Puzzled - Crank "locks up"...

[fanaudical]

While partially-assembling P&C's to my case today, I discovered that I can only rotate the crank through a small degree of rotation before something is binding in the case. Here's the situation:

- 2.4 7R mag case bored for 2.7L P&C's
- Larger oil pump installed w/ all necessary case clearances
- Before installation of P&C's, crank rotated smoothly (provided all rods were held with small end on center of piston throws)
- Installing P&C's from flywheel end and working forward (Goofy engine stand I'm using prevents installation from normal end, but I don't see how this would matter.)
- I find that I can't rotate past BDC of center two cylinders when attempting to position the next set of rods at TDC (I have 4 P&C's installed, still have the last two to do)
- I have verified that timing chains are not binding/bunched up.
- No issues when installing pistons into cylinders (I have no concerns about messed up rings or anything like that).

Any suggestions before I start disassembly? I didn't find any very similar issues using search.

Thanks for the help.

[brp914]

Skirt contacting case? Piece of crud in IS/crank gears? Can you rotate opposite direction?

[fanaudical]

Piston skirt hitting the case is a good check point. I'll see if I can find some way to view that.

I inspected the gears and all seemed clean, but will check again. It will rotate in the opposite direction to about the same location.

I have already verified that I didn't leave any bolts in the periphery of the flywheel.

[racing97]

Some times the case is not bored with sufficient clearance and it can bind the cyl into the poston
Also the machinist may have not bored the case down at the bottom the same as the ID of the cylinder and the piston is hitting at the bottom of the skirt. Always use a radius on the boring cutter or the case will crack at the time or after doing the operation

Regards

[fanaudical]

Confirmed - the case wasn't bored deep enough to accept the larger cylinders and clear the piston skirts. (Skirts are hitting at the bottom of the stroke.)

As far as repair goes - Is it at all advisable to have the piston skirts trimmed? Or is this a situation in which everything comes apart to have the case modified (again)?

I'm quite surprised at this issue - I spent a lot of time reviewing case mods with the shop (and they are quite reputable, too). Is there something else I could be missing here?

[Eagledriver]

This is a very common upgrade. The shop should have known how far to bore the spigots out. While trimming the skirts would be much easier, I wouldn't do it. Shorter skirts means more piston rock/slap. It sucks but I'd take it apart and get the spigots bored deeper (for free from the shop that did it wrong).

-Andy

[fanaudical]

Does anybody have the correct depth for how deep to bore the spigots (cylinder mounting surface to bottom of step) when opening up the case? I'd like to check against what I'm measuring.

[cstreit]

Check your cam chains for binding up around the layshaft sprocket.

Check to make sure your rods are really free.

Too long a bolt in your flywheel? (This one has gotten me a few times, haha)

[fanaudical]

Thanks for the replies. I pulled P&C's today and it looks like root cause is no step-bore to clearance the skirts. Grrrr - I should have caught that BEFORE assembly...

[racing97]

I can tell you that more than one person has equally marked the corner of the piston and taken a die grinder and notched it a little, probably not more than 3mm or so. I know the piston balance people will go crazy but remember the piston does not reciprocate so this is not too risky a venture, but yes it is a little shade tree. The center of the skirt is where the rock is compensated from you will be on the outside of that.
regards

[JohnJL]

Originally Posted by racing97

marked the corner of the piston and taken a die grinder and notched it a little, probably not more than 3mm or so. I know the piston balance people will go crazy but remember the piston does not reciprocate so this is not too risky a venture, but yes it is a little shade tree.
regards

Not arguing this wouldnt work in the short term, but why do you say the piston doesnt reciprocate? Its just about the most reciprocatingest thing in there?

[fanaudical]

In talking with several different people today, I'm finding that clearancing the end corners of the piston skirt is fairly common. (Not sure it's right, but several seem to have done so.) I can understand this working if pistons are carefully modified so as to maintain mass balance side-to-side on opposing throws. What's not clear to me is the long-term affect on piston slap and the like.

Looking further at my case, I'm not sure how one would cut deeper without getting into the bearing webs. I think I'm going to get out the modelling clay and do some serious interference checks before I go any further.

Does anybody out there with serious experience in modifying 2.4-to-2.7L cases care to weigh in here?

Thanks for the help.

[racing97]

Sorry, reciprocate is a poor choice of words the Pistons motion is translating (linear) rather than spinning about the crank.

[MBEngineering]

HI fanaudical
what crank have you in the engine 66mm or 70.4mm???

regards mike

[fanaudical]

Mike - I'm using the 70.4mm crank.

[fanaudical]

All fixed. I modeled the case/piston interference in 3D CAD and found some very minor "shaving" in the case took care of the problem. All back together again and proceeding well...

[Flieger]

CAD is only as good as your measurements. I do not think I would trust it without disassembling all the parts to confirm that the parts are all to spec which seems like too much work. But if you are confident then that is all that matters.

[fanaudical]

You're correct on the CAD bit. I measured/modeled VERY carefully. All better (or so I think...).