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MG MGB Technical - Engine stiff when turning over

Hello,

I am having problems reassembling my engine. I have inserted pistons 1 and 2 ok. When the big ends are on and torqued up the engine still turns freely. However When I torque up the big end bolts on piston 3 the engine becomes very stiff. However if left loose the engine turns over fine. I am guessing that the clearance between the crank shaft and bearings is too tight. What is the best way to resolve this issue? I am sure the big ends are connected to the correct pistons.

What torque should be required to turn over the engine?

Many thanks

Alex
Alex Davies

Alex,

Try bolting the big end up on the crank without the piston being in the bore. If it is still tight I think you must revisit the machine shop and get them to rectify the clearance problem as there seems to be insufficient clearance in this bearing.
Iain MacKintosh

Sounds like insufficient clearance or a too undersized shell, or a bent rod. undo the bearing and measure the bore and the journal. Does it tighten up at the last tweak of the wrench or before it gets tight? Is the shell sitting correctly?
Art Pearse

It is ok until the final torque up. Would it be worth swapping piston#3 with piston#4 as there may be a slight difference in the crank shaft diameter? The machine shop suggested it didn't matter which piston went where as long as the big end cap stayed with the piston.

Any ideas on the advisable torque to turn the engine round?
Alex Davies

Look for bright spots on the bearing surfaces - this will show where it is binding.

Check the sizes - not unknown for rogue bearings in the packet!

Make sure there are no burrs or any dirt on the shells or the rod.

Make sure you have the right end cap on the rod - check the numbers stamped on the side.
Chris at Octarine Services

Enginge should be EASY to turn over with a 12" ratchet and socket. Most drag is from the rings. There should be nowhere it feels worse than the rest of the circle.
Art Pearse

One further check to make that I haven't seen suggested above - try turning the bearing cap around and see if that loosens things up. Cheers - Dave
David DuBois

Alex
Make sure you hav'nt marked the crank bearing surface when inserting the piston and rod down the bore as i did this and the motor seized when attempted to start the car.
Its quite easy to ding the crank when pushing the rod down to meet the crank.

Pete
PJ Thompson

If you don't have the tools to measure the crank journal and rod bearing, you can get some Plastigage from your local parts store to check how much clearance you actually have at the bearing. It's inexpensive and easy to use. Instructions are included. RAY
rjm RAY

I had a similar problem when I built an 18V engine for my Magnette.
The engine was so stiff when the bearings were torqued up that a 4' long bar was required to turn the engine!
I had built an identical engine for my MGB shortly before, which turned easily.
After stripping and checking several times I finally decided to start it. The additional torque of a starter motor spun it and it started immediately.
This engine now feels better than the MGB's with high oil pressure and is free-revving.
Perhaps a combination of tolerances all on the tight side?

Regards,
David
David Overington

If the crank stiffens when the mains are tightened, they could be out of line. This isn't the case with a rod. It can only be a tight bearing.
Art Pearse

Alex, as Chris said it's not the big end cap that stays with the piston but with the conrod. It must be matched and fitted the correct way round with the numbering matching. Also a speck of dirt between the shells and the rod or cap will seriously affect the clearance. Which piston is fitted to the rod will make no difference to the overall fit. Look for bright markings on the shells after turning the engine over with the cap tightened and then try the conrod on the crank without the piston. Then its back to the machine shop I'm afraid.
Iain MacKintosh

Alex,

Reminds me a misadventure that had been happening during these last 9 months to a friend. Exactly like you he had isolated each rebuilding step and noticed its issue on a specific cylinder. Finally, culprit was found on the new conrod bearing shell. He wiped it off carefully with a smooth file. After a cautious cleaning, everything had come good. The new bearing shell on its ouside face was smally burred and when torqued stiffened rotation movement.
Hope that help.
Guy RENOU

This thread was discussed between 12/04/2010 and 15/04/2010

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