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MG TD TF 1500 - When tightening the con rod bolts they're stuck

I am rebuilding an XPAG engine.

When I tighten the connecting rod bolts to the value in the workshopmanual( 3.7 m./kg) I can't move the conrods on the crank.
If I tighten to 2.0 m./kg only, there is no problem, they turn freely.

Is it a problem to leave them at 2.0 m./kg?

If not, what should I do??

Erik

Erik vanHardeveld

Yes, there is a problem. Did you have the crank ground undersized? You are not trying to use over sized berrings to account for a worn crank, are you?
I would check all three main bearings and all four connecting rod berings with Plasti-gauge torqued to the proper value.
If you had the crank reground I would bring it back to the machine shop and have a chat with them. Something is wrong.
-David
D. Sander

Another possibility is that you have inadvertently overlapped the ends of the upper and lower bearings on one of the con rods.

Of course, if you have done this, you will need new bearings and you should check the crank journal for damage. Don't ask me how I know...

In any case, David is absolutely correct. You have a problem that changing the torque won't correct.

David Littlefield

Hello,

Thank you for your comments sofar.

I use a newly ground crankshaft (ground at 0.040) with new 0.040 bearings.
As far as I can tell the crank is correctly ground at 1.731 inch (43.97 mm)
I have flexigauged the bigends and they definitely are 0.002 inch (0.050 mm)
The bigend openings are not oval and just the last
draw on the torque spanner makes the conrod stuck.
If I unloose it just a little, it is free.
The main bearings are all-right.

Erik
Erik vanHardeveld

Hi Erik,
when you say:"i flexigauged..." did you do this with the 3.7 torque?
And was the play 0.05 you found on the top of the bearing.
Remarkable issue however.
Greetings and see you soon in Houten, Huib
Huib Bruijstens

I would bring the crank, the bearings and the rods back to the shop that reground the crank and ask them to have a look. It sounds like the rod journals may not have been properally ground/polished. Fortunately, if this is the case, they can remove a little more...
-David
D. Sander

Itotally agee with above posts. Some of the replacement bearing shells have very square edges, and reground cranks curved or beveled edges. This results in binding at the very edges of the journals. You may be able to see bright marks on the shells at the very edge. David's idea is the best. The shop can also torque the nuts and actually measure the inside diameter all around to make sure there is not some high spot or something. Lastly, I think my machinist had an issue with the notch or tang that fits into the slot on the rod/cap being an issue. That could be it also. George
George Butz

Hello,

Huib: Yes I flexigauged with the 3.7 torque

David: The problem is, I had it all machined 2 years ago. So going back now is a bit of a problem.
Moreover: all 4 journals are exactly
1.731 inch (43.97 mm) as they should. So the machinist did a good job

Will putting some paper in between perhaps give some
tracks?

Erik
Erik vanHardeveld

Hi Erik,

You might want to check the conrod bores for ovality. The last 2 engines I rebuilt had the same problem you describe and I was able to rectify it by machining the bores.

Measure the bore in two places, across the split and at 90deg to it.

Cheers,

David
David Tinker

The machine shop will have a gauge they measure the inside diamter with. On my rods, the cut-out notch for the tang in the bearing shell (to keep it from rotating) was not deep enough or long enough, and when fully torqued it would slightly squeeze the shell outward and then it was too tight right at that area. The square shell edge to crank bevel/fillet is a known issue as well. Your overall clearance may be fine, but if one of these things could be the problem. George
George Butz

Hello

Does anyone know what the exact inside diameter of the big end bores is?

I think I read somewhere that it should be 1.915.
Mine are 1.915 horizontal(bolt tot bolt) and 1.910 vertical.
(How accurate can one be with a electronic sliding caliper)
So that is oval but very, very little.
Could this be the problem though?

Erik
Erik vanHardeveld

Eric, I would recommend that you lightly coat your bearing shell with some "machinist's blue" or "prussian blue" and then check for high spots and trueness on your bearing assembly to identify the area of interference. You would just tighten the assembly very slowly until you feel slight resistance as you simultaneously rotate the crank through a few degrees. The transfer of the pigment on the worpiece will indicate the high spots, once you have disassembled. This should identify any ovalness or tightness at the fillet or cut out tang etc., as previously suggested . Cheers Phil
Phil Atrill

Not sure at what point the crank stopped moving....
Before getting too upset, did you check the rotation of the crank, after tightening each con rod?...Maybe only one is causing the problem...It is possible, that only one of the bearings is causing the problem....
Try each one separately, until you find the offending rod or bearing. Be sure to have all the bearings properly greased with engine-building lube.
Edward
E.B. Wesson

This thread was discussed between 16/12/2011 and 19/12/2011

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