Conrod weight

[DJinCA]

As some may recall, I am putting together a pile of parts into a 51/52 Golden Flash.  This is a budget project.  I have a freshly ground large journal crank.  I have four connecting rods, two left and two right.  Unfortunately, one of the left rods has a bit of twist to it.  The other 3 appear to be straight and in pretty good shape.  I weighed each of them, and the found that the left rod weighed in between the two right rods, but there seems to be ~10 grams between each rod pair. 

There are pretty noticeable dimensional differences between the two right rods, but I don't really see any place to remove that much weight.  physics.nist.gov says the density of aluminum (aluminium) is just under 2.7gr/cc, so I would need to remove over 3 ccs of material to match weight.  That doesn't seem reasonable.

I know about fatigue in aluminum alloy, and that the right answer is new modern rods, but that simply is not in the budget for this rescue from the scrap heap project.  I am spending my limited money on building an RGS.  I am just trying to make this one a runner, but would like to avoid building a paint shaker.

I tried doing some searches on this topic, but didn't find much useful.  I would appreciate any advice.  I hope to look through a friend's parts pile for rods,and will take my scale in hopes of finding something closer to matching one of mine.  Thanks in advance.

DJinCA

[Swarfcut]

Bit of a conundrum. It depends how perfect you want the motor to be. In an ideal world the conrods should match, weight for weight, but also the distribution of the mass should also be the same. In other words, the weight of each assembled end should also match. In practice we assume big end shells, bolts washers and nuts all match, and just compare the bare rods with small end bushes fitted. Worth swapping over these the parts if there is any noticeable individual weight difference. Further perfection costs more, which is fine with money no object, but these old machines were never super smooth to start with, the basic design means they will only be smooth in a particular speed range. This is the domain of the professional  engine tuner, so the bucks will start to mount up.

 Big Journal Rods are marked 67 1160 followed by a forging number, eg R4Z1R which differs depending on the manufacture source of the rod. The rods should match size wise, and appear almost identical, so dimension differences noted by eye are unexpected....are they all big journal rods?

  Sourcing a closer matched rod is probably the cheapest option, followed by matching your nearest existing rods. Converting  timing side to drive side rod by drilling the bleed hole can be done. There is debate as to whether the hole is necessary or effective, so take your choice here. Removing the forging flash and polishing  the heavier rod is a good place to start, but get expert on the spot advice as to further lightening.  10 grams difference may be acceptable  for the bike's intended use.  Many folks have built motors which despite every known machining and assembly perfection turn out to be just as rough runners as backyard lash ups.

 Assure yourself that the crank sludge trap has been removed entirely and is super clean after its trip to the grinders.

 Swarfy.

[KiwiGF]

https://www.a7a10.net/forum/index.php?topic=6086.msg41615#msg41615

The above may help as background info. and has pics, it makes a difference which end of the rod is heavier. Gudgeon pins can make a difference! 10g sounds quite a lot 

I guess oem rods are not worth much nowadays so you could probably find some of the correct weight cheaply albeit the problem may be finding some (I’ve got a few a10 small journal rods I assume will collect dust forever anD I guess I’m not alone in that!)

[JulianS]

Not all large journal rods are quite the same - see below - the later ones for big end bolts with self locking nuts have modified end caps. The service bulletin says altered thickness. But I doubt if the weight difference would be very much.

[Swarfcut]

Julian. Again another gem from the archives.  From the wording it appears later A7 engines would use similar bolts and nuts and so presumably required slightly different machining of the big end caps to accommodate the larger hex of the Simmonds nut and access space for the spanner, when compared to earlier rods fitted with castle nuts and split pins. Noticed the part number for a later complete A7 rod changes from the 1953 parts list, but this could be due to it being supplied as a complete, loaded unit, with different spec from early models rather than a change to the basic forging. Anyone Know?

  Always wondered whether it was a good idea to open out the spot facing on my early rods, rather than make thicker spacer washers to move the nut away from the cap to get room for a spanner to fit on a later Simmonds nut. Former weakens the cap, latter alters the balance factor, both theoretically. So you can't win either way. What do other folks do?

