Author Topic: Replacing conrods  (Read 2255 times)

Offline mark

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Replacing conrods
« on: 04.08. 2009 07:28 »
The chap that is rebuilding the bottom end on my 55 road rocket has suggested that it is wise to replace the conrods as alloy ones only have a limited life. Any thoughts on whether this is really necessary. The ones in it have a couple of tiny nicks which should polish out.
Regards
Mark
55 road rocket




71 Norton Commando Roadster 750


Australia

Online BSA_54A10

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Re: Replacing conrods
« Reply #1 on: 04.08. 2009 10:29 »
Cast alloy rods have a theoretical fatigue life of 10 to the 12th ( 1,000,000,000,000 ) stress cycles.
One revolution is a stress cycle, you do the math.
 
Bike Beesa
Trevor

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Re: Replacing conrods
« Reply #2 on: 04.08. 2009 10:37 »
G'day Mark,
               if in doubt get them checked for twist or bend. If ok polish out the nicks and use new bolts & nuts. Alloy rods are a lot stronger than a lot of mechanics think. Under normal use they will do 100,000 miles. I have used std rods in race motors with 14:1 compression, the rods were fine, not so the crank or barrels.
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Online Triton Thrasher

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Re: Replacing conrods
« Reply #3 on: 04.08. 2009 10:54 »
Cast alloy rods have a theoretical fatigue life of 10 to the 12th ( 1,000,000,000,000 ) stress cycles.

 

Hmm, so is that at 6000 rpm or 3000 rpm.  It must make a difference.

Online Triton Thrasher

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Re: Replacing conrods
« Reply #4 on: 04.08. 2009 11:00 »
G'day Mark,
               if in doubt get them checked for twist or bend. If ok polish out the nicks and use new bolts & nuts. Alloy rods are a lot stronger than a lot of mechanics think. Under normal use they will do 100,000 miles. I have used std rods in race motors with 14:1 compression, the rods were fine, not so the crank or barrels.
Cheers.

Oddly (or not) 54A10's figure comes to about 100,000 miles too.No it doesn't! Nothing out of ten.

In real life, these rods are usually broken by seizing bearings.

Online RichardL

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Re: Replacing conrods
« Reply #5 on: 04.08. 2009 15:20 »
Trevor,

I'm curious, where the 10 to the 12th number comes from.

TT,

Using my bike as an example, 4000 RPM gets 60 MPH or, one mile/minute (perhaps better said as 4000 revs in 4th gear gets a mile regardless of RPM).  1,000,000,000,000/4000 = 250,000,000. Sometimes I'm fumbleheaded and miss the obvious, but this seems to be saying 250 million as opposed to 100,000 miles. Of course, this is all in 4th gear. It's possible one might only get 125,000,000 miles. Drat!

If I am correct, barring other calamities and assuming Trevor is correct, I think this means my rods will survive stress cycles for my remaining period of interest.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline beezalex

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Re: Replacing conrods
« Reply #6 on: 04.08. 2009 16:17 »
The point remains the same.  Unless they are otherwise damaged or incorrectly installed, the rods will not fail in normal operation.
Alex

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Online Triton Thrasher

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Re: Replacing conrods
« Reply #7 on: 04.08. 2009 16:44 »

TT,

Using my bike as an example, 4000 RPM gets 60 MPH or, one mile/minute (perhaps better said as 4000 revs in 4th gear gets a mile regardless of RPM).  1,000,000,000,000/4000 = 250,000,000. Sometimes I'm fumbleheaded and miss the obvious, but this seems to be saying 250 million as opposed to 100,000 miles. Of course, this is all in 4th gear. It's possible one might only get 125,000,000 miles. Drat!

Richard L.

I had it wrong. I got it into hours and then divided by 40 instead of multiplying by 40 to get miles (I was only doing 40mph).


