Bent Crank Shaft?
Discussion
I am rebuilding a Lotus Twin Cam, crank was reground by previous owner with +10 mains, and all the journals are good and round, but when I bolt up the main bearings caps to the right torque the crank is far too tight. I'm asking myself if the crank is bent.
I'm thinking how to check this at home, if I remove the bearings and caps from 2, 3 4, and bolt up bearings 1 and 5, and put a dial gauge in turn on 2, 3 5, any run out on these bearings would indicate, either that the crank is bent or that the regrind was not done in line.
Is my thinking right or is there some other DIY check.
Thanks
I'm thinking how to check this at home, if I remove the bearings and caps from 2, 3 4, and bolt up bearings 1 and 5, and put a dial gauge in turn on 2, 3 5, any run out on these bearings would indicate, either that the crank is bent or that the regrind was not done in line.
Is my thinking right or is there some other DIY check.
Thanks
It would be a good trick to regrind something and make it bigger but we'll assume you mean -10 mains 
Anyhoo, yes you check for straightness by resting the crank in lower shells on 1 and 5 (no need to put caps on) and checking 2,3 and 4 with a dial gauge. Ideally it should be dead straight but anything over 2 thou runout is a big problem. 1 thou I could probably live with. It's very unlikely that even a semi competent person would bend a crank while grinding it, I've never seen it for instance, but a crack somewhere can cause it. Hold the crank up by the flywheel mount in one hand and tap a crank web lightly with a hammer with the other. It should ring like a bell. If there's a dull flat sound this is bad n'kay. I suppose if one doesn't really know what sound to be listening for this might be a problem though.
Also obviously check journal sizes properly with an accurate mike, that the caps didn't get mixed up position wise (seen it so many times) and then just fit one cap at a time to see which if any locks it up.
If in doubt about the caps having gotten mixed up or even being from the wrong engine it's surprisingly easily to check from scratch with no other information and it's a skill every engine builder should learn. Fit a cap without bearings and then examine the honing marks inside it under a good light. You should be able to see the honing pattern continue across the split line with each "score" mark lining up exactly with its continuation on the other side of the housing. Like growth rings in a piece of wood or fingerprints the pattern on each journal will be unique in terms of the spacing and the variation in depth of the score marks and if they don't line up then it's the wrong cap or it's the wrong way round. You should also not be able to feel a lip on either side of the split line if it's the right cap. Have a practice by putting a cap in a known wrong position and you'll see what I mean.
Anyhoo, yes you check for straightness by resting the crank in lower shells on 1 and 5 (no need to put caps on) and checking 2,3 and 4 with a dial gauge. Ideally it should be dead straight but anything over 2 thou runout is a big problem. 1 thou I could probably live with. It's very unlikely that even a semi competent person would bend a crank while grinding it, I've never seen it for instance, but a crack somewhere can cause it. Hold the crank up by the flywheel mount in one hand and tap a crank web lightly with a hammer with the other. It should ring like a bell. If there's a dull flat sound this is bad n'kay. I suppose if one doesn't really know what sound to be listening for this might be a problem though.
Also obviously check journal sizes properly with an accurate mike, that the caps didn't get mixed up position wise (seen it so many times) and then just fit one cap at a time to see which if any locks it up.
If in doubt about the caps having gotten mixed up or even being from the wrong engine it's surprisingly easily to check from scratch with no other information and it's a skill every engine builder should learn. Fit a cap without bearings and then examine the honing marks inside it under a good light. You should be able to see the honing pattern continue across the split line with each "score" mark lining up exactly with its continuation on the other side of the housing. Like growth rings in a piece of wood or fingerprints the pattern on each journal will be unique in terms of the spacing and the variation in depth of the score marks and if they don't line up then it's the wrong cap or it's the wrong way round. You should also not be able to feel a lip on either side of the split line if it's the right cap. Have a practice by putting a cap in a known wrong position and you'll see what I mean.
Edited by KiaDiseasel on Sunday 23 October 16:21
Berw said:
I am rebuilding a Lotus Twin Cam, crank was reground by previous owner with +10 mains, and all the journals are good and round, but when I bolt up the main bearings caps to the right torque the crank is far too tight. I'm asking myself if the crank is bent.
