Scraping white metal bearings

[Colin Wilks]

When I disassembled my RP engine to sort a savage oil leak I was surprised to find that one con rod was very tight on the journal and another one had been encouraged to "falls under its own weight" state by the expedient of leaving the castellated nuts not much more than finger tight, although captured by a split pin.

I have spent many hours so far blueing the journal, nipping up the rod and cap and then scraping the blued areas to remove a British Standard Gnat's. My scraper is sharp and well lapped so taking tiny amounts is not difficult, and I debur with a rag from an old denim trouser leg. I am ensuring the absolute minimum of blue on the journal, so I am confident I am colouring only the points of contact, with no "squidging". Interestingly, I get a very different contact picture if I rotate the rod and cap on the journal separately. My MO is to nip the the nuts up with a short ratchet each time, so I'm scraping what will be there in practice.

As a first venture into the art of scraping, I am happy with the results I am getting, but wondering if I am missing a trick, as it does take forever, although the whole experience is quite zen like.

How do others perform this task, and are there any short cuts I am missing? 

[Howard Wright]

Hi Colin

If you are missing a trick....so am I.  It took several days on and off to get the bearings on my car to “fall under their own weight”. Then when everything seemed just right I nipped up all the nuts to the final torque.....only to find one rod sticking as I turned the crank in the engine. Eventually I got things as I assumed they should be.

Good luck,  it will run nicely if you persevere 

Cheers

Howard

[Bob Culver]

It is unusual for a rod significantly tight to remain that way. The purpose of running in was to melt the high spots and the surface usually shows signs of smearing if originally tight. On the other hand the oil film attains immense separating pressure.
Hardly dare ask but any chance a cap get mixed or reversed? The bolts run slack must be replaced as this is the condition which causes fatigue. (As does repeat tightening!)
If the crankpins are at all oval contact assembled only occurs with the unworn high part, which may give a different pattern than when the halves are tried separately on the sides. Slight distortion occurs when the cap is tightend, esp if the caps not perfectly filed.
Experienced persons can quickly fit rods of quite seriously different size. The main art is avoiding corrugations and keeping the rod square. (Expereinced persons can also reduce ovality of the crankpin!)
Modern practice is to maintain a running clearance which makes the final fitting somewaht blind.
It was considerd good practice to fit down the caps and lightly scape good bearings. This removes the abrasive layer of embedded dirt which eventually builds with no filter.
The oriinal Austin bolts are a very precise fit, i dunno about replacements.

[Colin Wilks]

Bob, I'm interested in your comment "Slight distortion occurs when the cap is tightend, esp if the caps not perfectly filed.", which would explain the different contact patterns I described. I have not really considered trying to improve the alignment of the rod and the cap as I'm not sure where I would start, and taking any material off the jointing surfaces will make the rod tighter on the journal. How would you go about working out which shoulder to reduce, or are you meaning just a debur and polish? I wondered if the rods were fouling the face perpendicular to the journal as I could get only a .006" feeler blade in. I bolted the rod and cap together and filed a total of about .010" off, so the outside babbitt faces are true to one another. I am still a long way from Woodrow's recommended maximum of about .060".

I am as confident as I can be that the rods and caps are correctly numbered and orientated, and have test fitted them all in different permutations, so I am starting from how the engine was before disassembly and from the best starting point.

The car has done less than 1,000 miles since a orofessional rebuild ten years ago. It had done only 350 when I got it. The babbitt in the tight rod had blurred across the joint as you describe and I reckoned I was lucky not to have run the bearing, so in fact the oil leak did me a favour!

[Colin Reed]

HI Colin,
Is this on a rebuilt Engine or one that has a few miles on the clock,
I am assuming the crank is out of the crankcase .

Colin Reed
NZ

[Stuart Giles]

A good way of seeing what's really going on with big end clearance is to use stuff called Plastigauge. With a 2 bearing motor you really want a bit more clearance at the outer edges of the bearing than at the centre. Plastigauge comes in round strands, bolt up the bearing with a strip in there and then you can use the supplied card to tell exactly how much clearance you have.



This pic is from a V8 that I've just built for a mate, can't find one of doing an A7 motor at the mo' but the stuff obviously works the same way.

[John P]

Many years ago I white metaled my own rods initially machining the bore and then scraping generally as you discussed.

The only thing I do recall is that I had some similar issues to you which eventually turned out not to be the bearing diameter itself but the radius where the big end white metal is machined to clear the radius on the crank.

I eventually slightly scraped this radius and all was well.

John.

[Colin Wilks]

HI Colin,
Is this on a rebuilt Engine or one that has a few miles on the clock,
I am assuming the crank is out of the crankcase .

Colin Reed
NZ

As described, the engine has about 1,000 miles on it. This should have been a simple replacement of the camshaft bearing retaining bolt, but having raised the block a little I decided to take it off and check the pistons, which I am replacing. The engine is now out of the car, but I have left the crank and flywheel alone as there is no discernible play so I'm leaving well alone. My objective is to get the car back on the road asap, so I am not going to strip the engine further just now. The crank has been reground and maximum ovality of the big end journals measures less than .001".
Of course, if you have a compelling reason to take the crank out I can do so.

I've got two sevens, both without engines at present, so my plan is to acquire the bits to build up a good spare in order that I can keep moving. I enjoy the mechanicing, but what I really want to do is use the cars!

[Colin Reed]

Hi All,
If remetalling rods the surface between cap & rod are machined or filed after white metalling and assembled then machined to suit reground crank which means the two thrust faces also have the correct clearance and (No scraping is required ) 
When rebuilding an engine it does not make sense to remetal the rods and leave the crank as is 
 normal practice is to grind all big end Journals to the same under size  and machine rods to suit end result perfect fit.

Colin 
NZ

(PS ) do not fit tight bearings hoping the high spots will melt down

[Bob Culver]

.060 seems an absurd side clearnce, likely to knock if the rod is pushed sideways on each stroke. Some clearance is necessary to allow oil to escape, although the holes in top largely acheive this.
Plastigauge is intended for modern bearings with a definite clearance. Likely to damage a bearing with no clearance. Beware old stock as goes hard. For a stock car unlikely to exceed 50 mph or equivalent paralledl big ends fitted to fall easily seem to work and last just fine.