Swarfy.

[berger]

" what do other folks do " stick em in and open the throttle

[DJinCA]

Thanks for all of your replies.  I went to the large journal crank because the small journal rods I had were trash.  There was galling under the bearing shells that lifted thin bits of alloy like feathers.  I had never seen anything like it before.  What appeared at first look to be a good pair of large journal rods had one twisted rod, so it was left to do something else.  I noticed the different forging marks Swarfy mentions and assumed that was some of the reason for the differing weights.

As to Swarfy's sludge trap caveat, excellent advice.  I thoroughly cleaned the crank, including trap removal (still out of the crank) before sending it out to be groud and will clean it again before assembly when it comes back from the machinist.  I had the grinder grind the timing side journal down to where it was clean and round, rather than a fixed dimension, and the crank went out with the cases to get the new TS bush line bored.

Somewhat to KiwiGF's comments about the weight of bearing shells, gudgeon pins and rod bolts, I stripped the rods (except the small end bush) and weighed them.  The bolts, nuts and shells were near enough identical.

Julian's Service Sheet is interesting and somewhat enlightening, but not surprisingly, BSA didn't offer any dimensions, etc. to identify the different caps.

As Swarfy said, the weight differences may be acceptable for the bike's intended use.  I will see if I can find a closer matching rod to pair with one of mine, clean a little forging flash off the heavier, and in the end, probably follow Berger's advice.  Thanks again all.

[trevinoz]

When you match the rods, you have to weigh each end and adjust the heavy one to the light weight.
Metal is usually ground from the end of the rod for the pointy end and ground out of the cap for the blunt end.

[duTch]

 After reading Trev's comment, I wonder if any had the big-end hole rebored, which involves shaving the joint surfaces before reboring the hole- that'd lose a few grams

[DJinCA]

Thanks Gentlemen

Trev, I know that generally it is as you say, but it didn't seem practical to remove >3 cc's of material from the rod, even working from both ends.  I didn't get down to measuring the distribution of weight given the disparity of weights between the rods.  If I find a one to mate one of my current rods that is closer in weight, I will get into greater detail.

duTch, looking at the big ends, there was no suggestion that substantial material (if any) had been removed from the end caps to true them up.  The bearing shell locator keys appeared unmodified.  Meanwhile, the big ends measured out pretty round.  While this is not meant to be any sort of absolute, simple comparison showed the I-beam sections of the rods were measurably thinner on the the lightest vs. the heaviest of my rods.

I guess I was mostly surprised at how great the differences were between the rods that I had.  I will spend a little more time on looking at improving my choices, but will in the end move forward with what I have available at the time.  While I want to build the best I can, it will have to be within the existing constraints, budget being at the top of the list.

DJinCA

[ironhead]

Thanks Gentlemen

Trev, I know that generally it is as you say, but it didn't seem practical to remove >3 cc's of material from the rod, even working from both ends.  I didn't get down to measuring the distribution of weight given the disparity of weights between the rods.  If I find a one to mate one of my current rods that is closer in weight, I will get into greater detail.

duTch, looking at the big ends, there was no suggestion that substantial material (if any) had been removed from the end caps to true them up.  The bearing shell locator keys appeared unmodified.  Meanwhile, the big ends measured out pretty round.  While this is not meant to be any sort of absolute, simple comparison showed the I-beam sections of the rods were measurably thinner on the the lightest vs. the heaviest of my rods.

I guess I was mostly surprised at how great the differences were between the rods that I had.  I will spend a little more time on looking at improving my choices, but will in the end move forward with what I have available at the time.  While I want to build the best I can, it will have to be within the existing constraints, budget being at the top of the list.

DJinCA

If you're  really worried about the weight difference, fit the lighter rod on the left side if the weight difference is at the bottom. ( rotating mass )There is a difference in the weight of the sludge trap bungs ( left one has a locating spigot for the tube, so weighs a fair bit more than the plain right one ) However, BSA didn't seem to think this a problem at the time.

[DJinCA]

Thanks Ironhead,

It seems so obvious after someone more clever points it out.  It makes complete sense.

DJinCA