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#556827 - 08/05/14 12:02 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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zoe Offline
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I just bought a 1966 A65L motor that I'm going to rebuild. I read and hear that there are modifications concerning the side to side wandering of the crankshaft and balancing the crank that need to be addressed to make these motors reliable and better runners. Are you guys making these modifications, or any others, on these engines?

Bob

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#556835 - 08/05/14 12:39 pm Re: 1967 Lightning Piston Seized While Riding? [Re: John Healy]  
Joined: Oct 2013
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Waymon3x6 Offline
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Waymon3x6  Offline
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Connecticut, USA
Originally Posted By: John Healy
So, exactly what do you consider HIGH rpm.

Is it possible for you to consider that at 50 mph in high gear you are no where near high rpm.

If the previous owner installed a larger transmission sprocket, as it is common to do these days, the rpm at 50 mph would be even lower.

Can you understand the concept that you are measuring dynamic cylinder pressure when you use a compression gage. Can you understand why the gage measures a lower pressure when the throttle is closed? Can you make the step in understanding what you are doing when you open the throttle from the 1/3rd, or less, throttle opening normally required to maintain 50 mph in high gear and open the throttle to 3/4. Do you understand shifting to a lower gear acts as a lever, or torque multiplier, and reduces the strain on the engine, just like your car does when it automatically shifts down a gear in the same conditions you describe.

Add to this some other problem, even if ever so slight, and you have the perfect conditions for detonation, especially if the fuel doesn't have a high enough octane rating to prevent it.

Remove your plugs, and with a small focused beam, look at the cylinder bore. Do you see the pock marks on the top of the piston consistent with detonation? Do you see any scuff marks on the cylinder bore?

And yes use the clutch as a diagnostic tool. When it seizes pull in the clutch. Is the wheel still locked? If not your problem is in the engine. If it is look at the clutch and transmission.


The best way I can describe high RPM (because I'm uncertain that the RPM gauge is accurate, although it does move between 2 and 3k rpm, but nothing higher as 3k is about as high as I'd rev it) is to compare it with the noise you would hear when you are riding next to a harley or any other cruiser on the high way at 65 or 70mph. So it's not bubbly-low, but the pitch is in that mid to upper range.

I check the plugs this weekend - I should be able to see exactly what you're talking about with just a flash light, right?

When I pull in the clutch, the rear wheel moves freely just like it's supposed to. I always fill it with 93 octane at the gas station.

#556837 - 08/05/14 12:56 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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Rickman Offline
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Ohio
Let's repeat this:

These BSA's are NOT hardleys, they should NOT be allowed to let their engines run hard at low RPM's, lugging the engine.

At 50 mph in 4th gear, you are lugging the engine. BSA twins don't like low RPM.
If you are running the engine only within that 2-3k window, you are lugging the engine.
But I'll admit the wrong tach may have been installed, but that would point more toward the bike not being quite as original as you may have been lead to believe...

This said, I had another A65 decades ago, fresh bore job, and the engine got hot at times and heat seized.

Sure, check the timing, absolutely, but you may end up taking the cylinder off to have the clearance checked...
You may want to look closely at the ring end gap also. I've heard of rings getting too hot, expanding too much and seizing the "piston" in the bores also.

If the engine is vibrating your hands, "unsatisfactorily", and you are lowering the RPM to get the vibration away from your hands, you are hurting your engine.

Go find that guy with another A65 who wants to go for a ride, and the two of you check each others ride out, see if'n the two of you can figure the problem out?
Or find the local BSA meet, and see if'n the fellers can sort your bike out?

HTH....

#556847 - 08/05/14 1:24 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Rickman]  
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Waymon3x6 Offline
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Waymon3x6  Offline
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Originally Posted By: Rickman
Let's repeat this:

These BSA's are NOT hardleys, they should NOT be allowed to let their engines run hard at low RPM's, lugging the engine.

At 50 mph in 4th gear, you are lugging the engine. BSA twins don't like low RPM.
If you are running the engine only within that 2-3k window, you are lugging the engine.
But I'll admit the wrong tach may have been installed, but that would point more toward the bike not being quite as original as you may have been lead to believe...

This said, I had another A65 decades ago, fresh bore job, and the engine got hot at times and heat seized.

Sure, check the timing, absolutely, but you may end up taking the cylinder off to have the clearance checked...
You may want to look closely at the ring end gap also. I've heard of rings getting too hot, expanding too much and seizing the "piston" in the bores also.

If the engine is vibrating your hands, "unsatisfactorily", and you are lowering the RPM to get the vibration away from your hands, you are hurting your engine.

Go find that guy with another A65 who wants to go for a ride, and the two of you check each others ride out, see if'n the two of you can figure the problem out?
Or find the local BSA meet, and see if'n the fellers can sort your bike out?

HTH....


Thanks for the reply. I'm certain that the RPM gauge is all original. It's just 47 years old and hasn't been tampered with or taken out of the housing and cleaned/adjusted.