I'm thinking how to check this at home, if I remove the bearings and caps from 2, 3 4, and bolt up bearings 1 and 5, and put a dial gauge in turn on 2, 3 5, any run out on these bearings would indicate, either that the crank is bent or that the regrind was not done in line.
Is my thinking right or is there some other DIY check.
Thanks
Are the caps in the correct location and orientation ?I'm thinking how to check this at home, if I remove the bearings and caps from 2, 3 4, and bolt up bearings 1 and 5, and put a dial gauge in turn on 2, 3 5, any run out on these bearings would indicate, either that the crank is bent or that the regrind was not done in line.
Is my thinking right or is there some other DIY check.
Thanks
Correct bearings ? Are they just 2 bolt mains ? Or are all the relevant fasteners tightened up ?
If you tighten each cap on its own...does it make a difference ? Is there one particular cap that really tightens it up ?
If the crank was bent, good chance it will mark some of the bearings as you rotate it...are there any markings on the bearings ?
I suspect the regrind is not straight, i.e. not all the bearings have been ground on the same axis, that's what I meant by 'bent when it was reground'. i don't know who did the regrind.
The Elan is one of only three in S E Asia, the previous owner spent a fortune, spyder drive shaft, mick miller water pump conversion, uprated cams, etc, but it was built by a butcher, water ways blocked with sealer, metric nuts forced on UNF studs, and the rebore was not in line.
Block has been rebored correctly. I was looking for an easy check on the crank, with what I have in the garage, so 'turning' the crank with a dial gauge is a first step as I have these, everything I read talked about doimg it on a surface plate with v blocks, but I thought it could be done in situe.
Plasgauge is next but that means a day trip to drive out to a specialist supplier.
After the previous owner had the car rebuilt, it never ran right and stood, he then died and the car stood for 7 years under cover in Mellaca, before I bought it. Rust is not an issue, but there were 5 carrier bags of bones on the 'transmission tunnel' where the rats lived.
All the bearing caps are correct and aligned right, they where all marked on the original casting, and were stored in order.
Ok -10 or +10, bearings are refereed to as +10, so lets not get pedantic.
Your advice appriciated
The Elan is one of only three in S E Asia, the previous owner spent a fortune, spyder drive shaft, mick miller water pump conversion, uprated cams, etc, but it was built by a butcher, water ways blocked with sealer, metric nuts forced on UNF studs, and the rebore was not in line.
Block has been rebored correctly. I was looking for an easy check on the crank, with what I have in the garage, so 'turning' the crank with a dial gauge is a first step as I have these, everything I read talked about doimg it on a surface plate with v blocks, but I thought it could be done in situe.
Plasgauge is next but that means a day trip to drive out to a specialist supplier.
After the previous owner had the car rebuilt, it never ran right and stood, he then died and the car stood for 7 years under cover in Mellaca, before I bought it. Rust is not an issue, but there were 5 carrier bags of bones on the 'transmission tunnel' where the rats lived.
All the bearing caps are correct and aligned right, they where all marked on the original casting, and were stored in order.
Ok -10 or +10, bearings are refereed to as +10, so lets not get pedantic.
Your advice appriciated
Berw said:
Ok -10 or +10, bearings are refereed to as +10, so lets not get pedantic.
On the contrary, I have found that a large part of getting things perfect, every time, is to always be exceedingly pedantic about even the tiniest details which is why I invariably am. Bearings can be sized to fit not just smaller crank journals after a regrind but also larger housings when the block has been line bored which was a common practice at one time to reclaim out of tolerance machining at the factory, particularly with Ford.So a main bearing might be stamped -0.010" for the journal and also +0.040" for the housing for example. I once briefly knew a right numpty who thought he could rebuild his own Cosworth engine with basically zero knowledge who had to do it three times over in the end trying to find out why it had no oil pressure. It transpired he was fitting std o/d bearings into a line bored block because he'd never measured the housings to check. The result was about 40 thou bearing clearance.
Doing the reverse of course, line bored bearings into a std block will lock your crank straight up!
Assuming the crank passes the crack test which is thankfully quick and easy, then I would double check that you have the correct sized bearing shells to suit the crank.
Plastigauge would help but you have everything at hand to check the basics which could save you a days driving.
