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MG MGB Technical - Crankshaft Won't Turn When Torqued

The problem is simple but the solution is not clear! I am in the process of putting my 79 MGB 1800 series engine back together. Crankshaft was turned to .10, bearings were ordered to .10 (they also are marked as .10) when I torque the main bearing bolts and the pistons down to the required torque settings I can not turn the crankshaft by hand. If I back the piston nuts to 26 ft. lbs and the main bearing bolts to 50 ft. lbs. I can move the engine. When I try to go above either setting the engine/crankshaft will not turn by hand. Question is! what is causing the engine to bind-up (I checked the torque on all bolts associated to the crankshaft and worked my way up/down the scale till I reached the torque values noted above). Can I, dare I, put the engine together knowing the torque values are not to spec? And if I do will it ruin the engine? I do not run the car hard nor have I selected to up the performance levels for the engine, i.e., I am returning the engine to stock condition minus have the engine bored to compensate for wear and tear. Also the machine shop I used has a good reputation with high ratings from everyone I talked to. He used my manuals to make sure of the specifications when he turned the crankshaft.

Thanks,

Jim
Jim McDowell

Did you check bearing clearances with plastigage after receiving the crank from your machinist or inside and outside mikes? Did you use assembly lube on all rotating bearing surfaces?

It's possible that the radius on the crank throw is too large and is getting clamped on the edge of the rod bearing. Same goes with the main journals. Or there is some dirt under a bearing shell.

You need to isolate where the problem is. Loosen the rod nuts and tighten to snug only. Everything turns? Tighten one main cap (to the correct torque) at a time and check for binding. Add another main cap to correct torque and check for binding. When you run out of main caps without binding you know the crank isn't bent (much) or the bearing clearances too tight. May be too loose, though. Check clearance for crank end float. OK so far? Tighten one rod to correct torque - check for binding. Check rod side clearance from each side. Do another, and another, and the last. You should be able to tell which rod is binding this way. If the first one binds, loosen and do another. Maybe they will all bind individually. Same with the main caps.

Wayne
Wayne Pearson

Wayne,
Thanks for the quick response! In sequence to your questions:
1. No I did not check bearing clearances after receiving the crankshaft back from the machine shop (should have done it)
2. Yes I used assembly lube on all rotating bearing surfaces.
3. Yes I did check the crank end float clearance, it was within tolerance.

I loosened all rod and main cap bolts/nuts then starting with the main cap bolts begun tightening them from front to rear going to torque then trying to hand turn the crank, back off on the torque till the crank turned. I did this on all main cap bolts. Proceeded to the rod bolts, front to rear and did the same. As stated the torque value I could get to and still turn the crank was 26 lbs. on the rod nuts and 50 lbs. on the main cap bolts. The values are consistent for the main cap and rod nuts. My next step, unless someone can come up with another item to check, is to pull the crankshaft and check the measurements.

Jim
Jim McDowell

You don't need to pull the crank in order to check it with plastigage. All you need to do is set a short strand of it on the journal before you install the bearing and cap. Then torque it down like normal. Then remove the cap and bearing. A scale is printed on the paper that the plastigage comes in. The clearance is measured by how much the plastic squishes out. Sounds like the clearance is too tight but this will definitely tell you where and by how much when checking caps individualy as Wayne suggests.

-Jared
Jared Snider

Jim, try removing all the connecting rods and see if the crank rotates freely when torqued. If it doesn't turn easily, have the block align bored and the crank straightened. Then check each piston individually to make certain you have the proper .004" (I believe that's what Peter Burgess recommends) cylinder wall clearance in each respective cylinder. If they are not correct, have the cylinders torque plate honed to fit. Then be certain to check the ring gaps in each cylinder and adjust as needed.

Actually Jim, you need to remove all the oil from your crank and bearings in order to properly plastigage them. Do yourself a favor and go through each step thoroughly - and if you don't have an accurate set of micrometers to measure all your clearances, bring the engine to someone who does. I cannot stress how extremely important this is, unless you want to rebuild the engine again next year.
Good luck and keep us informed!!!
Jeff Schlemmer

Jared. Note that Jim states he has used assembly lube. That means he does have to pull the crankshaft and con rods, clean everything well, then assemble to use the Plastigage.

Jim. Clean and Plastigage all the bearings and the journals. All of the clearances should be within specification. The way I am reading your post is as follows:

1. You can tighten the main bearing caps to full torque specification and the crankshaft turns freely.

2. You can tighten the con rod bolts to some lower specification and the crankshaft will turn freely and the pistons will move up and down in the bores. But, when the con rod bolts are tightened to full torque, the crankshaft is binding and does not turn.

If my reading of your information is correct, it would indicate that there is no problem with the main bearing journals. (Still a good idea to check their clearances.) Nor, is there a problem with the piston rings being too tight since the pistons move when the con rod caps are at something less than full torque. (Again, it is an excellent idea to check the end gap of each compression ring as one prepares to assemble an engine.) This leaves us with one of two possible problems:

1. The connecting rod bearings are not a correct fit on the crankshaft or the crankshaft needs the con rod journals turned slightly more. Inspect with Plastigage and, if the clearances are shown to be incorrect, measure the journal lobes and determine if it is the journal(s) which have been incorrectly ground. If not, measure the thickness of the bearings using a micrometer and a ball bearing or a piece of rod as per the photo in Peter Burgess' book.

