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Crank clearances in an alloy block with iron caps


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#1 mariner

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Posted 13 August 2020 - 19:54

I never though about this before as I have no reason to but reading about the very fine line boring tolerances on an alloy engine ( Jag V 12) I noted it has cast iron main bearing caps.

 

I think most alloy engines without a lower saddle like a DFV etc do have cast iron main bearing caps.

 

So, if main bearing bores are machined to very tight tolerances but alloy expands much more than iron , how do you keep the bearing bore fully round when the engine is working and hot?

 

Anybody know please? 



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#2 Greg Locock

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Posted 14 August 2020 - 00:20

The bearing is lined so there's no step at the join. The crankshaft is steel, so it expands  like the cap, ie the clearance increases when hot. Short answer, I don't know, I'm just thinking it through.



#3 Charlieman

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Posted 14 August 2020 - 09:18

Just working out aloud too.

 

Aren't we forgetting about the bearing shell?

 

The shell comprises two metals and if I recall correctly, it isn't round. It is designed so that there is a 'wedge' of oil under pressure on which the round crankshaft runs. The shell may be drilled or slotted. What is the bearing shell fit within the caps? What is the contact area between the upper and lower shells and caps?

 

I'm presuming that the upper crankcase is bored precisely to ensure that there is no crimping of the shells after the lower caps are fitted, whilst ensuring that the shells are fully supported. After that, the shells can do their magic.

 

Don't forget about engines with spherical and roller bearing cranks!



#4 mariner

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Posted 14 August 2020 - 15:18

The bearing is round but smaller than the crank journal. The action of the crank spinning in the shell drags oil into the gap and provides the dynamic oil film essential to engine operation. 

 

At start up the crank is resting on the lower half of the journal, thus creating the wedge into which oil is dragged.

 

Incidentally this is one of the objections to stop - start. Each stop drops the crank into hard contact with the shell and each start has metal to metal drag. If stop start wears out the starter fitting a new one is easy. If the crank wears goodbye engine!

 

As far as I know the bores for the shells are bored circular , unlike pistons which can be ground oval. The shells are flexible and very, very slightly oversize. It's the "crush " as the main bearing caps are pulled up which forces the thin wall bearing into intimate contact with the block and cap. 

 

That is the key to heat being dissipated from the shell  bearing sucessfully. IIRC Clevite in Cleveland USA invented the thin wall bearing which Tony Vandervell of Vanwall fame made under licence or similar in the UK.

 

So, I think the differential expansion question still remains?



#5 Fat Boy

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Posted 14 August 2020 - 17:34


 

So, I think the differential expansion question still remains?

 

Racing engine builders hone both the cylinders and bearing journals at operating temperature. This is one of the reasons it's vital to get your cooling correct within a few degrees while on track. The engines need to be pre-heated first thing in the morning to allow the expansion and insure the proper clearances at start-up. If you spin a bearing on these types of engines, it's almost always a case of starting it up when it's too cold.

 



#6 mariner

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Posted 15 August 2020 - 09:19

Racing engine builders hone both the cylinders and bearing journals at operating temperature. This is one of the reasons it's vital to get your cooling correct within a few degrees while on track. The engines need to be pre-heated first thing in the morning to allow the expansion and insure the proper clearances at start-up. If you spin a bearing on these types of engines, it's almost always a case of starting it up when it's too cold.

 

Than k you, now I get it, they must ahve to use some sort of pre heating then machine quite fast I guess!!



#7 kikiturbo2

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Posted 16 August 2020 - 15:54

They will pre-stress the block by putting on a torque plate and fit all the mains, and then circulate water at operating temp during the whole boring/honing procedure... This will also limit thermal expansion and deformation of the bores that happens on blocks that are not heated/cooled during machining..



#8 Kelpiecross

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Posted 17 August 2020 - 09:57

Than k you, now I get it, they must ahve to use some sort of pre heating then machine quite fast I guess!!

 

 There are so many alloy block/steel cap engines about  that I doubt if pre-heating/boring etc. would be economical.  I suspect the caps are made a smidgen bigger in diameter so that when the engine is hot everything lines up.  



