Big end bearings replacement

[Mohamster]

Hi There, I have two related questions:

1- Is it possible to check whether or not big end bearings need replacement without taking apart the crankcase?

2- Is it possible to replace the big end bearings without taking apart the crankcase? I know you can get the rods off without dissassembly of crankcase, but I'm wondering about actually getting them back on and fastened to the correct torque.

I've been replacing my head gaskets and while I am at it I thought I'd replace as many components as I could without splitting the crankcase.

Thanks for any advice!

[itchyfeet]

It's a good question, it never occurred to me before when I had taken heads off to do this.

You would find it hard to measure the journals with a micrometer but you could measure the old shells with a bore guage in case it has been reground in the past.
Early DG does not have stretch bolts in con rods so you can reuse ( worth checking they are not stretch, may not be origional )

[Mr Bean]

Due to the laws of trigonometry any play in the big ends results in lost motion of the piston in the crank rotation which is minimum when the crank is at right angles to the cylinder center line and maximum when the crank is in line with the cylinder center line at top dead center.
If all this is too complicated for you then ignore it and do perform the following test:
Remove the spark plugs and on each cylinder at a time find top dead center and rotate the crank slowly back and forth over dead center. ideally you would use a digital probe down the plug hole to sense piston motion. The crank rotation which results in no piston movement over top dead center is a measure of the big end wear. If you had the maths capability or knew someone who has you could actually calculate the big end play in this way however remember that the maximum forces are at play around TDC so the lost motion will be small to virtually nil when the crank is at right angle.
Could cheer you up or break you heart/bank
Good luck
CS

[Mohamster]

On balance, at this stage I think I should short of going into the big end bearings. I know that perhaps I'm taking a risk and missing an opportunity, but I need to get this van back on the road.

That said, I like your method CS as its non-invasive! .. feel confident enough with the trigonometry to give it a go. Guessing it can't really be relied upon to yield good results without the pistons in place because of the lack of realistic forces at play?

Itchy thanks for your thoughts I've been following your engine rebuild thread with great interest and admiration for the amount of attention you're giving to the work. You've done a lot more than me .. but I think we'll both be rolling again around the same time ... fingers crossed

[itchyfeet]

I think we'll both be rolling again around the same time ... fingers crossed

I'm already rolling with a spare engine, never going back to working on the only engine I have

https://club8090.co.uk/forum/viewtopic.p ... 0#p8146661" onclick="window.open(this.href);return false;

[kevtherev]

It's becoming a trend now I think

Re bearings
If they are sensibly quiet I would suggest that shell replacement would not be high on my list
Oil pressure is generally considered an indicator of excessive wear.

[Mohamster]

I'm already rolling with a spare engine, never going back to working on the only engine I have

Bit of a noob . . . Next project I guess

Re bearings
If they are sensibly quiet I would suggest that shell replacement would not be high on my list
Oil pressure is generally considered an indicator of excessive wear.

Aahh .. *penny drops* more wear means less resistance to oil flow around crankcase means lower oil pressure *facepalm*. Gonna invest in an oil pressure gauge as part of this job and keep a record of oil pressure for internal benchmarking purposes

Thanks for all the useful inputs

[Mr Bean]

Yes oil pressure gauges along with regular oil changes etc, are the best way to keep tabs on the condition of your bearings etc and in some cases will highlight problems not related to bearing wear before the bearings are damaged.
I tend to avoid oil pressure gauges on my road vehicles though as I can't stand the way the pressure drops off on tick over when the engine is hot. I guess the low oil pressure warning light is best for me as it self tests every time you start the engine.
After many years of messing with engines maybe I am an Oil Pressure Hypochondriac!
CS

[Ian and Lins]

It's a good question, it never occurred to me before when I had taken heads off to do this.

You would find it hard to measure the journals with a micrometer but you could measure the old shells with a bore guage in case it has been reground in the past.
Early DG does not have stretch bolts in con rods so you can reuse ( worth checking they are not stretch, may not be origional )

If I read Itchyfeet correctly you can take out big ends/mains through the crankcase? If so would this help at all?

http://plastigauge.co.uk/" onclick="window.open(this.href);return false;

[Mohamster]

If I read Itchyfeet correctly you can take out big ends/mains through the crankcase? If so would this help at all?

http://plastigauge.co.uk/" onclick="window.open(this.href);return false;

I think that's right, but I've opted not to because I feel like I'm just courting extra work from a precautionary angle when actually I had no prior indication that those bearings were worn. Also, i've been good with oil changes as was the previous owner.

That said, those plastiguages look pretty nifty! Good tip

I feel like I got the answers I need plus some nice extra info on big end bearing issues. So, I'm thinking to write a final post summarising the issue and extra stuff learnt. Perhaps could add to the Wiki too?

[itchyfeet]

If I read Itchyfeet correctly you can take out big ends/mains through the crankcase? If so would this help at all?

http://plastigauge.co.uk/" onclick="window.open(this.href);return false;

Thats no good they are imperial, VWs are metric

interesting it goes form 1 thou to 7 thou

[Mohamster]

Thats no good they are imperial, VWs are metric

interesting it goes form 1 thou to 7 thou

Well spotted. Could have ended up with another Space-X like failure due to unit inconsistencies

Nonetheless, really interesting operation principle

http://plastigauge.co.uk/how-plastigauge-works/" onclick="window.open(this.href);return false;

Put the plastiguage onto the journal surface, then put the bearing on and pressure. The area covered by the compressed plastiguage and its width then dictates the clearance. I'd be a tad worried about propagated errors in measurement, but still it's pretty cool!

[Mohamster]

Although they do specify that it's particularly good for measuring large bearings such as in marine applications.

[Ian and Lins]

Been around for ages. I remember hearing about them in the 80's, maybe even earlier? This was about the time I found either the big ends or mains had gone on my Triumph T150V Trident. The signs were a large hole in the bottom of the crankcase and half a con-rod poking through. Bit of a give away, that.

[Mr Bean]

Those were the days! When Molyslip came out I used it in my BSA A10 and big ends came undone! I guess I was a big green in those days and probably didn't use new fixings. On our first run up the new M1 many of our "Group" damaged out engines.
CS