Piston Hole Identification - Reassembly

[barber1303]

I'm in the process of rebuilding the engine from my '85 (2nd gear repair). When I pulled the pistons/rods, I put them in individual bags with tags identifying which holes they came out of. Unfortunately, the residual rust protectant I had on them dissolved the ink I'd used on the tags. Stupid I know...lesson learned...external tags/labels. Any tips or tricks I could employ to try to figure out which holes they came out of?

For what it's worth, this is a 34K mile engine with original std pistons and rings.

 

[Fire-medic]

My suggestion is to use a set of feeler gauges, or better-yet, an inside micrometer, and an outside micrometer. (or a tool to take the inside bore measurement, and then use an outside micrometer) Measure the pistons at 90 degree angles, top, middle, bottom. Do the same thing for the bores. If you're handy w/the product, make an Excel spreadsheet. That should help you to see where the pistons belong, or to provide you with info on wear, to determine if you need oversize pistons/rings, and a bore job.

LR=piston #1
LF=piston #2
RR=piston #3
RF=piston #4

To get you started in the right direction:

See p. 3-8 in http://vmoa.net/VMX12-Service-Manual01.pdf
Engine disassembly ch. 3

Also pp. 3-28 to 3-32 for piston and bore measurements.

 

[MaxMidnight]

As the big end bearings are sized to the journal to give to correct running clearance I would also be inclined to us a Plastiguage to check this is correct.

PS, the technical term is 'bore', not hole.

 

[[email protected]]

Most folks don't have the knowledge, or tooling to perform measurements that would be precise enough to determine which piston came from which cylinder (unless you have some major wear issues happening).

It would be nice to put them back in their original position but you probably won't be able to find a way to determine which came from where. Remember that when the pistons are manufactured, they are all built to the same tolerance . The cylinders are all bored to the same dimensions. Sure, they wear together but, if you hone the cylinder and you have new rings, I can't say it would matter much.

As far as the rod bearings, it should go without saying that you'd be checking for proper clearance.

 

[sdt354]

My mentor, who was a great engine builder, used to throw everything in a basket unmarked when rebuilding a motor. I never followed that procedure, but everything he built ran beautiful. Of course he carefully measured everything during reassembly.

 

[barber1303]

Thanks for the advice fellas! I've rebuilt many engines, just never made this particular mistake before. Looks like I'll need to pick up some plastiguage. I think I have two pistons identified based on barely legible tags and can put them back in their respective bores (sound better Mr. Midnight? ). I'll check bearing clearance all around and maybe that'll differentiate the remaining pistons.

Fire-medic...thanks for the detailed advice. I have the tools to execute the procedure, but perhaps not the skills/precision required to detect the minute differences. I might give it a try anyway if I'm still in doubt.

Thanks!

Chris

 

[Fire-medic]

barber1303 said:
"I'll check bearing clearance all around and maybe that'll differentiate the remaining pistons."

For the life of me, I cannot comprehend how that would be possible. You think you have two of the correct pistons in the correct bores. Now your chances of getting it wrong are 50%, which also is the possibility of getting it correct! My suggestion is to just measure the (bare, no rings) piston to bore clearance with a feeler gauge, and try to put the pistons in the holes where they come closest-to being in the 'middle' of the minimum/maximum values.

If you're gonna measure the crank journals as MM suggested, don't stop there, measure the rod big-end clearances, too. You wouldn't want to-have gone to all this work, only to find that you developed a rod knock.

 

[MaxMidnight]

barber1303 said:
"I'll check bearing clearance all around and maybe that'll differentiate the remaining pistons."

If you're gonna measure the crank journals as MM suggested, don't stop there, measure the rod big-end clearances, too. You wouldn't want to-have gone to all this work, only to find that you developed a rod knock.

Aw c'mon pay attention at the back! That is what I suggested.

 

[Fire-medic]

Sorry, I thought you were speaking of the crankshaft main journals. Please this one time, forgive my lack of reading comprehension.

 

[barber1303]

You guys are great! I'll get out the micrometers and the bore gauge and see if that illuminates the situation any.

 

[MaxMidnight]

Sorry, I thought you were speaking of the crankshaft main journals. Please this one time, forgive my lack of reading comprehension.

OK, I'll let it pass this time but it's smacked botty for you the next time!

 

[Toolman]

Can you still see the colour of the rod big end bearing size? if you can then go to the bearing calculation chart and figure out what size should be used for that rod. just a shot in the dark as they all could be the same. also the rods as they are positioned on the crankshaft have to be turned a certain way this could also be a clue in sorting out the proper location of the rods.

good luck
Toolman

 

[02GF74]

Ok, stupid question but unless the engine has been rebuilt, then surely all the bores and hence pistons would be made to the same size, and I would expect the same applies to the main journals?

Surely it is cheaper to make stuff the same size as opposed to making items ever so slightly different in size, which would mean adjusting the machinery and then having a person measure and fit different sized bearings and pistons?

 

[MaxMidnight]

Ok, stupid question but unless the engine has been rebuilt, then surely all the bores and hence pistons would be made to the same size, and I would expect the same applies to the main journals?

If we lived in an ideal world yes but parts will be specified with a maximum and minimum size because during the manufacturing process machines go out of adjustment and tools wear. There will also be variances caused by ambient temperature or humidity of if the machine has had maintenance.
There may also be several lines making the part or the parts being assembled may have been made a different times.

These discrepancies within the tolerance range between the component parts will, in most cases, be small but significant if you want to the maximise performance, efficiency and durability of the engine.

During the assembly process the manufacturer will match components; for example a bore that is near to top end of the tolerance range will get a piston and rings also near the top. Putting a lower end piston with a top end bore or perhaps more concerning a top end piston with w bottom end bore would not be ideal.

Given that for many year manufacturers have been able to work to very tight tolerances then you could possibly re-assemble the pistons and rods on any bore BUT you probably won't be getting the best from the engine or maximise durability.

If rider would notice is a moot point!

 

[one2dmax]

If I were trying to ensure I was putting them back to the same positions I would first understand what bearing color codes were on each journal. It's not very common for the 2 crank rod journals to have the same final codes when calculating the crank and rod numbers. Look for the numbers on rods as well as crank to see what they should be. The rods are usually 6's or 7's while the cranks are usually 1 through 3. THEN look on the edges of the bearings for a small splash of color.

This may help you narrow down what goes where.

Of course you need to make sure that you get the radius sides of the rods facing the radius of the cranks (flat sides to flat sides of rods and radius to crank sides).

 

[Toolman]

Sean explained it in more detail but we are on the same page