#9 Fat Boy

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Posted 17 August 2020 - 21:27

Of course. Pre-heating a street car engine prior to firing it up is also a non-starter. Street engines are set up artificially loose, as you say, and then have a bunch of oil shoved at them to keep the bearings happy at temp with overly wide clearances.



#10 Greg Locock

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Posted 18 August 2020 - 00:00

When i worked on engine noise i found that matching the main bearing diameter and the big end diameter gave a small but measurable (and subjectively discernible) improvement in sound quality.The plant hated me. That got canned fairly quickly.



#11 mariner

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Posted 18 August 2020 - 08:53

WAY back when I was a school leaver to be I applied to Vauxhall (GM UK) for a training course, finance not engineering, but as part of the induction we were given a question sheet and let loose around the Luton plant. I didn't get a job but the plant visit was great for a young petrolhead.

 

On the engine line a guy used a range of plug gauges to work out the right piston size for each bore. Than another guy picked the required set which travelled with the block to installation - hopefully in the correct bore. I suspect grinding tolerances are much better now.

 

 

The other one was the seat build area down in the plant basement. Frames had  the covers fitted using hog rings and very big plies. The joke was that when I eagerly asked what the next step was to score points on my questionnaire the foreman replied " no idea , they go up through that hole and I've never seen up there since I started!



#12 Greg Locock

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Posted 18 August 2020 - 23:26

Dunton used to length match conrods to block height for diesel - where it is obviously more important. 



#13 mariner

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Posted 01 September 2020 - 18:23

It's probably a bit tacky to try to extend your own thread but by chance I found some comments by Wally Hassan, the Coventry Climax design chief on bearing clearances which are interesting. I am no engineer but he certainly is!

 

In his talk to the SAE in Cleveland he covered the development of the FPF four cylinder engine leading to the 2.5 litre version ,achieved with a bigger stoke . This resulted in some big unbalanced forces due to limits on crank balance weight space. This caused the main bearing diaphragm to split above the bearing hole.

  

To fix it they switched from light alloy main bearing caps to steel ones for more rigidity. This worked but "also as previously mentioned, has had the effect of maintaining a higher oil pressure at higher temperatures than was the case with light t alloy bearing caps , due, no doubt , to the smaller expansion of the steel cap." " Differences of 10-15 lb per sq. in. have been noticed between otherwise identical engines except for the bearing cap materials" 

 

So I guess the holes were not perfectly circular when hot!



#14 Kelpiecross

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Posted 02 September 2020 - 04:39

Of course. Pre-heating a street car engine prior to firing it up is also a non-starter. Street engines are set up artificially loose, as you say, and then have a bunch of oil shoved at them to keep the bearings happy at temp with overly wide clearances.

  

 

  I meant pre-heating before machining - not start up.        



#15 Lee Nicolle

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Posted 05 September 2020 - 05:45

Iron caps is defenitly no. Steel caps yes. 

I suspect some engines have alloy caps too. 

I have heard of preheating blocks to bore maintunnels though to me  torque plate honing the cylinders warm would be more important. And very hard as well.

I own a V8 petrol Landcruiser [alloy block] which rattles badly when cold.  But quietens down within about 45 sec. It does sound like an engine with a broken piston skirt. And having experienced many of them a bit scarey.

All engines I have ever built were production blocks and caps of several brands.

Alloy sprintcar blocks use steel caps. As do most iron ones as well. And to my knowledge are align honed the same on the same machine.

I have used synthetic oil for near 30 years. And have built engines with  bit less clearance on the crank. 1.8 to 2 thou and have not had a bearing problem. But would if using a premium 25-50 mineral oil. Those were 2.5-3 thou. And still marked up bearings. Using synthetic means more power as the pump does not work quite as hard, which in turn puts less heat in the oil. And less oil pump drive gear/ belt wear as well.

As for road car engines having more oil clearance? Sorry. A racing engine of any sort has a lot more clearance than  factory tolerances. See above. Factory is generally around 1.5 thou and sometimes less. Though with these modern turbo and s/c engines I suspect that is opened up a bit.


Edited by Lee Nicolle, 12 September 2020 - 08:58.