Online groily

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Re: Replacing conrods
« Reply #8 on: 04.08. 2009 20:34 »
I'm there/here Richard, but no idea at all how many miles you get out of a rod!! All my rods, all 21 of them in things from 45 to 70 years old, seem to be whirring around OK and clocking up the miles. Not thinking of changing any of them - but nor of going Drag Racing, to be fair.
Currently eyeing alloy rods and a few other parts in a marque not so renowned for endurance as our BSAs, aka Royal Enfield twin, but even there, they seem to be fine, in alloy, with a ridiculously long stroke of 90mm odd and 70mm bore, 6250rpm available and 50 bhp . . .  so they say. Scary. Had it running and it was fun, quick and nicely noisy, but, ahem, a lot of oil everywhere ..  . . so it's in bits to see what can be done to render it continent, or at least reasonably so. We think we have breather trouble with Beesas . . . ?! Try an Oilfield - quite capable (apparently) of blowing its primary side into the nearest ditch having ingested petrol-ly fumes through the crankcase breather into the chaincase and exploding . . . a fair few mods are essential, I'm told. In hand.
Thee and I know we can get nearly anything for an A from the various excellent folk who support us, but I have never yet come across anyone quite like my new best friends Hitchcocks, the UK RE people . . . absolutely anything and everything - a lot of it re-imported from your side. If we could all get BSA pistons for 40 pounds a pop with pins and rings in any size from standard to max . . . wouldn't life be nice? A key source of AMAL parts btw for anyone looking, including pre-Monobloc stuff.
Temporary pre-occupation with oily bits from Redditch has not, however, precluded some decent mileage on the Beesas . . . Meteorological Offices various should be sued for issuing false weather forecasts which have led to a damp crotch too often this European summer. But despite the worst of the elements, no BSA has embarrassed me in ages. I was only saying to a mate this afternoon that apart from oil changes and a gear-change return spring, I haven't laid a serious spanner on my scruffy A in gawd knows how long (like a year and a lot of miles), and the same goes for the off-topic B (which is quite simply one of the finest classic machines ever made if you're not in any kind of a hurry).

Soon as the Oilfield is sorted I'll probably be plagued by something terminal in the Small Heath part of the shed, but no matter how hard I look I can't find anything wrong with the darn things. Brilliant adverts for the whole classic movement, especially as they aren't at all smart. Just as well they work unremarkably actually, as the Domestic Management is starting to notice the proliferation of stuff in the shed and wondering whether there's any plan to reduce the stock in some way. I plead necessity, addiction and general insanity.
There's no good answer to an insanity plea, which is why it should always be added to any list of excuses. Can't underline that maniacally enough!
Bill

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Re: Replacing conrods
« Reply #9 on: 04.08. 2009 21:22 »
beezalex wrote
Quote
Unless they are otherwise damaged or incorrectly installed, the rods will not fail in normal operation.

The innermost truths are often presented with very few words... ;)
e

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Offline MikeN

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Re: Replacing conrods
« Reply #10 on: 05.08. 2009 12:11 »

               if in doubt get them checked for twist or bend.

Mark,
 If you are going to use them again ,make sure you or your mechanic checks them for alignment as stated above.
  Unless he is an A10 specialist he might not have the correct fitting test bars or equipment to check them.
 Alloy rods are frequently bent by careless (previous)owners .usually by hammering out the gudgeon pins.
  Im talking about thous here ,not something you can detect by eye.
Misalignment causes friction and wear on bearings and pistons.
Mike

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Re: Replacing conrods
« Reply #11 on: 05.08. 2009 14:41 »
Groily,

Even though hitting the topic just briefly or tangentially, your report and musings are a good read and quite entertaining, as usual. Thanks for that.

I wonder, do the new Enfields have the same oil/breather problems, or is yours of the India-made variety anyway? (You might choose to reply in "Stories and Chat".)

Finally, now slightly and facetiously back on topic, it would seem all there is left to do about the comments from Mark's mechanic, having, as a mob, refuted them thoroughly, is to grab our pitchforks and storm his shop.

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline mrshells

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Re: Replacing conrods
« Reply #12 on: 05.08. 2009 19:01 »
put new bearings in and run them again
if it aint broke dont fix it  *smiley4*

Offline trevinoz

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Re: Replacing conrods
« Reply #13 on: 05.08. 2009 23:06 »
Quite often there is minimal bending, as Mike states.
Assemble the rods to the crank, obtain or machine a rod the same diameter as the gudgeon pins and long enough to pass through both little ends. If the rod passes through easily, you can assume the rods are straight.
If there is slight misalignment, a judicious twist should bring everything into line.
Trev.

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Re: Replacing conrods
« Reply #14 on: 08.08. 2009 15:05 »
The stress cycles figure comes from the aluminium developement association.
It is the fatigue life of the alloy used.
The number is when the rods would be expected to fail due to the aluminium fatigueing so represents the maximum service life in revolutions.
It was posted just to give you an idea that the rods will last for a very long time before they "just fail".

It was to be followed up with the little provisos that a notched sample halves that fatigue limit, but I have been other wise engaged all last week.

The cruxt of the matter is that defects are 1000 times more important than operating hours so rods that are physically in good condition are fine to reuse regardless of the age and usually the rod will need to be scrapped because of twists, bends, scratches or cracks well before old age is a problem.
Bike Beesa
Trevor