Ym2000 piston help

bridgeport13b · Nov 27, 2018

Hi so I have a 2tr20 engine with 88mm pistons and I was wondering if I should get rotating assembly balanced for the 90mm pistons used in the 2tr20a? My engine suffered from a rod knock, so I bought new pistons and bearings and a used replacement connecting rod. I am also boring out stock 88mm bore sleeves for 90mm pistons. I just don稚 Edmonton want to put engine back in and find out that the engine is way out of balance with new pistons. Also is the crank, front pulley, and flywheel balanced togething? are the rods and pistons weight matched so that they cancel each other out?
Thanks
Richard

California · Nov 27, 2018

Just thinking theory here - the crank throws are 180 degrees apart. I expect it will balance the same as before so long as the big end of the new rod matches the remaining rod. So no need to take in the crank.

A balance shop might want to match the rods and pistons into perfect twins. That should be pretty simple.

Anybody know if this will make any practical difference?

bridgeport13b · Nov 28, 2018

California said:
Just thinking theory here - the crank throws are 180 degrees apart. I expect it will balance the same as before so long as the big end of the new rod matches the remaining rod. So no need to take in the crank.

A balance shop might want to match the rods and pistons into perfect twins. That should be pretty simple.

Anybody know if this will make any practical difference?

That痴 what I figured but I wasn稚 for sure, I know on in-line 4 cylinders they make rods and pistons same weight so that they cancel each other out. I guess I wanted to make sure there was not anything else that would cause issues.

winston1 · Nov 28, 2018

Over my head. :confused3:

Jtullis13 · Nov 28, 2018

bridgeport13b said:
Hi so I have a 2tr20 engine with 88mm pistons and I was wondering if I should get rotating assembly balanced for the 90mm pistons used in the 2tr20a? My engine suffered from a rod knock, so I bought new pistons and bearings and a used replacement connecting rod. I am also boring out stock 88mm bore sleeves for 90mm pistons. I just donç¨š Edmonton want to put engine back in and find out that the engine is way out of balance with new pistons. Also is the crank, front pulley, and flywheel balanced togething? are the rods and pistons weight matched so that they cancel each other out?
Thanks
Richard

The rods and pistons should be balanced for your crankshaft. Since you went up in size, even though it wasn't a huge jump, I am not sure what the calculation should be for that, but they should be balanced. I know that these little engines aren't running 5,000 rpm or higher but still would be a good idea. Hate to have you put it back in and have an oh S*** factor come into play. I just know from past experience on gas burners that I had to have pistons and rods balanced with my crank when I bored out the block, again these were high running RPM engines too and I bored more than 2mm. Maybe someone else can chime in on this. rScotty I think was an actual diesel mechanic, maybe he can give some light on the subject.

normde2001 · Nov 28, 2018

Hot-rodders have been boring and using oversized pistons and rings for decades with no effect on balance.

careyo63 · Nov 28, 2018

I've Bored and use over sized pistons on many gas engines 4-6-8Cyl. And a 6 2cycle motor. And only oversized 1 of the cylinders for that matter. Worked in a Eng. machine shop when I was 18. I know for a fact my, And do mean BA. Mec. 6Cyl. outboard was with no effect on balance. Bought it with a bullet hole in it. Yup!! Replaced the head and ran it for Yrs.. 2 Pistons were damaged we new of. Which one finally broke yrs. later. Used a 175 block bored 60mm over after that. With fiberglass valves. Kept the 150 cowl and man did it hurt some feelings. Like I said I was well versed on 2cycles...Which I owned boats before that one also. Mec #1 on the water.............LOL...

bridgeport13b · Nov 29, 2018

I decided to take engine to machine shop for boring the cylinder for new pistons anyways he said that on in-line type engines that they are internally balanced and that the crank, flywheel and harmonic balances are balanced together and the rods and pistons are weight match to each other and cancel each other out when they run. So basically as long as pistons and rods match in weight they are balanced as long as crank, flywheel and harmonic balanced haven稚 been changed.

Jtullis13 · Dec 1, 2018

bridgeport13b said:
I decided to take engine to machine shop for boring the cylinder for new pistons anyways he said that on in-line type engines that they are internally balanced and that the crank, flywheel and harmonic balances are balanced together and the rods and pistons are weight match to each other and cancel each other out when they run. So basically as long as pistons and rods match in weight they are balanced as long as crank, flywheel and harmonic balanced havenç¨š been changed.

Sounds like you are good to go then. Let us know how it performs after you get it all put back together!

rScotty · Dec 1, 2018

bridgeport13b said:
I decided to take engine to machine shop for boring the cylinder for new pistons anyways he said that on in-line type engines that they are internally balanced and that the crank, flywheel and harmonic balances are balanced together and the rods and pistons are weight match to each other and cancel each other out when they run. So basically as long as pistons and rods match in weight they are balanced as long as crank, flywheel and harmonic balanced havenç¨š been changed.

That's right. As a mechanic about all you can do is match the weights of the reciprocating parts - the pistons & rods - so that both pistons are equal and also that both rods have the same weight.

There are other types of balance as well and they are fun to think about. For example there is a rocking couple between the front and rear cyiinders which is minimized if both cylinders have about the same compression, weight of parts, and make the same power.
And there is also a target percentage balance (never 100% & sometimes much less) between the reciprocating parts and the rotating parts that is RPM dependent....in fact, that is why some crankshaftshafts have oddly shaped lobes. But as a mechanic you really can't do much to affect those types of balance except to match the weights just as your machine shop said.

Most small Yanmars don't have a harmonic balancer, and the flywheel + clutch assembly should be symmetrical around the rotational axis, so those parts are automatically in balance.

It sounds like you are on the right track.
rScotty