The thing that I seem to be failing at describing is the fact that I am not lugging the engine. The throttle is 80+% open when the engine seizes. I do not have the tranny in the last gear and only 10% on throttle as your post implies.

I suppose my best bet would be to take it for a ride, first without the choke and see if she seizes. Then take out the plugs, and try to see if there are any scrapes inside where the piston is. Not entirely sure what to do after that but I'm open to any other suggestions.

Last edited by Waymon3x6; 08/05/14 1:25 pm.
#556849 - 08/05/14 1:28 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
Joined: Mar 2005
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John Healy Online content
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Boston, Massachusetts
Quote:
I read and hear that there are modifications concerning the side to side wandering of the crankshaft and balancing the crank that need to be addressed to make these motors reliable and better runners.


Setting up the end float on an A50-A65 crankshaft during initial assembly is part of the job. It isn't a modification!

Giving the budget allows, having the crankshaft dynamically balanced is always a good idea.

Toughening the combustion chamber to detonation by making sure there are no sharp edges to become "glow" plugs and increase the chances of detonation, or worse, pre-ignition is manditory and should not be considered a modification.


#556854 - 08/05/14 1:38 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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Alex Offline
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Alex  Offline

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Seattle
Originally Posted By: Waymon3x6

The thing that I seem to be failing at describing is the fact that I am not lugging the engine. The throttle is 80+% open when the engine seizes. I do not have the tranny in the last gear and only 10% on throttle as your post implies.


I think a good rule of thumb should be more like don't apply more than half throttle BELOW 3000 rpm. The sweet spot is between 4000 and 5000. You should feel it really come alive at 4500. If you want to be conservative, shift before 6000.

Seriously.


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#556855 - 08/05/14 1:40 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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run990 Offline
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lancashire united kingdom
silly question but the clutch is not slipping is it ,is there free play in the cable .if its slipping when the bike comes on cam it could be over reving & causing the lack of top speed & seizures .if its geared right 50 mph uphill is 3rd gear territory .i dont think it would run well if it had seized as many times as you think it has.are there any british bike owners local to you ,if so ask there opinion ,way cheaper than blowing it up thru lack of knowledge

#556864 - 08/05/14 2:41 pm Re: 1967 Lightning Piston Seized While Riding? [Re: run990]  
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Fullminator Offline
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Draper, Utah
After reading all the previous posts from this thread, I am fairly sure you have experienced a light seize of the piston in the bore. The seize is most likely on the thrust face of the piston skirt. Aluminum pistons in iron cylinders make for some technical calculations to figure the different rates of expansions to get the piston to bore clearance, and this is highly dependant on knowing the quality of the pistons installed in the engine. All the rest, high manifold pressure, mis timed ignition, choke on or off, low RPM with high throttle demand, poor oil circulation, all contribute to higher heat in the cylinders, and when the pistons expand more than the bore size of the cylinders, no amount of more twist of the throttle is going to make it go. When you let it sit, it cools and starts fine again and the bike will probably run like this for a long time, however it will become noisy as the piston and bore no longer fit together as they should, and it will start to burn oil and lose compression. You may be able to see evidence of this with a bore scope, but initially most of the light seize evidence will be on the piston. Only after repeated seizings will the bore show signs of it.
Cheers.
Fullminator.

#557125 - 08/06/14 9:59 pm Re: 1967 Lightning Piston Seized While Riding? [Re: Alex]  
Joined: Jan 2006
Posts: 1,838
JD Offline
Moto-Amish
JD  Offline

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Posts: 1,838
Maryland
Originally Posted By: Alex
I think a good rule of thumb should be more like don't apply more than half throttle BELOW 3000 rpm. The sweet spot is between 4000 and 5000. You should feel it really come alive at 4500. If you want to be conservative, shift before 6000.

Seriously.


+1!


Josh
#557802 - 08/11/14 8:58 am Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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Waymon3x6 Offline
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Waymon3x6  Offline
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Connecticut, USA
Well I took it out for a ~100 mile ride over the weekend... Zero problems. I brought it up to about 60 a few times but cruised around at ~45 and had no issues.

Choke cable was tight this time, must have been the choke. Stupid. Thanks for the help guys, I definitely learned a lot from this thread.

#557968 - 08/12/14 8:54 am Re: 1967 Lightning Piston Seized While Riding? [Re: Waymon3x6]  
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Dick Harris Online content
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Dick Harris  Online Content
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East Bethany New York
I'll bet that even after all the help in previous posts, that you cruised at 45 mph in "last gear" Ah well. Dick

#558262 - 08/14/14 7:59 am Re: 1967 Lightning Piston Seized While Riding? [Re: Dick Harris]  
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Waymon3x6 Offline
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Waymon3x6  Offline
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Connecticut, USA
Originally Posted By: Dick Harris
I'll bet that even after all the help in previous posts, that you cruised at 45 mph in "last gear" Ah well. Dick


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