Hi

Advice wanted in laymans terms.

I have the barrel off my M20 and need to check if the Con Rod is twisted or bent.

I have the minimal of measuring tools, steel rule and vernier/calliper.

anyone talk me through it in stages.

All advice appreciated.

John B

Unless you are thinking of doing the full "Ian" to it find a length of dowel the same diameter as the piston pin.
Slip it through the little end and use your eyes .
Top down for twist
Side on for bend
Front on for bend.

email (option): bsansw1@tpg.com.au

Or just use the gudgeon pin itself and settle it down on some 1" machine blocks each side. As stated by Trevor, you'll have to use your eyes to look for any distortion.

Ron

email (option): ronpier@talk21.com

Thanks for the advice %20void(0);

Thanks for the advice Ron

Assemble the engine without piston rings and cylinder head. Using a feeler gauge, check clearance along the length of the piston, on each side, in TDC and BDC. This will give you a good idea whether you have a problem. You should have some clearance, I would guess at least 0,003" and ideally 0.004" to 0.006", along the length of the piston for all positions. Bear in mind that when you measure one side, the piston should push over to the other side of the bore.

email (option): pvlietstra at gmail.com

J-B ...... was the engine running before you stripped it?

Was there anything which makes you think the 'rod is bent?

What do the wear (witness) marks on the piston say ?

Re - latter question, is here any obvious irregular wear on the piston, such as bare metal at the top of the piston and a similar area of bareness, diametrically opposite?

Publish a pix for us to comment on ?

Checking the straightness of a 'rod, in an assembled crank or set of crankcases can only really be done with a long length of rod which fits snugly (no play) into the small-end of the rod, preferably when the bush is removed (otherwise you are measuring if the bush is concentric with the rod) - ok if you can machine something to suit?
The longer the rod the better as with a rod over 12" or so, any bend/twist is visible with the eye - particularly if you place a straight edge across the crankcase mouth and compare the two for parallelness (is there such a word?).

As for using parallel's, v blocks, or such - you don't have them and to be honest as any measurements will so close to the con-rod, any bend will be measurable in thou's and difficult for you to measure.
You need to magnify any defects so you can visibly see it - so i guess having a nice long bar or something (get creative - studding clamped inside the gudgeon pin ?) is the solution for you?

The above would show any bend, as for twist - does it make any difference ?
otherwise using the rod through small end - try and compare if it is parallel to the crankshaft ..... not certain how you can do this easily with the crank in crank cases?

Parallel bars on the crankcase and a gudgeon pin fitted into the rod (with a good small end bush) are a quite adequate means of determining whether the rod is bent in line with the axis of the crank...
Deflection can usually be seen with the naked eye and can easily be verified using a thin feeler gauge..

It is not necessary to actually measure the degree of deflection at this stage, only to determine whether the rod is bent.
If it is the rod will have to be removed from the engine to straighten it accurately and actual measurements can be taken accurately at that point...

If you are intending to work on engines, a set of parallel bars are a good investment and can be bought fairly cheaply...

You have to have tools...Ian

email (option): ian@wright52.plus.com

And know how to use them .......

I think i could give 6 of my long time bike owning mates a bore micrometer and get six different answers - in fact, come to think of it, I KNOW i would get 6 different answers........

Have bought the parallel's, had a quick measure from crankcase to under gudgeon pin each side, about 2 thou difference.
Seems ok!
Should have gone to spec savers?
JB

You could give them to 6 qualified mechanics and get 6 different answers.

At a VMCC president's day they had an engine set up by a metrologist.
We were then "challenged" to determine the valve lash with the supplied feelers.
Less than 10% actually got it right.
Nearly everyone "measured" the gap a lot wider than it was.
Oddly enough , more wives & girlfriends got it correct than actual owners.
Must have made for interesting comments next time the bike broke down.

A couple of years latter they did the same with some maggys.
The results were somewhat similar however more got it right so some of us learned from the experience a few years earlier.

A tip for all you who organize family & field type activities.
Not as much fun as an pillion egg & spoon race, but a lot easier on the public liability insurance.

The best wee did was the kick start reliability challenge.
Where you nominate how many kicks to start your bike, cold & hot.
Then you all line up & kick, the ones outside their number of kicks being eliminated till only 1 bike is left.
We used to run a "guessing" competition in parallel for how many successful starts the winner would do.

email (option): bsansw1@tpg.com.au