Connecting rod balancer home made.

[linuxrob]

Rod balancing
This is the first time I have had to do more than just clean and inspect a conrod. My 400 /4 rods were in the same position as when it left the factory so they all went back in the same as they cam off. I did measure the big and small end clearances (well in spec at 36000 miles) but that was it.

The Z500 conrods according to the manual will have weight marks (big capital letter) stamped on the big end caps. Mine, even though this is it’s first strip since the factory have no marks at all! So I am not going to assume that the the rods 1 & 2 are the same weight range, same also for rods 3 & 4 as the manual says they should be.
Out comes the cheap digital scales I bought for my dry chemicals. It can do up to 2Kg and tolerance of 0.1g. A general rule, I am led to believe, is that the rods should be no more than 0.5g different in weight.

With conrods you don’t just match the overall weights but need to separate out the big end and the small end weights then match them up too.
To get good repeatable results you need to support one end solidly on the scale and have the other end suspended level and supported in such a way as to eliminate friction while holding it in line. Then repeat the process weighing the other end.

There are many YouTube videos available on how to make a conrod balance jig. Most use small chain or suspended brackets to support the “non weighing" end.
I had some bit of wood spare (as you do) so cut a square of flat 18mm MDF as a base to fit the sale and support structure.

 

Then cut a smaller square to fit the bed of the scales.

 

 

The Z500 conrods big and small ends are approx 36mm and 14mm respectively so I cut 2 discs from what was left of the 18mm MDF with hole cutters and files them to make dowels of just under these sizes to bare on the big and small ends.

 

The next job was to screw a 90 degree angle bracket to the small square block resting on the scales. An M6 x 30mm set screw was inserted though the top hole in the bracket and secured with a nut and washer.

 

 

I also fixed another piece of spare wood to the base board with angle brackets for the suspension arm.

Not having any suitable chain to make the suspension arm I attached one angle bracket to another via 2 x 75mm long M6 screws, nuts and washers. The connecting holes between these two were drilled 8mm to allow for plenty of movement. This was then tested with a spare Z500 rod.

 

It is vital that the conrod be positioned horizontal and support arm as vertical as possible.
With the rod mounted I took the reading for the big end first. I then removed and refitted the same rod a few times to get an idea of how repeatable the measurement was.

 

 

 

Unfortunately there was as much as 2grams variation in the measurements. I knew this was not the fault of the scales as I had tested them with the full weight of a rod on this base a few times and the results were within 0.1g of each other.

After making sure I placed the rod in the same position on the jig each time I checked that the rod was level and also that both supports were in line. No difference. I could only conclude that it must be the friction between the rod and mounting dowels and to some extent, friction or restriction in the suspension arm used.
Most jigs available for sale were using a plate type suspension arm with ball bearings in. And also in the mounting points for the rod. I looked up the best bearing sizes for these rods and settled on the following:

Small end: 686 open ball bearing, size - 6mm x 13mm x 3.5mm from eBay £2.55 with free postage.
Big end: 6300 open ball bearing, size - 10mm x 35mm x 11mm from eBay £2.60 with free postage.
Suspension arm: Same as small end.

I had a small piece of 3mm thick Aluminium plate left over from making the P.A.I.R. blanking plates on my Bandit and would be perfect for a new suspension arm. A piece of thin hard wood would also be good but the 686 bearing would probably have to be glued in place.

 

A starter 12mm hole was drilled in the plate for the bearing and 6mm hole drilled at the other end for the M6 set screw to mount the conrod. Using a small round file I slowly and evenly opened the 12mm hole to make the 686 bearing a press fit. The 6mm hole got the same treatment to make it a very close fit on the M6 set screw (6.3mm).

 

The lower mount will be used for the big and small ends of the rod so a spacer (washers) then another 686 bearing for the small end. The big end uses a piece of old 10mm fuel pipe cut to 10mm long and opened out to 6mm to enable the 6300 bearing to sit tight on the set screw. The same hardware setup can be used on the solidly mounted set screw mount on the scales.

To mount the suspension arm securely I drilled a hole in the vertical board (in the right place to make sure the rod is level) 6.5mm. Then a 55mm set screw with washers and spacers (made from M6 inside diameter Aluminium tubing) and through the second 686 bearing was fitted.

 

 

With this setup the attached suspension arm could swing back and forth very freely.

With the Aluminium tubing working so well clamping the inner race of the 686 bearing on the arm mount I modified the small end rod support hardware to incorporate 2 spacers on it’s 686 bearing in the same way as the arm.

 

 

To support the big end of the conrod on the suspension arm the small end spacers and 686 bearing are first removed. An M6 nut now secures the set screw, then a M6 penny washer followed by the rubber tubing is placed onto the screw. The bearing slips over the tube and is held in place by another M6 penny washer and a M6 nut. The M6 penny washers are the exact size to clamp on the inner races of the 6300 bearing. The 6300 bearing turns very freely

 

The jig is now set to measure the weight of the spare conrod small end.

 

With the addition of bearings there has been a marked reduction in friction all round.

I have a stop point now for both ends of the rod, big end goes so the bearing is flush and the small end sits on against a M6 penny washer.
I could now fit and remove the rod as many times as I like and get 0.1g max difference each time.

I then swapped the hardware round to measure the big end weight of the spare rod.

 

Luckily I got the same 0.1g max variation in weight so concluded that the jig is fit for use.
I have since replaced the set screws with M6 shouldered bolts with long enough shouldering  to get a tighter fit on not only the 686 bearings but the rubber tube under the 6300 bearing. One more mod I will make is to fit a second 686 bearing to the small end support and modify the spacers accordingly. This will give a better stable support for the small end either on the scale or on the suspension arm.

Even though I have a stable jig I'll take 5 measurements of the big and small end weight. I will then disregard my highest and lowest value then average the other 3. This is just a good precaution to make sure that I get as accurate a reading as possible.

The important point to note is that when we have the fixture set up, we want the conrod sitting stable and horizontal on the scales.
Also make sure that the support arm for the small end of the conrod is hanging very close to vertical. All this will improve the chances of getting valid, repeatable results.

 

Rod        Big        Small        Total
1        269.2        58.3        328.7
2        268.9        58.7        328.5
3        267.7        58.7        327.7
4        268.7        58.7        328.9

 

places to remove material from only

 

Not conclusive results, will wait for that extra 686 bearing to arrive this week de grease the rods and remeasure. At the moment No3 rod is the lightest so the others need to come down to that.

[Mr Fro]

Very nice!

[Mississippi Bullfrog]

And very clever. Your perseverance is commendable.

[Fozzie]

I've never seen one of these home made before. Well done!