Replace con rods without splitting cases?

Head is off but not cylinders, yet. Once the cylinder block is removed, is it possible without further disassembly to install new con rods?

No.

No. There's no way to access the con rod nuts. You'll have to split the cases.

BrianK said:

Head is off but not cylinders, yet. Once the cylinder block is removed, is it possible without further disassembly to install new con rods?

Click to expand...

Why do you want to replace the Conrod's?

BrianK said:

Head is off but not cylinders, yet. Once the cylinder block is removed, is it possible without further disassembly to install new con rods?

Click to expand...

You could saw a big hole underside of cases to access the conrod big wnd nuts i guess...but that creates other issues...

Thanks folks. No surprise, but I didn't want to assume.

To Concours' question: Pistons need replacing, and if it could be done without splitting the cases I'd spring for some of Jim Schmidt's shortened pistons/longer con rods. Not to be.

Appreciate the help. - B

Pistons are one thing. Con rods are another.

Not unless it's a BMW boxer motor.

You could use just his light weight pistons designed for stock rods. I think reducing piston weight would always be a good idea.

IF you do not know the history of your motor it would be a bit silly to not split the cases and at least clean out the sludge trap, to me it is preventative maintenance.

possm said:

IF you do not know the history of your motor it would be a bit silly to not split the cases and at least clean out the sludge trap, to me it is preventative maintenance.

Click to expand...

... And check the big ends.....

Adding JS light weight pistons to stock rods and not getting the crank rebalanced for the weight change might not be ideal. Have not done it that way myself, so mostly talking maybe. However, with a Commando the rotating mass being a touch off balance might not be as noticeable as it is on a pre-Commando Norton.

Per JS, for both 750 and 850: "because of the lighter pistons this means your motor will shake less vertically and the same horizontally – a win win situation. Rebalancing is not necessary." https://jsmotorsport.com/pistons-for-stock-rods/

Lighter pistons move the balance factor towards a better value for performance rather than comfort.

acotrel said:

Lighter pistons move the balance factor towards a better value for performance rather than comfort.

Click to expand...

But it's the lighter pistons that achieve that. Moving the balance factor to what is achieved otherwise with the lighter pistons would make it worse in the Isolastic frame.

hhh said:

Per JS, for both 750 and 850: "because of the lighter pistons this means your motor will shake less vertically and the same horizontally – a win win situation. Rebalancing is not necessary." https://jsmotorsport.com/pistons-for-stock-rods/

Click to expand...

That same philosophy is supposed to apply to the JS long rods and pistons and it is close but a rebalance of the crank would probably be better at lower RPM cruise. That's based on actually installing the parts and using them. The lighter pistons, as good old Al mentioned, do make a difference at very high RPM where not many members here reach unless they are racing or enjoy thrashing their bikes once in a while.

Commando owners wouldn't notice any difference because of the isoelastic mounting of the engine.

By the way, I'm not the only one that believes rebalancing would be beneficial. However, I might be the only one that will say so on this website.

Static Rebalancing is something you can do yourself if you're so motivated. I've laid it out in my race manual with illustrations. You need a gram scale (a simple mail scale should work). Level edges to set the crank on. Carpet thread or fishing line, wire and sockets to use for hanging weights. A calulator, some basic math skills and a good eye.

When I built my 850 motor for racing, the first thing did was rebalance the crank - I filled hole in the counter-weight with a threaded steel plug. A billet crank would be a much better way to go. The balance factor came out to be 72 %, and I expected it to be around that figure. Ashleigh suggested that I had guessed that figure. I had a steel frame with one end in a vise, and the other supported by an adjustable rod - I levelled the frame and rolled the crank on it. I had weights on the ends of the rods, and used an electronic balance and a calculator. The method is in Phil Irving's book 'Tuning For Speed'. In the old days, every racer read it. If you are revving the motor to 8000 RPM, a balance factor of 80% works better. It is not about vibration for comfort reasons. A smooth running motor delivers more power. Your Commando with a low crank balance factor is not designed to run at high revs.
If you do not believe me, rebalance and do a before and after assessment. There is a significant difference in performance.
Isolastics are a cheat, but seem to work in a road bike. They do not change what the crank does to the motor. With race bikes - rubber mounted motors always seem to make the bike slower, but it might the feel of less vibration that creates the impression. If I was racing a normal Commando, I would raise the balance factor to suit the intended rev limit, regardless of isolastics. Even on the slowest corner on a race circuit, my motor never drops below 5.500 RPM and peaks at 7300 RPM.

Your post above does not comport with the laws of physics, sorry. And you can't cheat physics, but you can employ novel means of deploying it to advantage, and that's exactly what the Norton engineers did with the Isolastic system. What you did with your 850 was correct for your frame, not for an Isolastic frame.

I also believe in balancing the motor to suit JS rods and pistons.

One reason is this: if using a 50+ year old stock crank, you do not know your starting point, so all the calculations and estimations are based on sand.

I have it on good authority from back in the day that Norton botched the crank balancing on Commandos. My hypothesis is that the isolastics worked so well, they dropped the ball on crank balancing.

Plus, an engine could have been re-balanced by some unnamed person, at some unknown time, using some unknown theories.

I’ve built 3 JS equipped engines now and have had them properly balanced each time and the results have been fabulous every time.

The most recent was my Seeley with a 1007 motor. I believe it was balanced incorrectly when first built as the vibration was horrible (ie I thought it would brake things), but then a 1007cc, 11:1cr, solid mounted twin ain’t ever gonna be turbine smooth of course.

I canvassed opinions and got a range from 72% to 78% as the most recommended factor for a rigid mounted Seeley. So, knowing no better, I aimed in the middle and went for 75%. The re-balance and the use of JS rods and pistons TRANSFORMED the bike.

Yes, I know I did 2 big changes at once so cannot put all credit to the JS parts, but they removed over HALF A POUND of reciprocating weight. Just think about the stresses involved with that going ‘up, stop, down, stop, repeat’ over 100 times per second !!

Unsurprisingly, the engine produced more power as a result as well as being much smoother, so def a win-win.