Crankshaft rebuild, big end replacement repair.

In rebuilding the crank, I'm being guided by a very useful Youtube video https://www.youtube.com/watch?v=9h-AbRQLGgA
and this instructable: https://www.instructables.com/id/Rebuilding-a-pressed-pin-crankshaft/

I'm trying this job simply to see if its possible- its either this or junk the crankshaft I have simply for having worn bearings! However, bear in mind that any rebuilt crank will need to be reassessed for shimming requirement because it's unlikely to be reassembled to exactly the same width. Most untouched cranks have a width of 1.19 inches measured between the crank entry points.

Before even starting to dis-assemble the crank, its important to have an eye to the rebuild. Firstly, assess the crank you have. Make sure the threads are good and if they need redressing sort them out. Check the key way making sure it's not enlarged or uneven and that the key fits tightly and doesn't wobble. Finally check the bearing lands or contact areas and the regions where the oil seal lips will run. Minor scratching often occurs on the lands during fitting or removal, it's not particularly serious although it can make a shaft stick during assembly, just polish it off with a light file or emery. Scratches or pitting in the oil seal lip running area is more serious as the seal needs to be gastight. Unevenness, or pitting here will promote leaks and rapid oil seal wear. Check any remedial action has been successful before even trying to change the big end.  Check it runs true and that the shafts arent bent.
The cost of a conrod/big-end kit is about £80. A complete new crank is £130 so not quite twice the cost, and it comes with a more modern little end roller bearing. Consequently unless you are sure your crank is basically sound, it's probably better to buy the new one. However, I wanted to try a rebuild so in the spirit of experiment I moved on.

When tested at the journals very close to the webs (and inboard from the bending) my shaft was trued pretty much perfectly with a run-out of the order of 0.02mm. Despite this the webs weren't perfectly aligned- there was some swivel obvious around the crank pin

Webs slightly swivelled

There was also some non parallel alignment between them, although strangely this was more obvious side to side rather than top to bottom.

The gap between the webs varied from 7.88 to 8mm depending on where it was measured.

Distance between webs

Inter web distance.

Interestingly, I think these various out-of-expected orientations were somehow self-cancelling as the the shaft ran true despite them. The gap between the webs is about 8mm and an 8mm brass rod or flat bar slipped between them. I decided I could use the bar as a depth guide during the re-pressing stage; it should slip snugly snugly between the webs to keep them parallel without over-pressing.

8mm brass bar tested as spacer.

 Measuring between the webs at the crankpin showed the gap here should be about 14.2mm

There is a groove at the top of the webs where the thrust washers locate- distance between webs here...14.24 mm

When dis-assembling, the first problem is how to support the crank in the press. I could see two approaches; best would be to support one web such that you can press the pin through that web whilst supporting, and thus pressing only on that web. To do this I made use of the grooves machined inside the webs above the crankpin, these are hidden when the crank is intact but I can show them below on the crank once disassembled.

Inside of crank webs, note the edge grooves (3mm deep)surrounding the crankpin locating hole. Square indentation at the top is where the thrust washer locates.

I used two lengths of flat steel bar to make a suitable support, 4mm and 8mm thick, width isn't critical but 40-50mm should be fine. I cut 30cm lengths from each. The 8mm bar will simply slip between the webs below the crank pin and needs no modification.

I cut a 26mm slot in the centre of the 4mm thick bar such that this will slip down between the webs and into the grooves flanking the conrod. Positioning this bar is hard to describe when the crank is intact so here its shown on a disassembled one. Probably best to use a wider bar so the slot doesn't weaken it so much. Hey, you use what you have!

4mm bar with recess showing how its intended to locate in the internal grooves at the top of the webs.

This slot will need to be filed for a good fit.
The bars and crankshaft are assembled on the pressing table as shown. Since the 8mm bar is thicker and not recessed into the web it lifts the web higher at this side.  The 4mm bar then needs to be lifted to ensure that the crank is held level. For my first attempt I used two lengths of 8mm round or hex bar. It worked, but I think better is to replace them with two pieces of flat 5mm bar shaped to fit more closely against the conrod for more even support and these are shown below. In fact if you calculate the thickness needed; a 4mm bar recessed into a 3mm groove should project only 1mm and thus need a 7mm flat bar to pack it up to the same height as the 8mm bar between the webs, but it seemed to work OK at 5mm. I can demonstrate that below- again on a disassembled shaft.

Manner of assembling the packing pieces prior to pressing, shown here on a separated shaft, the 4mm bar lies in the grooves and the packing pieces fit around the conrod and protect it from the pressure.

However, I couldn't tailor these plates until I had separated the shaft, so for my first attempt I used the rods assembled as shown. This did work but led to some bending in the 4mm bar, hence my wish to use the flat bar for support in my next attempt.

Thicker bar slipped between webs and close to crank pin, thinner bar slipped down above the conrod but engaged with the internal ridge on the webs

Supporting the upper web using flat bar instead of rod.

