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DIY Crankshaft?
- Thread starter Brant
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93mustang97sea
New Member
As far as i know on 787 motors at least. To rebuild the crank it needs to be broken.. Yes broken in half to get to the bearings and such... Thats why I dont think people just do it on there own. When they can get one rebuilt from sbt or hotrods for cheap even new for kinda cheap.
Dr Honda
Well-Known Member
I've done a bunch over the years... but there is an art to it... and you need the tools to make sure it's indexed, and runs true. A crank that isn't perfect can eat the engine in short order.
You will need a press, V-blocks, a dial indicator, and a couple hammers. You want the run out to be better than 0.001". (spec is 0.001" ~ 0.002")
You will need a press, V-blocks, a dial indicator, and a couple hammers. You want the run out to be better than 0.001". (spec is 0.001" ~ 0.002")
Totally agree with Dr Honda. I have done it, but wont mess with cranks again. You have to be a very good machinist to start, and make a bunch of special tools to be able to get the right dimensions set. You need a 50 ton press to get them apart, and have to make some special press tools to get the crank pins pressed out as there is not much clearance to get at the pins.
In the end, it is much better to buy a rebuilt one.
In the end, it is much better to buy a rebuilt one.
Dr Honda
Well-Known Member
If you have an understanding of precision work... then have at it.
As an FYI... I own most of the tools needed, and the only time I will do a crank is if it's not available commercially. So... that is normally for restoration work. (like old 2-stroke street bikes) I will do single cyl dirt bike cranks... but it's extra work to do a multi cyl engine.
Honestly... it's not worth my time when I can pick up the phone and get a complete hot-rods crank that is normally better than OEM. (with a warranty)
There isn't anything "Mystical" about rebuilding a pressed crank... it just takes time, and tools that 90% of the home mechanics don't own.
Just my 2 cents.
As an FYI... I own most of the tools needed, and the only time I will do a crank is if it's not available commercially. So... that is normally for restoration work. (like old 2-stroke street bikes) I will do single cyl dirt bike cranks... but it's extra work to do a multi cyl engine.
Honestly... it's not worth my time when I can pick up the phone and get a complete hot-rods crank that is normally better than OEM. (with a warranty)
There isn't anything "Mystical" about rebuilding a pressed crank... it just takes time, and tools that 90% of the home mechanics don't own.
Just my 2 cents.
The reason I won't do it again is pretty simple. It is a lot of work to make all the spacers and such, and the cost of decent parts. On an older crank, I wouldn't think of just replacing a rod bearing. It is likely that the inner seals are either already bad or doing to go soon, and to do the whole crank is a lot of work. Takes too long, its hard work, and the amount you will save doesn't compensate for the trouble.
You aren't going to budge the rod pins with a 12 ton press, and most 12-20 ton presses don't have the throat width to fit a SD crank anyway.
If you have the crank out, take a good look at it. Notice that the end bearings are spaced out. This need to have the proper spacing when you are done putting new bearings on. This means you have to fab a spacer to the precise dimension to get them back on the right spot.
Now take a look at the rod pins. To press them out, you need to work outward. Notice that you don't have full access to the pin to press it out; the inner bearings obscure access to the full diameter of the pin. You only have about 50% of the diameter to work with. You need a long pin to get at it, and it needs to reduce near the crank lobe for clearance. This needs to be strong steel, and it is dangerous pressing as the tool can fly out at great force if you don't have a strong setup.
And this is the easy stuff. Have you ever worked with a pressed crank. Takes a long time to learn the tricks to get it properly in place. Without a fixture, you need to align it closely, press the journal on slightly, then check it in v-blocks for alignment, beat it with the right technique to get it aligned properly, press it a little more, check it again, etc. Again, you need the proper spacer to make sure you have the right clearance for the rods. You could press it a little, measure, press, measure, but if you go too far, the whole thing needs pressed off again and start over.
If you have a lot of time, have the chops to work with precision stuff, and just want to do it for experience, go for it. Just be aware that if you get it apart, and can't get it back together right, you won't have a core for the rebuild....
Dr Honda
Well-Known Member
All good points above... but, most of the time... if you "go too far"... you've just crushed the thrust bearing/washer, and you will have to replace them.
Brant
New Member
Well! I tried pressing the crank apart yesterday. The 12 ton press didn't cut it. Also couldn't really get on anything very straight. So I made up a couple of shims today and borrowed a 50 ton press at the local machine shop. The rod bearings pressed out like butter but the center section took an amazing amount of force. It came apart with a bang and shook the building.... Holy Crap!!.....I'm now looking for another core crank with one good bearing to use for a trial..... Anybody got one ??....
Yep, not only is it scary, it can be dangerous as well. If your setup isn't straight and strong, the drift you use to press the pins out with can shoot out sideways like a rocket...
At least you understand why I insisted you needed the 50 ton press! (I know this simply because I couldn't get one apart using a 30 ton press....)
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