Replace pistons or not?

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freefly103

freefly103

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Recently took the head off my 70 Commando to investigate an oil seal leak on the right hand inlet. While there, took the barrells off to inspect the pistons and rings. They are standard size Hepolites with the AE stamp and look good. The do have the long slot underneath the ring. For the relatively low cost of new pistons and rings, I am inclined to change these to new standard size pistons with new rings.

Replace pistons or not?


Replace pistons or not?


Replace pistons or not?


Replace pistons or not?


Replace pistons or not?
 
Re: Replce pistons or not?

My understanding is you can't just drop in newer type pistons without boring to fit because of tolerances varying in manufacturing. Correct me if wrong. I would put the money toward JS rods n' pistons If I had to rebore. :|
 
Re: Replce pistons or not?

Yes, I would replace them. I am surprised they have lasted this long. Jim
 
Re: Replce pistons or not?

CRAP those are the kind of old taboo pistons that flipped their top in my '72 Trixie ruining her engine, the first time, on a mere easy commute ride. On top of new piston/pin/ring set might consider Bore Tech treatment so barrels last a few more pistons/rings before new bores. About $350 and weeks processing though to cover a couple decades extra cylinder use, maybe.
 
Re: Replce pistons or not?

Hmmm - do I have to rebore the barrells? I thought I could drop in new 73mm standard size pistons to replace these standard size Helpolites.

There's no Boretech where I am (Malta - that little island between Italy and North Africa).

I don't really want to take apart the bottom end, so replacing the rods is not an option unless I absolutely had to.
 
Re: Replce pistons or not?

I just happen to be online before others, so measure bores up & down to get sense of their condition to re-use, even w/o a hone touch up if lucky, then will know if can just stick in pistons rings and go for it as bottom end is pretty robust and nothing to service there like cleaning sludge trap that can't get packed up to matter in Commandos anyway. Ya need to allow for ~.0045" piston/bore clearance to decide. IF rods don't have much slack to tip sideways and no detectable wiggle slack on shells then run with it no worries until rod knock noise some decade or more down the road.
 
Re: Replce pistons or not?

From what I have read about these Pistons I would replace them... And I am just about to on my 68. Like Hobot said measure up the bore and see what condition it is in. If it is outside limits get it bored and go next O/S, if it's ok I would hone it so the new ring can bed in correctly. Honing only removes about 0.0001" per 60-80 strokes, but keep it to a minimum to achieve a nice even 45 degrees x-hatching. Then clean it a few times and check it with a white cloth and make sure no black on it. If so clean again. Most important to remove all the grit.

Also have a good look at the spigots at the bottom of the barrel were they go into the crankcases. Mine are cracked from the top of the radius of the cut out for the conrods. It is a known fault. If it's not cracked I would file the sharp machined edge off to stop it cracking in the future.

Cheers
D
 
Re: Replce pistons or not?

Reboring is only needed if its outside wear limits, you can fit new pistons and hone for the new rings.
 
Re: Replce pistons or not?

Honing is only done after re-boring. Deglazing is done [ or used to be done ] when doing rings etc. Think about it. When you deglaze, you remove material from the bores and make them bigger and older. The modern school of thought is to just put the new pistons in. The only consideration is the step worn at the top. If you can feel this, it probably needs re-boring though. My last job where I blew a piston, I just renewed the std pistons and then took the bike on a hard ride. No problems. no oil burning and no blowby. Bore taper is the killer. 1 1/2 thou taper is ok.
When I was told a good 850 can do 350,000 miles before reboring, I knew I had nothing to be concerned about.

The most important thing about replacing pistons and or rings is the running in process. Dont fuc around nursing your bike. Give it shit and it will not let you down. Do several short but hard rides.
Dereck

what would I know, I am only a mere motor mechanic.
 
Re: Replce pistons or not?

