Piston wont fit connecting rod

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Piston wont fit connecting rod

New postby Campuslounger on Wed Aug 09, 2017 1:26 pm

So I am in the process of doing my top end. I ordered an original piston and rings, ported and polished cylinder and honed the cylinder. I'm putting it all together and all of a sudden the piston will not fit over the connecting rod. The rod has washers on each side of the rod bearing. When I measured these they were 29.98mm. When I measured the inside distance on the piston it came up with 28.46mm. I also measured the old piston and that was 30.08mm and fit as it should.
I noticed on the old piston the edges of the piston where I measured were flat and sharp. On the new one they were kind of rounded. I will have to file them down in order for them to fit but how do you know how much to file on each side? Any ideas would be very helpful.

Connecting rod with washers.JPG
Connecting rod measurement with the washers on both sides of bearing 29.98mm


New piston.JPG
New piston measurement 28.46mm


Old piston.JPG
Old piston measurement 30.08mm
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Re: Piston wont fit connecting rod

New postby kbzx6r on Wed Aug 09, 2017 2:15 pm

It sounds like your original piston had the inside of the wrist pin lugs machined and your new piston is "as-cast" there. Use your calipers to measure your old piston from the inside of the wrist pin lug surface to the OD of the piston. Do each side of the new piston seperately to the same dimension. Just curious, what year and model? I don't recall ever seeing wrist pin thrust washers on these old Suzukis. I'm wondering if that might be one of those WTF surprises (a.k.a. Previous Owner Modifications). Maybe someone couldn't get the right wrist pin bearing (too narrow) and tried to compensate by adding thrust washers???

Come to think of it, how do you even reassemble it? Lots of grease to hold the washers in place?
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Re: Piston wont fit connecting rod

New postby Campuslounger on Wed Aug 09, 2017 3:12 pm

Screenshot (3).png
Item 6


I got what your saying about machining those down. The machining will be yours truly with a file. I don't think I have to take too much off but I had never seen that before.

Thanks I'll let you know how it goes. Oh and it's a 74.
Last edited by Campuslounger on Wed Aug 09, 2017 3:12 pm, edited 1 time in total.
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Re: Piston wont fit connecting rod

New postby kbzx6r on Wed Aug 09, 2017 3:32 pm

Thanks for adding the exploded view. It appears Suzuki added the thrust washers to the TS400 in '73 and the TM400 had them all along. Learn something new every day! Or maybe I already knew it back then - but just forgot 'cuz I'm getting old :(. Anyhow, careful hand filing should work OK there. The math for your original parts comes out to .1mm (.004") total clearance. I wouldn't go less than that but wouldn't worry about a few thousandths over either.
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Re: Piston wont fit connecting rod

New postby kbzx6r on Wed Aug 09, 2017 3:40 pm

Come to think of it, I do remember them now. The washers are weird little things with a counterbored ID in the inner face to fit over the ends of the needle bearing. That's why the grease came to mind. Without the grease they could fall down into the crankcase and you would have to go magnet fishing.
Drive it like ya stole it
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Re: Piston wont fit connecting rod

New postby Butcher on Wed Aug 09, 2017 11:50 pm

Bloody hell! Never knew those washers existed! I had a bad day with I think it was one of my TF125 motors. Or my 185? Anyhow, I struggled to get the wrist pin (gudgeon pin) in and gave it a love tap. I ended up fudging the small end roller bearing. PITA!

Yea small file and some care. I always thought that the small end would find it's own center. The 400 is rather large and I suspect the early designed ones chewed up pistons in there? Or the designers were paranoid?


Check con-rod for sideways play. Should be less than 3mm total.


After thought. Chewed out small end might be because of lack of lubrication (ran out of oil). Or PO used premix and bypassed the SUPER RELIABLE pump!


Stuff a rag into the crank case to catch them slippery washers :) I wish you luck. don't get frustrated, your almost there!


Oh and on engines. Let engine warm up for good minute then take it easy for next minute. Reving the living daylights out of engines when cold is one of my pet hates. There is no need for it (showing off may be?). Eutectic pistons (look that up) NEED to expand evenly when warming up.
Last edited by Butcher on Wed Aug 09, 2017 11:50 pm, edited 1 time in total.
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Re: Piston wont fit connecting rod

New postby kbzx6r on Sat Aug 12, 2017 6:50 pm

@Campuslounger - How did your piston mod turn out? Everything go back together good? Happen to have any pics?
Drive it like ya stole it
Currently - 75 TS250, 02 Kaw ZX-6R
Previously - 70 T350, 71 TS250, 72 GT380, 72 TS125, 72 TS250, 75 Yam YZ360B, 72 GT380 diy cafe racer, 80 GS750, 73 TS185, 82 GS1100
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