When someone unknown has done work then always triple check for cock ups. Wrong shells right packaging, grinder with wrong info, change of spec in production run, mix up in workshop, anything can happen.
Plastigauge would help but you have everything at hand to check the basics which could save you a days driving.
When someone unknown has done work then always triple check for cock ups. Wrong shells right packaging, grinder with wrong info, change of spec in production run, mix up in workshop, anything can happen.
Berw said:
After the previous owner had the car rebuilt, it never ran right
If it's already been run then you would have had the evidence in front of you. Doing a PM on an engine is like doing one on a body, there will be clues and witness marks everywhere and the old bits should be kept for reference. If the crank had misaligned journals or was bent then was run you would have seen abnormal wear marks on the bearings and also the caps could be a sloppy fit.Plastigauge isn't really the tool for the job as you need to spin the crank, but it could be utilised to give you some references, obviously you'll need to tighten the whole lot up 2 - 4 times with the crank turned 1/4 to give you an idea of what is happening, but could well confuse more than teach. A dial gauge and mag stand are pretty idiot proof.
Edited by 227bhp on Monday 24th October 09:36
99hjhm said:
Slated and hated by many but it's pretty idiot proof. Sounds like a clearance problem or caps as Kia suggested.
And if the crank was bent....it probably wouldnt show anything for that same idiot.And of course if nothing is known about the engine at all..maybe the block is screwed and the crank is fine.
Lots of checking to be done.
Berw said:
I suspect the regrind is not straight, i.e. not all the bearings have been ground on the same axis, that's what I meant by 'bent when it was reground'.
Vanishingly unlikely. Once a crank has been set up between centres all the journals are ground in the same operation. Any error in centering will mean the crank doesn't rotate about its true axis, i.e. it'll be a tad out of balance, but it won't affect how it spins in the block.KiaDiseasel said:
Berw said:
I suspect the regrind is not straight, i.e. not all the bearings have been ground on the same axis, that's what I meant by 'bent when it was reground'.
Vanishingly unlikely. Once a crank has been set up between centres all the journals are ground in the same operation. Any error in centering will mean the crank doesn't rotate about its true axis, i.e. it'll be a tad out of balance, but it won't affect how it spins in the block.227bhp said:
It is possible though, I don't know how as my knowledge of the actual machine is zero, but I have seen the evidence first hand.
The odds are millions to one against. Unless the crank is cracked I'll wager it's dead straight. The problem could be all sorts of things. Bearing sizes, journal sizes, housing sizes or even the OP just thinking the crank should spin more easily than it does. It takes a lot of effort to get a crank to spin really freely in new bearings, like so freely you can give it a turn and it'll just keep spinning for a few seconds. I've been there with race engines trying to save the last ft lb of drag, polishing the crank with metal polish to make it mirror smooth, tickling the main caps to get the perfect fit. All wasted effort really because engines settle in after a bit of running to much and such the same end condition but you never stop trying to achieve perfection.stevieturbo said:
And if the crank was bent....it probably wouldnt show anything for that same idiot.
And of course if nothing is known about the engine at all..maybe the block is screwed and the crank is fine.
Lots of checking to be done.
True, but how many times have you seen a bent crank? And how bent does it need to be before it doesn't turn? I sent (admittedly a longer straight 6) crank for grinding and it was 15 thou bent, straight from a running engine. Of course it could have been dropped as could the OP's. And of course if nothing is known about the engine at all..maybe the block is screwed and the crank is fine.
Lots of checking to be done.
99hjhm said:
True, but how many times have you seen a bent crank? And how bent does it need to be before it doesn't turn? I sent (admittedly a longer straight 6) crank for grinding and it was 15 thou bent, straight from a running engine. Of course it could have been dropped as could the OP's.
I havent seen it often, but I have seen it.Likewise I havent seen many twisted blocks...but I have seen them. And either will cause a crank to jam up.
On a funnier note the first engine I ever built was a complete shambles. It was a Rover v8 and was so tight I couldn't turn it over unless I used an extension bar and some leverage. |The starter motor sorted it out, eventually and it was fine after that. I've no idea what was wrong but highly likely to be an incorrectly fitted cap somewhere down the line. That was a very very long time ago,
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