No compression, maybe broken connecting rod

[Handy Don 12,235]

I've often wondered what sort of jig holds a crankshaft so that the rod races can be machined. Anyone have an image?

[richmondred01 2,237]

27 minutes ago, Handy Don said:

I've often wondered what sort of jig holds a crankshaft so that the rod races can be machined. Anyone have an image?

they are large machines to handle any crankshaft. 
These little engines that we fool with only half the machine is used.

 

 

[Handy Don 12,235]

2 minutes ago, richmondred01 said:

they are large machines to handle any crankshaft. 
These little engines that we fool with only half the machine is used.

 

 

Both of these images seem to address machining the main journal bearing surfaces that are in line with the shaft. How does it align the offset rod journals?

[squonk 41,153]

The mounts move in an ellipitical motion and the rod journal stays centered against the wheel.

 

 

Edited August 1, 2021 by squonk

[Handy Don 12,235]

2 hours ago, squonk said:

The mounts move

Very cool seeing how the chucks can be offset so precisely and how to get the shaft chucked so the top/bottom of the stroke for a rod journal aligns with the chuck offset adjustor. And the counterweight is so straightforward but so integral to smooth operation. 

I especially liked as he began the second side of the rod journal watching the grinder level out the out-of-roundness!

What a cool combination of great skill and precision tooling.

Thanks @squonk!

Edited August 1, 2021 by Handy Don

[lynnmor 7,308]

2 hours ago, squonk said:

The mounts move in an ellipitical motion and the rod journal stays centered against the wheel.

 

 

Maybe I missed something, did he just indicate a worn rod journal so it will be ground to the same error as before?

[squonk 41,153]

The way understood it was he was centering the journal to the machine's plane. The few thousands off he was talking about was the actual stroke length where there would be no noticeable difference felt between one cylinder to another. The journal will be ground true. Think of it this way. If the engine stroke was 3.000" exactly and1 cylinder was actually 2.996" and another was 2.997" you would not notice anything. Just the piston travel would be a couple of thousandths shorter.

[Handy Don 12,235]

51 minutes ago, lynnmor said:

 

Maybe I missed something, did he just indicate a worn rod journal so it will be ground to the same error as before?

 

Restating @squonk, what I think I saw was that he indicated two things:

1) making sure the chuck offset was within a reasonable distance of the spec'd stroke (since that offset would become the post-grind stroke distance) -- here I thought he was "splitting the difference" between wear on the journal "top and bottom" when setting his offset/new stroke/new journal diameter setting

 

2) making sure that the rod journal was centered correctly in the machine's chuck adjustment plane (to be sure TDC and BDC are where they belong) -- this was when he shifted the crank in the chuck, though he did not show how he did that alignment

 

Are you pointing out that by indicating against the existing, worn journal, the centering could have been affected by uneven wear on the sides perpendicular to the piston travel? If yes, I agree. Is there another way to assure that the journal is correctly centered in the machine's adjustment plane?

 

[lynnmor 7,308]

7 minutes ago, Handy Don said:

 

Are you pointing out that by indicating against the existing, worn journal, the centering could have been affected by uneven wear on the sides perpendicular to the piston travel? If yes, I agree. Is there another way to assure that the journal is correctly centered in the machine's adjustment plane?

 

 

I believe that the rod journal offset needs to be measured from the main journals to get the correct stroke.  It also needs to be indexed to the flywheel key, but the correct degree of offset may not be known.  What is being done is simply making the rod journal to a correct under-size with a good surface and not in a precise location.  I  think that the process in the video is done using worn surfaces to assure that will clean up, putting it in the correct position may not allow that to happen.  Yes, it may be good enough, especially if done on a single cylinder engine where the relationship between multiple rods in not a concern.

[Handy Don 12,235]

2 hours ago, lynnmor said:

putting it in the correct position may not allow that to happen

Yes, I can see that.

Getting the rod journal to it's correct rotational angle relative to the fly wheel (and the other rod journals) would be ideal but your point that grinding at that position may not yield a clean surface is very relevant. Is it correct, then, that the worst that could happen for a still-usable crank is that the rod journal could be out of place rotationally by ±.02" and have up to .02" longer stroke? For a 3" stroke that's ±0.2% of rotation or ±0.76º--is that significant?

Edited August 2, 2021 by Handy Don

[lynnmor 7,308]

3 minutes ago, Handy Don said:

Yes, I can see that.

Getting the rod journal to it's correct rotational angle relative to the fly wheel (and the other rod journals) would be ideal but your point that grinding at that position may not yield a clean surface is very relevant. Is it correct, then, that the worst that could happen for a still-usable crank is that the rod journal could be out of place rotationally by ±.02" and have up to .02" longer stroke?

 

It would probably be worse than that because the indicated low side may need material removed in order to get a true surface.

[Handy Don 12,235]

6 minutes ago, lynnmor said:

 

It would probably be worse than that because the indicated low side may need material removed in order to get a true surface.

I was thinking in terms of replacement parts being 0.01" or 0.02" offset. Taking away more material assumes that you can get a rod for the smaller diameter journal, right? If you can't, then the crank is unusable even if you can get a true surface.

Edited August 2, 2021 by Handy Don

[lynnmor 7,308]

4 minutes ago, Handy Don said:

I was thinking in terms of replacement parts being 0.01" or 0.02" offset. Taking away more material assumes that you can get a rod for the smaller diameter journal, right? If you can't, then the crank is unusable even if you can get a true surface.

 

That is correct, I was only talking theory.  Different manufacturers have different sizes available.

[Handy Don 12,235]

We are on the same page (whew  )!

Thanks again @squonk for the video and @lynnmor for chiming in, I learned quite a bit with this thread!