Friday, January 29, 2016

Big End Running Clearance

So far things with the new engine build have been going rather well for a change, both the crankshaft running clearance and the endfloat have been measured and are within spec. Now it's time to measure the big end clearance, will it be three out of three...

When it comes to conrods, I've always loved the wonderfully literal terms 'big end' and 'little end' as it tells you everything that you need to know right away. No messing about with fancy names, just straight to the point. I'm sure the naming process went exactly like this: "These new thingies we've invented Bob, what shall we call this end? I'm thinking the 'capacious end'" - "Err, let's just go with big end hey Jim". "Okay, but what about 'diminutive end' for the other side then Bob?". "Mmm... maybe we should just go with small end!"

Anyway, nonsense aside, I need to make sure that the running clearance is acceptable and to do that, I need to know what acceptable is. All the specifications are of course available in the Haynes Manual and in this case I'm aiming for a clearance between...

As for the asterisk: well considering that there's no definition of what a later model actually is, coupled with the fact that the only way to find a Rover dealer would be to hop into your nearest Doc Brown converted DeLorean, I'm going to have to go with the numbers highlighted above.

To measure the clearance, I'll be using almost the exact same 'Plastigauge' method I used when I measured the crankshaft running clearance here. Not only is Plastigauge cheap, it's easy to use and, so I'm led to believe, very accurate.

To make the measurements, I obviously need to insert the pistons. What's not so obvious though is which way round they go! Thankfully all the conrods and caps are labeled up '1' to '4', which is great, however for some reason they are back to front! Yup, piston '1' goes in cylinder '4', piston '2' goes in cylinder '3' and so on. Crazy I know, but that's how they came.

One reason for the mix up could be the way the mini engine sits in the engine bay. Normally we count things from left to right so would naturally count cylinder '1' as the one nearest the clutch and number '4' being nearest the fan. Seems logical until you realise that the fan side of the engine is actually the front of the engine and thus the side you should be counting from. This leaves you with the pistons being numbered '1' to '4' from right to left...

So why is this so important you might ask! Well it has everything to do with the offsets! Each of the conrods has an offset so that the big end will correctly align with the crank. Put the wrong piston in the wrong cylinder and you'll realise pretty quickly that there is just no chance that it's going to push into place.

If you look closely at the picture above though, you'll notice that piston '1' and piston '4' are in fact mirror images of each other, so if you have put the wrong piston in and it hasn't aligned properly, what's to stop you from just taking it out again, rotate it 180° (to put the offset on the correct side) and stick it back in?

Well... nothing in fact... you can indeed put piston '1' in cylinder '4' and it will correctly align onto the crank and bolt up and will seem fine and dandy. Any sense of achievement will soon be crushed though when you rotate the crank only to find that the big end bashes into the side of the engine block!! At which point you'll curse your own stupidity, go away, get a cup of tea and then come back take it out and do it correctly.

Oops!! That's not supposed to happen!

As you may have guessed, I did get a little confused as to which piston went where and which way round they go. To be honest, the shonky labelling didn't help, but I really should have engaged my brain before my hands. So after I did engage my brain, I eventually got correct piston (number 4 in my case) the correct way round for cylinder '1', which as can be seen below, has the offset to left. (In the picture by the way the fan would be to the right)

Before inserting the piston, I pressed a new bearing into the big end and also the cap. I did contemplate using the the old bearings, but one of them looked a bit too scratched and failed the 'fingernail' test so I decided it was safer to go with a new set.

Locating my piston ring compressor, I inserted the first piston into cylinder '1'. As I'm a bit clumsy, I put a plastic bag on the big end to avoid scratching the cylinder walls on entry. As a further precaution, I also make sure the crank journal I'm aiming for is positioned as far away from the cylinder as possible to avoid ramming it with big end as I push it in.

Once the piston was in place and resting on the crank journal, I placed a spit of wood behind the conrod as a means of stopping the crank from rotating while I worked.

Placing a small length of Plastiguage on the journal, I put on the cap and torqued it down to the required 37lbf ft which is 50Nm.

As soon as it was tightened up, it was undone again to reveal that I had unfortunately placed the Plastigauge in a position that corresponded with an oil hole in the bearing, meaning it didn't squash it properly!!

Oh well, no great worries, just clean it off and try again in a different position away from the hole which resulted in a much more evenly squashed Plastigauge pattern. Measuring it with the supplied gauging card, it was somewhere between 0.038mm and 0.050mm.

Just over 0.038mm
Just under 0.050mm

After examining the results a few times, I'd say it was about half way between the two putting the running clearance at 0.044mm which thankfully is well with the spec of 0.025 - 0.063mm.

Working my way across the other pistons, they were remarkably consistent as they all gave the exact same Plastiguage squash pattern meaning the same 0.044mm of clearance.

Piston 2 - also came out at 0.044mm did piston 3
and finally piston 4.

So there you have it, all of the big end running clearances are thankfully well within spec. Although this was a bit of a faff on to sort out, it's good to have the peace of mind that comes with knowing it's been done. Had I have built the engine without checking the running clearance, there would have been that niggling doubt in the back of my mind that I should have checked when I had the chance, so I'm glad I did.

So along with the crankshaft running clearance and crankshaft endfloat, I can now tick the 'big end running clearance' off the list of things that need checking and as this is the last thing on the list, I'm finally in the exciting position where I can start to assemble the engine, so watch this space!!

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