I replaced my cracked piston like yours in 1975 with another similar pair because at the time that was what the Norton shop sold (£26 for a pair, a weeks wages as an airforce recruit) I just replaced them again 18 months ago with about 60,000 miles on. The bores were shot but the pistons are in remarkably good condition. If you are going to be exploring the upper rev limit every time you ride then replace them but if you are a normal sane motorcyclist where 5000rpm will give you 80 mph leave them in. They haven't failed yet so you might as well wear them out. As a matter of interest, I rebuilt my engine and left the mains as they were, they have about 35,000 miles on them. I would seek out someone who can measure the bores for you and that might give you a steer as to rebore or not.
 
Re: Replce pistons or not?

Here's one of your flip top pistons:

Replace pistons or not?
 
Re: Replce pistons or not?

As mentioned before get your bores "sized/checked/inspected" and if worn over factory specs then you will need to rebore ~~~ .010? or what ever is required. If thats not what you want to do, then put it back together and just look at it, start it up now and again for the prospective buyers, because it will fail sooner than later...

IMO, + 010 thou rings n pistons.. (forget the rods unless your racing or a thrasher). Riding with mechanical respect will give you 10k's + with a simple piston/ring/ valve grind overhaul...
 
Re: Replce pistons or not?

You can get a fairly accurate indication of bore wear with a piston ring and feeler guage. Put the ring in at about where the wrist pin would be at top dead center. Use a piston pushed against it to make sure it is square. Measure the ring gap with a feeler guage. Now move the ring to the bottom of the bore and square it up with the piston and measure the gap again. Any difference more than .005 between the bottom and top numbers will probably indicate a rebore is needed. Not as accurate as having a good machinist measure it but it sounds like in your location finding someone competent to measure it is a challenge.
 
Re: Replce pistons or not?

Not sure about your maths Htown. Circumference is Pi x diameter therefore divide the increase in circumference by Pi to get the wear. Your example of 0.005" divided by 3.142 is 0.00159 so a bore wear of 1.5 thou (of an inch). Try and find +0.010" pistons in the non slotted design. You'll be lucky. If the bike has done any decent mileage you will also be lucky if a 0.010" rebore cleans it up. Get it measured properly and then decide on the way ahead.
 
Re: Replce pistons or not?

My tale of woe from back in 1975. I bought my Commando for £350 from some bloke in Barry South Wales the next night after work I went to start it and when eventually it did start, both exhaust pipes went bright cherry red. I took it back to the vendor (that's being polite) and he said the timing had slipped and reset it for me. I rode home that weekend 140 miles and burnt a complete tank of oil in the process. And another getting back on Sunday night. One of the slotted pistons had cracked from the hole beneath the slot to the bottom of the skirt. I suppose I was lucky that it didn't drop off completely and also that the oil shortage didn't lead to it seizing solid.
Changing the pistons round the back of a barrack block in February has mentally scarred me for life. Still got it though :twisted:
 
Re: Replce pistons or not?

Oops my bad, I meant .005 bore wear or taper. That equates to slightly more than .015 difference in the measurements of the gap between the top and bottom or the bore.
 
Re: Replce pistons or not?

Ok, can't feel any step in the bores, so will manually check ring gaps as suggested by using a ring at various intervals and if within tolerances will install these little JCC babies:

Replace pistons or not?


Cheers,

Joe
 
Re: Replce pistons or not?

I was suggesting the ring gap method as only a means to get a rough estimate of wear not as a replacement for proper measuring to fit pistons.
 
Re: Replce pistons or not?

Increased ring gap is more an alert to check if its mostly the ring wearing or the bores, by far usually just rings. Its not the big ring gap that causes the leaks that matter its the rounding of ring edges not scrapping and sealing. Not a hobot opinion but world wide engine references. Last Trixie ring set was .022 and .024 but bores still good and still slightly undersize for .020" over bore. Almost exact same findings in Wes's '71 found a year later, just needed new rings to stop oil use and smoking. MIne was d/t cracks in ir boots valleys, Wes's was d/t only using k/n filter w/o the 1" thick faom wrap I gave him now.
 
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