Pistons tilted

Douglass Harroun · Dec 9, 2014

Rebuilding an 850. The rods were re-sized. At TDC each piston top height varies side to side ~.002" on one piston, .004" the other. Swapping the pistons and rods around changes the side and amount of difference. Was as much as .007" on one. There seems to be a tiny difference in the rod straightness and crank throw straightness. I'm thinking this will put a side force on the piston pins/ circlips. Have seen other pistons come out with ridges on one side of pin grooves, Maybe due to this. Anybody else notice this? What would Comnoz say?
Doug

pete.v · Dec 9, 2014

Are both pistons tilted in the same direction as in a non centered crank.

SteveA · Dec 9, 2014

Douglass Harroun said:
Rebuilding an 850. The rods were re-sized. At TDC each piston top height varies side to side ~.002" on one piston, .004" the other. Swapping the pistons and rods around changes the side and amount of difference. Was as much as .007" on one. There seems to be a tiny difference in the rod straightness and crank throw straightness. I'm thinking this will put a side force on the piston pins/ circlips. Have seen other pistons come out with ridges on one side of pin grooves, Maybe due to this. Anybody else notice this? What would Comnoz say?
Doug

Jim might say...what exactly do you mean by 'resized'?

comnoz · Dec 9, 2014

Douglass Harroun said:
Rebuilding an 850. The rods were re-sized. At TDC each piston top height varies side to side ~.002" on one piston, .004" the other. Swapping the pistons and rods around changes the side and amount of difference. Was as much as .007" on one. There seems to be a tiny difference in the rod straightness and crank throw straightness. I'm thinking this will put a side force on the piston pins/ circlips. Have seen other pistons come out with ridges on one side of pin grooves, Maybe due to this. Anybody else notice this? What would Comnoz say?
Doug

It sounds to me like the rods are not perfectly straight. Possibly done when they were resized. The crank may be off a bit also -it's not uncommon.

I do not recommend resizing rods anymore. They usually came back worse than when I sent them.
If there is a measurable problem with the rod I replace them.

Jim

Douglass Harroun · Dec 9, 2014

Comnoz-
Will the motor last in this condition? How bad is too bad to risk running?
Doug

comnoz · Dec 9, 2014

Douglass Harroun said:
Comnoz-
Will the motor last in this condition? How bad is too bad to risk running?
Doug

I don't think I would run it with .004 difference from side to side on one piston. That is quite a bit of angle. It will wear the piston and bore off center and cause oil consumption.

I would not be as concerned about a small difference in stroke or small differences in piston height from one cylinder to the other.

Has the crank been turned undersize?
If it has I would remove the pistons from the rods and rotate the crank to BDC and measure the distance between the small ends of the rods. Then rotate the crank to TDC and measure again. The measurements should be pretty close. If they are not then the crank has been clamped too tight when the journals were ground. [common mistake on a Norton crank]
Jim

jaydee75 · Dec 9, 2014

Jim continues to amaze me. He knows things I didn't know I didn't know.
Jaydee

Hobot · Dec 9, 2014

Does comnoz know anyone personally that ever made such a crank machining over clamping error? Best wishes getting back on spec.

comnoz · Dec 9, 2014

hobot said:
Does comnoz know anyone personally that ever made such a crank machining over clamping error? Best wishes getting back on spec.

Yes, I have had it happen to me twice, once on my engine and once on a customers engine. Both times the grinder claimed no responsibility and I had to send them to another place to get them corrected.

That problem was what prompted me to decapitate a pair of pistons to demonstrate the problem. One the first occasion the rod small ends were 1/8th inch farther apart at BDC than they were at TDC.

Rohan · Dec 9, 2014

How does this show up in a running engine though ?

And thats why there is (some) room for the rods to slide on the gudgeon pin ??

comnoz · Dec 9, 2014

Rohan said:
How does this show up in a running engine though ?

And thats why there is (some) room for the rods to slide on the gudgeon pin ??

If the rod has to slide sideways on the pin when the crank rotates then the rod big end bearing wears on the edges. That was the big thing I saw first. The engine was noisy also. Jim

Hobot · Dec 9, 2014

Does Hi rpm crank flex mis-angle journals in big ends to cause small ends to push pistons more out of line?
If so could this contribute significantly to the long stroke friction losses?
If so could one mis clamp machine a crank out of square so when it flexed it relieved piston-rod-pin-bore bind?
If not, never mind.

acotrel · Dec 9, 2014

comnoz said:
Rohan said:

How does this show up in a running engine though ?

And thats why there is (some) room for the rods to slide on the gudgeon pin ??

Click to expand...

If the rod has to slide sideways on the pin when the crank rotates then the rod big end bearing wears on the edges. That was the big thing I saw first. The engine was noisy also. Jim

The bores might not be at precisely right angles to the centre line of the crank in the transverse direction. If you are reboring, it pays to check that the top and bottoms of the cylinders are parallel, and the bed of the machine is at right angles to the tool holder. If the bores are slightly canted, friction losses are higher.

SteveA · Dec 10, 2014

jaydee75 said:
Jim continues to amaze me. He knows things I didn't know I didn't know.
Jaydee

Me too, I didn't know I didn't know what is meant by 'resizing' rods......

I still don't know what is meant by it?

Jim, any one....what is it?

comnoz · Dec 10, 2014

SteveA said:
jaydee75 said:

Jim continues to amaze me. He knows things I didn't know I didn't know.
Jaydee

Click to expand...

Me too, I didn't know I didn't know what is meant by 'resizing' rods......

I still don't know what is meant by it?

Jim, any one....what is it?

Resizing or reconning rods involves cutting a small amount of material from the rod cap at a slightly diverging angle. This angle causes the bearing bore to pinch inward when the cap is tightened. Then the bore is honed back to the standard size in hopes of making it true and square again.
The problem is that the pin hones that are used to hone the bore back to size do not have any means of holding the rod perpendicular to the hone, so the hone just somewhat follows the hole that is there and the bore may not end up square. Jim

SteveA · Dec 12, 2014

comnoz said:
SteveA said:

jaydee75 said:

Jim continues to amaze me. He knows things I didn't know I didn't know.
Jaydee

Click to expand...

Me too, I didn't know I didn't know what is meant by 'resizing' rods......

I still don't know what is meant by it?

Jim, any one....what is it?

Click to expand...

Resizing or reconning rods involves cutting a small amount of material from the rod cap at a slightly diverging angle. This angle causes the bearing bore to pinch inward when the cap is tightened. Then the bore is honed back to the standard size in hopes of making it true and square again.
The problem is that the pin hones that are used to hone the bore back to size do not have any means of holding the rod perpendicular to the hone, so the hone just somewhat follows the hole that is there and the bore may not end up square. Jim

Thanks Jim, Very Clear. Understood.

New rods may not be cheap, but I am sure the 'cost and pain' of trial and error on this exercise can rapidly make new rods the 'cheaper option'.

Worth a try if you have all the equipment and a lot of free time, but otherwise not.

Pistons tilted

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