Cylinder Diameters

[Triam]

Ok, so I've had low compression in my cylinders, and when I added oil to the cylinders then the compression skyrocketed into spec, so I took my top end apart and this is what I found.
Cylinder 1:

Cylinder 2:

All others look like 1. I guess I'm safe to assume I need new pistons. The bore that pistons corresponds to looks like this:

I got out the bore gage and these are the measurements.

Cylinder 1:
Top
Vertical 2.7178
Horizontal 2.7172

Middle
Vertical 2.7181
Horizontal 2.7179

Bottom
Vertical 2.7181
Horizontal 2.7182

Cylinder 2:
Top
Vertical 2.7180
Horizontal 2.7174

Middle
Vertical 2.7184
Horizontal 2.7175

Bottom
Vertical 2.7183
Horizontal 2.7175

Cylinder 3:
Top
Vertical 2.7176
Horizontal 2.7178

Middle
Vertical 2.7181
Horizontal 2.7178

Bottom
Vertical 2.7184
Horizontal 2.7177

Cylinder 4:
Top
Vertical 2.7180
Horizontal 2.7173

Middle
Vertical 2.7177
Horizontal 2.7178

Bottom
Vertical 2.7179
Horizontal 2.7181

Do these look okay? From what I understand I can just hone the cylinders, buy new pistons and rings, put it back together and seat the rings. Can someone who knows more about these bore specs confirm this for me?

[Steve]

Going to have to convert all those inch measurements to metric, so they make sense to everyone here.

Just a hint, for those that might know the sizes, 2.718 inches is 69.0372 mm.

.

[Triam]

Conversion is as follows:

Cylinder 1:
Top
Vertical 69.032
Horizontal 69.017

Middle
Vertical 69.040
Horizontal 69.035

Bottom
Vertical 69.040
Horizontal 69.042

Cylinder 2:
Top
Vertical 69.037
Horizontal 69.022

Middle
Vertical 69.047
Horizontal 69.025

Bottom
Vertical 69.045
Horizontal 69.025

Cylinder 3:
Top
Vertical 69.027
Horizontal 69.032

Middle
Vertical 69.040
Horizontal 69.032

Bottom
Vertical 69.047
Horizontal 69.030

Cylinder 4:
Top
Vertical 69.037
Horizontal 69.019

Middle
Vertical 69.030
Horizontal 69.032

Bottom
Vertical 69.035
Horizontal 69.040

[Nessism]

Compare the numbers to those in the service manual.

That one cylinder seized. You need to replace the liner or bore that cylinder oversize.

[Triam]

They're all inside the service limit. They're outside the standard specs. Aren't the standard specs intended for the machinists though? Also, I couldn't find a spec for out of round or taper in the service manual unless it's cylinder head distortion which is .2mm which sounds like a lot.

[tkent02]

You also need to determine why this happened to #2. This type of damage doesn't just happen, there is a reason. Probably running lean, since #2 is the carb which supplies vacuum to the petcock a leaking vacuum line is likely the culprit. Could be a bad intake boot or something else causing it to run lean.
If you don't find the problem your brand new #2 piston will look about the same as this one (or worse) before too long.

[Nessism]

My understanding is that you don't want more than about .001" out of round or taper in any given hole. The Piston to cylinder clearance should also be determined and can't exceed the service limit. Again though, your number 2 cylinder is going to be scored all to crap, and bottle brush honing won't clean that up. Your best bet may be to find a new-used cylinder from a low mileage engine.

[Triam]

You also need to determine why this happened to #2. This type of damage doesn't just happen, there is a reason. Probably running lean, since #2 is the carb which supplies vacuum to the petcock a leaking vacuum line is likely the culprit. Could be a bad intake boot or something else causing it to run lean.
If you don't find the problem your brand new #2 piston will look about the same as this one (or worse) before too long.

Sorry I forgot about that part. You're dead on with what caused this. The #2 carb boot was coming apart so the mixture had been really lean. I've got a new set I'll put on before I put it all together.

My understanding is that you don't want more than about .001" out of round or taper in any given hole. The Piston to cylinder clearance should also be determined and can't exceed the service limit. Again though, your number 2 cylinder is going to be scored all to crap, and bottle brush honing won't clean that up. Your best bet may be to find a new-used cylinder from a low mileage engine.

At the worst spots the cylinder is .0009 out. I've got a nice professional hone at work, so I'll see how well it can clean up the scoring. If that doesn't work then I'll get on eBay and pick me up a used set of cylinders.

As always, thanks for the help guys.

[Triam]

K, after copius amounts of homing cylinder 2 looks like this.

It's time for a new cylinder or boring, right?

[tkent02]

This is for an 850?? If it is I have your cylinders.

[Steve]

If you want just one liner, I have a block with just three liners in it, no problem to remove another one.

Cheaper to ship just one liner, too.

.

[Triam]

This is for an 850?? If it is I have your cylinders.

If you want just one liner, I have a block with just three liners in it, no problem to remove another one.

Cheaper to ship just one liner, too.

.

Sorry, had I seen this earlier I'd take you up on it. I ordered some off ebay. If they end up not working out I'll buy them from one of you two.