (Dykes, Total Seal, normal or thin, flat or barrel), clearance, bore coatings, and so on.
Ken
I'm not an expert on the leakdown testing issue, but do have some experience with the technique, so I'll add what I can to the discussion. I've been using a tester that I bought from an airplane supply house several decades ago. I don't know what the orifice is, but I suspect it is .040". I think that was pretty standard back in the day. I've done a fair number of measurements with it on Commando engines, both race and street, and my experience has been pretty much like Jim's, although I haven't done anything like the number of measurements he probably has. I pretty much only use it for troubleshooting if an engine isn't performing properly.
It appears that leakdown testing is kind of like dyno testing, in that you can't necessarily compare measurements unless both are done with the same instruments and procedures. Still, my experience is that a good race engine will show about 5% when fresh, and I don't worry about it until it gets past 10%. That may seem like a lot, and it probably is if it's from the valves. It it's the rings, and the bike still runs good, I'm assuming that the ring seal is better with combustion pressure behind the ring.
Keep the results coming Jim. The whole piston design subject is fascinating, short skirt/long skirt, high pin/low pin, best alloy, ring material/coating, piston coatings, ring type (Dykes, Total Seal, normal or thin, flat or barrel), clearance, bore coatings, and so on.
Ken
- Joined
- Feb 12, 2013
- Messages
- 1,563
hobot said:
wASH YOU R MOUTH OUT PLEASE.
- Joined
- Dec 10, 2008
- Messages
- 7,253
dennisgb said:
I would ditch the pistons but I couldn't cure the pin height problem without new rods. New rods would need to be custom built as the rod journals are around 1.650 instead of 1.750.
The rods are currently 6.375 and the stroke is 2mm over so the pin is .420 higher than stock. That puts the side thrust very high on the skirt. Too high for the needed piston to bore clearance. Jim
- Joined
- Nov 26, 2009
- Messages
- 3,374
Something that has not been mentioned in this thread that was mentioned in an earlier thread about this same motor is that pressure vents (radial gas ports) in the pistons tops to the ring lands were used to give more ring pressure and that extra ring pressure against the cylinder walls resulted in rapid ring wear. The bore has a little wear now and that wear may be in part because of that extra ring pressure and wear. When the motor was first put together with a fresh bore, the leak down was good and measured 5% to 6% at 500 miles. This was with approx .006" skirt clearance or a little under. But the worn bore (which is no longer round) created leaks. Now it doesn't seal as well as it did after the initial build - even with new rings.
- Joined
- Sep 15, 2013
- Messages
- 246
Thanks for sharing that info and video. I'm new to Nortons but have been a life long Triumph guy and found that when using the "cheapie"pistons I get very good use out of them ,but set my clearance at .005 (never less) When I did my first Norton rebuild I read in the shop manual to use like .004 clearance. I thought my God are they nuts? Anyway I set mine at .005 with the "Cheapie" pistons and Hastings rings and she's running great with about 1000 miles on the clock since rebuild with no hint of wanting to seize up. Please keep the tech videos coming.....Skip
- Joined
- Nov 26, 2009
- Messages
- 3,374
gjr said:
The rods are not the problem. The problem as I see it is the radial gas ports that accelerated the wear on the first set of rings and helped to put wear on the bore. When the motor was new the leakdown was only at 6% and that's great. Now with the same rods and new rings it leaks too much and the only real difference is the worn bore. The radial gas ports in the pistons don't belong on a street motor. If it were my bike I would keep the rods, rebore and get new pistons without the radial ports. As Comoz said in his previous post - this is the best running motor he has had on the street. If it were mine I would keep it running that way. If you want high mileage and want to insure that the bore will not wear again, the best thing to do is send the cylinders to bore tech for carbide impregnation. It only a couple hundred odd bucks and I would do that for any top line motor no matter what is inside. This way you have the best of all options. Going to longer heavier pistons will probably break that crank with the smallish 1.650" dia journals. A broken crank could wipe out the whole motor and be very expensive. Shorter rods will not run as well and will increase stress and vibration.
Also - the Norton cylinder axis is already staggered about .090" to the REAR of the crank axis according to my measurements.
- Status