Use a thin shaft to press down on the pin and push it out through the conrod and lower web but make sure the shaft is long enough that it can press the pin out without allowing the press to contact the thread on the end of the crankshaft

Pressing on the crankpin

Pressing the pin through- had to stop here as the press is close to contacting the thread so switched to a longer  probe. Could then flip the webs over and press the pin out from the second web.

Pressing the crank pin out from the second web

This approach of supporting the upper web only did work but whatever I did the support bars always bent. I'm not happy with these bars being single use So after I'd dismantled one crank I tried a second using the press plate alone. I found the larger round plate hole could accommodate the shaft and crankpin together whilst supporting the lower web on both sides.

Press plate position above hole in th e anvil. Largest hole selected

Crankshaft can fit in hole, shaft passes through, crank pin is free to be moved downwards whilst the lower web is supported all around. Fit press pin and compress. Crank pin has to be moved through both webs and do takes more pressure. A 10t press was sufficient. 

Pressing on the crankpin from above now first closes up the webs and pinches the big end. Once that has tightened up, further pressing pushes the pin through both webs at the same time and so requires more force than the first approach. However it worked well, the pin exited the upper web, the upper web and conrod could be lifted off. The pin was then pressed further down and out through the lower web.

Removing the con rod obviously scattered the big end roller bearings everywhere. These rollers were held in a cage which was also detached. The crank pin shows clear deterioration and pitting where the rollers were positioned and this explains the wear I had detected.

Separated crank pin

I cleaned up the separate webs using the brass wire wheel to remove rust and clean out the groove for the thrust plates before reassembly. 

I checked that the webs hadn't been bent by the process and both internal faces still fitted well together.

Checking the webs for flatness and fit

To start reassembly I first pressed the crankpin into one web. This weas simple because I could just use the press plate allowing the shaft to hang through the opening.

Pressing the pin into the first web, use oil to ease it in.

This presses the pin into the table until its tip emerges flush on the other side so its hard to go too far. Its really important to polish the end of the pin and the inside of the web, they must both be absolutely clean. Use light grade oil to help ease them together.

Crank pin pressed flush

Position the thrust washer on the inside of the web and then fit the conrod and big end. Here I got a surprise as the, con rod and bearings were different from those I removed, the rollers were larger and the cage was absent.I could then press on to reassembly which is much easier to support because the web can lie flat on the press table as the pin is pressed in.

Big end roller bearing cage and its rollers (upper two). New bearings are shown below and are much bigger, the cage is not used.

I could slip the rollers inside the conrod packing them in motor assembly lube

New rollers slipped inside the conrod to form the new big end.

Apply the second thrust plate then clean and position the second web on the crankpin, ensuring all is clean and polished and using oil as before. The upper web needs to be positioned as closely as possible to alignment with the lower web and then pressed on.

Placing the second web- align with the first before pressing

Make sure the webs are aligned as closely as possible and press down. Once the web is on sufficiently that there is no possibility of the rollers falling out I removed it from the press and checked the alignment. It had skewed slightly  but I was able to get this back close to parallel by squeezing  the webs in the vise. I checked that the shafts ran reasonably close to true and then returned it to the press. I inserted an 8 mm spacer (rod or bar) between the webs and pressed down to complete the assembly, again the crank pin is pressed down to flush.

I pressed until the gap between the webs at the top of the thrust plate was 14.2mm

Closing up the webs over the new con rod

Crank pin flush in opposite web.

Finally, I had to check the shaft and re-true. I had hoped that my intermediate truing adjustment would mean that minimum adjustment would be necessary at the end. This didn't work out that way and I doubt I would bother with the interim adjustment next time. Despite my best efforts the crank was well out of alignment and needed a lot of manipulation to true it. The recommendation here is to use a heavy but soft-faced hammer to strike the webs as described. In fact I couldn't move them at all with a rubber mallet or copper hammer so in the end I was forced to tap with a lump hammer to  swivel them as needed. This did work and I ended up with a shaft that was trued within  acceptable limits. Wobble measured 5cm from the webs was less than 0.1mm (0.05 achieved).
At this stage I checked the crank width as above. I had over compressed the crank slightly and the thickness across the webs, measured at the shaft entry points was now 1.79“. I will use a 0.02"shim (smallest in the set) to compensate.

The con rod kit comes with a new small end bush which will need pressing in and reaming. It seems a good fit and is too small for the Gudgeon pin so reaming is needed. I've been supplied with some in the past that were too large even before fitting and accepted the pin as a loose sliding fit. I had assumed that these would tighten up due to "crush" when fitted but sadly this was not the case: fitting resulted in negligible compression of the Bush and so play in the small end wasn't eliminated. It had to come out again. The new one should be fine though and can be fitted as I've described under motor rebuilding.

New Bush right, lacks the splash lube holes present in the old. I will drill new ones after fitting and before reaming. 

 The only difference is that this bush lacks the 3 lubrication holes of the original and so will need to be drilled through, using the conrod splash lube holes as a guide... after fitting and before reaming.