6 replies to this topic

[nky_84]

Hi guys,

 

So i've just learned the hard way that fitting the big end caps the wrong way round is bad.  The car struggled to start and then wouldn't run very well once it did start.  I've since taken it to bits and realised my mistake....

 

The car hasn't moved, just been run up to temp twice, so maybe 10mins total running time.  The shells are all very shiny on one side and one of them is worn through the coating. The crank doesn't appear to have been damaged at all thankfully.

 

Before i order new shells, is there anything else that i need to check for damage etc when this level of dumb has occurred!

 

Also, if anyone can explain how / why getting this mixed up is such a no no from a technical perspective that would be smashing.

 

cheers,

Nick

[nicklouse]

If one has worn through to the backing get the crank looked at and measured. Then if it needs a grind you are not wasting money on bearings.

Why is it important. If they are not on the correct bore or are not the right way around they will not be aligned and could even be not concentric. Generally you find that the crank can not be rotated on assembly.

Same reason you need to fit the right Conrod caps on the conrods. And valves in the right guide etc if you don't thing do not work.

[Spider]

I'd be checking the crank as Nick has suggested, and while it may seem frivolous, I'd also be binning and replacing the Big End Bolts, they have been subject to a severe side load, which they are definitely not designed to do. They are not expensive and so cheap insurance.

[nicklouse]

I was thinking of an easy way to kind of explain why it is important. A over simple way is this. Take 3 oranges and cut them all in half. How many ways will they go together correctly.

Only one way.

[Spider]

 

Also, if anyone can explain how / why getting this mixed up is such a no no from a technical perspective that would be smashing.

 

 

Sorry, I've just read this bit now.

 

While the caps will usually fit over the studs (on a big bore engine) or step (on a small bore unit) either way, they only have one correct way of fitting back together.

 

When the Rods are manufactured, the meeting faces are first ground and then the tunnel (where the bearing shells fit) are machined and sized. The Bolts are usually 'off' one way or another and this can be for a number of normal manufacturing reasons but will only correctly line up one way only.

 

No doubt when you were assembling it, you noticed the small cut outs in the caps for the indexing of the bearing shells? These normally go to the same side. None the less, it's always wise to number the rods on disassembly and the caps at the same time and on the same side, so correct identification and orientation is readily (and universally) understood.

 

Also, sorry, but this does beg the question, when you were assembling the engine, you must have had to turn the crank a few times during the course of the build, if upon fitting bearing caps it was found hard to turn over (more usually from the caps being on back to front, they will lock up) it must have rung some alarm bells?      Sure, it will get 'stiff' when the pistons are ~mid-bore due to the drag of the rings but as they get around TDC / BDC you can normally turn them by hand fairly easy.

Edited by Moke Spider, 05 November 2015 - 04:28 AM.

[nky_84]

Thanks guys., that all makes sense.

 

I've had it all in a bits a few times now, got a bit cocky, and for some reason thought that the tangs went opposite sides and didnt think anything more of it.

 

I was attempting to replace the crank without taking the head off and although the crank felt tighter than i thought it should at mid bore, it still turned back and forth. I also couldn't fully rotate the engine as its a 16v head and the cams were still in, so valves would have met pistons. In hindsight, this wasnt a great idea, although it would have been fine if id got the caps the right way round....

 

Still, i wont be getting any of this wrong again, thats for sure!

[nicrizz]

Don't worry, you're not the only one.

I made this boo-boo too.

The car survived 5 months of hard use, including Avon Park, Castle Combe, and a rolling road day before I decided to take it apart to look for the mysterious knock that was only present around 2000 rpm.

I discovered one big end bearing was splaying out from the sides. I had to get the crank reground and I replaced the rod as it had gone a shade of blue from what I remember.

Here is the bearing:


It also appeared to move freely by hand when I built it up.

Like you say, I haven't made that mistake again since!!

Edited by nicrizz, 05 November 2015 - 09:03 PM.