2. If the bearing clearances check to be correct, you simply need to apply more force when turning over the engine. An engine that has been machines to acceptable tolerances is still rather tight when first being assembled. I have to use a long screwdriver and lever against the dowel pins at the back of the crankshaft when turning the crankshaft on my current rebuild (going in this week). This is normal and represents the piston rings dragging against the honed cylinder walls and the thick assembly lube on the bearings. I cannot turn the engine over with my bare hands like I can when the crankshaft alone is installed and torqued down. This is normal and it may be that you do not have a problem. But, for piece of mind, check the bearing clearances and ring gaps to make sure. Les
Les Bengtson

Jim,

This is a straightforward problem which you can do little to sort out and there is no point in taking any measurements at all. Take the whole lot back to the machine shop and get them to check and rectify. The bearing clearances/fillet radii are obviously too tight and you MUST NOT under any circumstances try to run an engine in this condition as it will immediately overheat and destroy the bearings. At the end of the day it may be that the dimensions you provided were at fault but you will have to wait and see when the machine shop completes it's measurements.
Iain MacKintosh

Before rushing to the machine shop, take the crank out and clean it and the bearings carefully, then check these things:

1. Are all the bearing recesses and caps clean and free of burrs?

2. Are the shells fitted correctly to the recesses and caps, tongue in the recess?

Fit the crank and the front cap, torque it down to full spec -

3. Does the crank turn under light hand pressure?

If not then spin it a few times and remove the cap - the bright areas of the shell will show you where it is binding. Use plastigage to measure the clearance.

If it turns OK then repeat for the rear cap then the centre one, then the two intermediates - make sure the word FRONT faces forward one the middle 3 caps.

Do the same for the rod caps, but before starting, check the numbers on the caps and rods - THEY HAVE TO MATCH!

The caps must also be the right way round with the lugs on the shells next to each other and the numbers next to each other.

Do things one at a time and note where the binding starts - that is where the problem is.
Chris at Octarine Services

Chris is absolutely correct but I gave Jim the benefit of the doubt in that by now he had checked all these points, had been scrupulously clean and had refitted fully dressed parts in their proper positions. Under these conditions it can only be back to the machine shop.
Iain MacKintosh

You didn't mix up the connecting rod caps did you? I had the same experience where I torqued down the rod caps and the crank wouldn't turn. After some investigation, it turns out that I hade mixed up a couple of caps. Take the bearing shells out of the rods and put the caps on. If you can feel the mating surface with your fingernail, chances are you've swapped rods/caps. You shouldn't be able to feel the edge of the mating surfaces because the big ends are honed to size. If I had stamped the numbers in the rods and caps, it wouldn't have happened.

Steve Mc

Hi,
Did you set the thrust bearing/crankshaft endplay?
A lot of people who assemble their own engines skip this step.You should be able to turn the crank by hand after the thrust bearing clearances have been set. See the engine assembly manual and perform the sequence as described. Basically tighten the main caps to snug and give the crank a stiff whack with a lead mallet/hammer to send it to the front of the engine. Then give it a whack to send it back towards the rear of the engine.. then tighten as per spec. This operation has the effect of lining up all of the reciprocating parts so that they do not bind against each other. Recently I saw an episode of Dream Car garage and they were building a L88 Corvette engine and this subject came up, when they were installing and setting the position of the crank in the block. They also mentioned that a lot of times this step is skipped as "not important" and as a result, when the bolts are torqued to spec, the crank will not turn or is difficult to turn. FWIW Alan
Alan

My first and only rebuild I blundered and reversed the con rod caps. Same problem as Jim,crank wouldn't turn without quite a lot of force. Lucky I didn't do any damage.

Corrected the mistake, and all OK.

Do the easy part- make sure you assembled the caps correctly and check clearances with plastic guage.

Phil Colura

Steve,

the 18V engine conrods come already stamped with a three digit number to identify rod and cap.
Chris at Octarine Services

Good to know Chris. Thanks. If I remember correctly, this one was an 18GC with "angled" rods. I had always made a habit of stamping the rods and caps on any engine rebuild, but on this occasion laziness got the best of me and I marked only the piston tops with an indelible marker. Shameful, I know. Haven't made that mistake again.
Steve Mc

Yes, early angled caps were not numbered and need to be marked on disassembly.
Chris at Octarine Services

Hi Folks:

Planning a 18GB engine rebuilding in the near future. The block has been prepared for a 0.010 oversize bore (have NOS 0.010 pistons)all new oil plugs, cam bearings and main bearing caps align checked. I plan to use a NOS crankshaft and NOS angled connecting rods. Since everything almost new, are there any pitfalls with this plan regarding clearance checks. Any thoughts would be appreciated.

Thanks: Rich Boris 67 B Roadster
Rich Boris

I don't care if they are all brand new off the shelf, all parts need to be checked - assume nothing!
Chris at Octarine Services

Thanks to all for responding to my original problem/request for help. In the end I removed the crankshaft, returned it to the machine shop were they measured it and informed me they did not take it down far enough when they machined it. They are going to machine it down by 1000th and give me a call when it is ready for pick-up. They apologized for creating the problem and said it should not have happened and they would take extra care the second time around.

Again thanks for the responses as I learned more about the engine by having the problem then if all would have gone well!

Safety Fast!

Jim McDowell
Jim McDowell

re:

Posted 14 January 2004 at 17:51:23 UK time
Chris at Octarine Services, Essex, United Kingdom, chris.betson@btinternet.com; http://www.octarine-services.fsnet.co.uk
I don't care if they are all brand new off the shelf, all parts need to be checked - assume nothing!

*******************************************************

These are words to live by! I replaced the fuel pump in my MGC about 1999. I had problems with the MG running like crap on my way up to Michigan....almost like it was running on three cylinders....I checked almost everything....except for the fuel pump...because it was new, off the shelf!

Guess what was bad.

My then 12 year old stepson, Tim, just HAD to remind me that one of my favorite lines was "Just because it's new, doesn't mean it's good!" .... Out of the mouths of babes....

rick

rick ingram

This thread was discussed between 12/01/2004 and 19/01/2004

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