I managed to get a pretty good start on the crankshaft, and I thought I'd document the progress on it so far.
First i cut the crank webs to rough dimension on the bandsaw and cut the to-be crankshaft and crank pin out of drill rod. I then trued up the crank webs on the mill and turn the round stock to the exact spec'd length and drilled center holes on each end of the crankshaft, just because I like the way that looks, there's no functional reason for the center holes at this point (at least I don't think so!). I'm not showing these ops as they are pretty straightforward.
Pix of the parts ready for the next op.
On the crank webs, I was very careful to get them to the same width and length by machining them together in the vise. To make sure that the crank would run true, I stacked the 2 crank webs together in the vise and spotted/drilled/reamed both holes simultaneously.
Setup in the mill vise using a square to make sure the webs are aligned in width.
Spot drilling the locations for the holes in the webs.
Drilling thru both locations. For the crank pin hole, i drilled 1/64" under .25" and for the crankshaft I drilled 1/64" under 5/16".
Reaming the hole for the pin and the crankshaft to .250" and .3125" respectively. I slowed the mill down to 150 rpm and liberally applied lube for the reaming op.
The pieces hand assembled on the print. The fit was very snug. I could hand assemble the parts but it took some pressure. I ended up polishing the round bits in the lathe with a bit of Scotch Brite just to ease the fit a bit. Rightly or wrongly, I decided that the fit was ok. Quick question. Does everyone try to get a really 'firm' or 'tight' fit or is it better to open it up to maybe a thousand over? I'd say the clearance I had was about .0002 to .0004 as best as I could measure.
Related to the crankshaft is of course the main bearings. I assembled them on the base plate and then made sure they were aligned with a rod that was about .001 under the bearing size, so i could slip it though. The pix shows this alignment.
The crankshaft would not go thru yet as I'd left the bearings just undersize. This pix shows reaming thru the bearing with an on-size (.3125) reamer. I put the reamer in a drill chuck and simply rotated it by hand and passed it through both main bearings. At this point the crankshaft would pass thru the bearing but was quite tight. It was certainly not freely rotatable.
Closeup of the reaming op. In this pix, i used a .001" over 5/16" reamer, again rotating it by hand.
Then checking the fit again with the actual crankshaft rod. Now the fit felt good to me. I could detect almost no slop or play but the shaft rotated freely in the bearings. The bearing material is SAE 660 bronze.
Just as a comment, in the future I don't think I'll try to ream thru both bearings like this again. With the small size of everything, there's quite a bit of flex as you try to insert and rotate the reamer, even being quite gentle. For a built-up engine like this, I think in the future I'll just ream the bearings .001" over and then assemble. There's enough play in the fastening arrangement to take out any small misalignment that may result. A cast engine assembly, I suppose would be different as you'd have to do the reaming in place.
I didn't show the Loctite being applied and the parts being assembled. Things were happening fast and i didn't have time to take any pix. What i did end up doing was finding a .30" block and clamping it between the webs to establish the spacing after applying Loctite to the crank pin. Then I applied Loctite just ahead of where the crank webs would go on the shaft and slid it into place, checking the position with a dial caliper.
This process worked, but I used Loctite 603 and it sets up VERY fast so I started feeling the parts getting very hard to move. After looking closely at the specification sheets and looking thru other build logs, it seems most people use Loctite 638, which is stated to more viscous with a much longer cure time. The strengths of both are almost equivalent. So, I'll for sure get a bottle of 638 to have on hand and depending on the circumstance use the appropriate product. I can tell you that I almost didn't get the parts in place before they locked up solid!
After curing overnight, and based on the wisdom of other builders, I went ahead and pinned the crank parts together as sort of a 'belt and braces' approach. The first pix shows 4 holes drilled in the crank assembled. I did not drill all the way thru the webs. I went to about .050" short of the opposite side. That way I will only have to clean up one side! I used a #49 drill (.073") and went and bought a small box of 4D finish nails. I mic'd a number of nails and selected 4 that were just under .073", say nominal .0735". That size could be easily pressed into each hole. I clipped off the points and ground a small bevel on each nail so they were pretty flat but still had a feature to ease the start into each hole.
Then I applied Loctite 603 to each hole and to each pin and pressed each one into place and lightly drove each pin in with a small hammer. You can see the Loctite that was displaced as the pins were driven in.
Then I clipped most of the exposed length off of each pin and peened each one a bit just to make sure they fit completely and tightly in each hole.
Right now I'm letting this assembly cure overnight and in the am, will file/grind/sand off the remainder of the pins and will see how well I can blend them into the crank webs. Then I'll mill out the center portion of the shaft and that should complete the crankshaft.
This crankshaft has been quite a learning experience so far. I'm not sure that I did everything correctly, but I think it will work out. My main question(s) is regarding the clearances that are best for the fit between the shafts and the webs and also the best clearance for the pins. Is is best to have have a hand fit on the pin or should they be tighter so as to have to be driven in firmly? Also I used soft iron nails but would it be better to use something like hardened dowel pins? Taper pins would probably be the ultimate but I didn't want to purchase a reamer as they seem to be around $40 for a 7/0 or 6/0 reamer. Opinions on the above?
All for today!
Enjoy!
Mike
