Big ends

[Steve kay]

To start with, the usual apology if the question is displaying technical naivety, or possibly ignorance but comments and help will be much appreciated. An engine is being put back together, onto a crankcase and crankshaft that may or may nor have seen much service. Con rods ar first glance were identified, at second glance marks were numerous and confusing. Therefore con rods and big ends were offered up to journals, but attempts to find four compatible matches was not possible. Two big ends were tight on each and every journal, one failed to tighten up, tiny but noticeable gap between the halves of the big end. Now how tight should the big end be? Tightening up to indicated torque should not end up with it virtually solid, or would that be eased off by rotation under power and with lubrication? How do the experienced forumists judge the correct tightness of the bearings?

[David Stepney]

First question is: shell rods or Babbitt rods? If Babbitt rods, you will need to blue them up with a light smear of engineers blue, tighten them until the will just turn and turn the engine over a couple of times. Remove the caps and see where the crank pins are touching. The blue will have rubbed off. Scrape gently where it has, blue up again and repeat the process. Keep on repeating the process until each bearing is no longer tight. The cognoscenti say that, with the crank and rods out of the engine, ,when the rod is put at right angles, it should just move to the bottom under its own weight. Personally, I have cheated a bit and scraped them with the engine built up until the engine turns easily with no sideways play in the rods. The rods should have a tiny bit of endwise movement (just a couple of thou) along the axis of the crank pin.

[Howard Wright]

Hi Steve

You would be extraordinarily lucky to get four unmatched con rods and ends to fit a different crank without some scraping.  But first you need to make sure that the rods and crank are both the same size.  The crank may have been reground so it is less than it’s true diameter and the rods may have been been bored to this reduced diameter.

If you can establish that both crank and rods are the same nominal size (micrometer) you need to blue the journal, fit the rod and cap and check where the blue is being removed or marked (I find I often don’t need blue as I can see the smooth interference marks on the white metal). The “high points” need to be carefully scraped using a sharp engineers scraper.  It’s a very long process.  The aim is that the rod should just fall under its own weight.

Hope this helps?

Cheers

Howard

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Hi Steve (again)

I think you really need to do some reading before attacking the crank rod assembly.  I think both Woodrow and Williams are helpful but there is also a lot of info on the web.

Cheers

Howard

[Robert Leigh]

Are you sure that the caps are matched with the rods to which they were originally fitted? I have hand scraped 10 thou undersized big ends to fit a new shaft; it takes time but it's cheaper and quicker than sending them for remetalling. Old rods often have hacksaw marks to identify 1,2,3&4. In such cases I reject them. If you plan to use full revs as I do they are not safe from breakage. When scraping to fit I tighten the rod dry onto the shaft to the point where it will just turn; after one turn the white metal is marked at the tightest points where a small amount of white metal needs scraping off. Repeat until it feels just slightly tight to turn when the bolts are fully tightened. At that stage loosen a bit and apply oil to the bearing before retightening. If you have things right the rod will just fall slowly under its own weight from horizontal; if not a little more scraping is needed until the gravity test works. If any of your rods will not tighten to turn without wobble they need remetalling.

[Chris KC]

I believe most of what's been said above is good advice; but I'm still slightly perplexed by the basic question.
Is it to be understood that these are a random pile of rods and caps which match neither each other nor the shaft? 
Or is there reason to believe they all ran together at one time?

Sounds like you've exhausted the combinations, but a forensic approach might help - for example can you see any continuous marks on any rod / bearing which match up to corresponding marks on any cap?

I'm no expert on white metal bearings nor the fitting thereof, but I do think the rods and caps must seat together properly under bolt tension and if that's not the case it should be addressed before any foray into bearing scraping. If the two halves of the rod have a visible gap between them trouble will follow.

[Colin Reed]

HI STEVE,
Before you do anything else measure all 4 crank pins ,
assemble the rods and caps and take at least 2 measurements per rod 
you need to know this before starting to do anything 
then make a decision on the best cause of action to take
this will save a lot of time

Colin
NZ

[Bob Culver]

If the caps have not been relieved full width at the parting line probably only the original combination will likely fit without a step. Also the side faces will likley only be flush on the matched pair.Even the crankpins may not all be the same size. And as above wear patterns and score lines will often identify pairs. Scraping was covered at length recently. Alamost anything tight can be fitted but not really for the raw amateur. Caps have often been crudely filed. Shims are not a sin and many cars had as standard. if the crankpins are perfectly round the falling by own weight notion is really too tight. If going to fully tighten bolts a myriad times best to use an old dedicated pair. Relatively inexpensive direct reading vernier callipers are readable to about .001 either way and can be used as external/internal comparators, although white metal easily damaged.

[Stuart Giles]

I'm no expert on white metal bearings nor the fitting thereof, but I do think the rods and caps must seat together properly under bolt tension and if that's not the case it should be addressed before any foray into bearing scraping. If the two halves of the rod have a visible gap between them trouble will follow.

Just to add to Chris's comment above, many big end caps will have been filed in the past to reduce clearance to the crankpin. Sometimes the caps have more filed of one side than the other. So it is well worth checking that the cap seats nicely on the rod before tightening the nuts down; otherwise the resulting bending load on the big end bolt(s) is asking for an expensive failure when the motor is run.

[Hedd_Jones]

Steve

Sounds like a proper bit of fitting, rather than swapping parts which is the norm nowadays.

With respect to tightness of a big end, and having done a bit of this sort of thing. I was taught that you want the rod to be able to rotate on the crank under its own weight. if it won't its too tight.

How loose it needs to be on the rod is more subjective!. Using something like plasti-gauge here works well in the absence of proper engineering grade measuring tackle.

With respect to the bolt tightness, there will be someone on here will quote some torque or another, and for highly stressed engines I will not disagree with the theory that it is good everything is equal and 'tight'. However when doing this sort of fitting work you will very quickly learn that the tightness of the bearing is often dependent on how tight you do the bolt up. I cant really convey on the computer what I think would be OK, I had my dad double check me when I was learning such dubious skills. Nylock nuts are a godsend in this application. What you need to remember is the bolts are in the general scheme of things pretty small, and a decent spanner puts easily the necessary torque on the fixings.

So how to get your tight big ends to fit. Do you have blue? blue them and see where they touch, scrape the high spots. Slowly slowly. a bit at a time. If you believe they are somewhere near, Timesaver lapping compound is good (so long as you keep it wet), if your bearings are really tight and seem to fit the journal well, you might need to shim the cap out.

I am fully aware that there will be forum readers swearing at the screen of their computers, frothing at the mouth ''you can't do that!'', but I would contend, that, yes, actually you can. People have been doing it for decades. Indeed when our cars were being maintained in the years immediately pre war and later, this would have been commonplace.

Please, if you bought that Speedy Crankcase on e-bay, and plan on 24hours at LeMans - don't do it, but if your building what might be a spare engine for emergency use out of 'sh1t' get stuck right in. If your somewhere in the middle, take a view.

[Chris KC]

Definitely subjective Hedd. I'm reaming some small end bushes at the moment and recall a few years back bottling the job and taking it to my nearest engine workshop. They came back just as loose as the ones I had condemned and replaced! 

On a sporting engine at least, I always feel a bit 'fast and loose' is better than micro-tolerance tight.

Bob also mentioned shimming the cap out and I let him get away with it too! But never on a shell bearing, please...