1975 Yamaha RD350 - Engine noise after engine seizure repair

Desi_RD350_TX

New Member
Hi All, I need your help in figuring out the engine noise i am hearing after installing the new pistons and freshly bored cylinder kit. My RD350 had an engine seizure due to lack of oil last month and i was hoping that the new piston kit and cylinder boring will take care of this issue. Please advise how to fix this. I can provide more details if needed. Thank you !
Note: 1) The bike was making a similar noise when i started it after re-filling the 2 cycle oil after engine seizure. 2) I did the installation myself by watching youtube videos as the RD Specialist mechanic is almost 2 hours away.
 

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teazer

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Sounds like rings catching on the ports. Did the machine shop chamfer all the ports after it was bored and honed?
 

teazer

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If you are absolutely sure that the chamfer is adequate, next step would be to check ignition timing on both sides.
 

Jimbonaut

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I had a similar sound in my newly-rebuilt engine. I over-bored a CB750 inline 4 cylinder engine to 823cc, but when I rebuilt the thing I installed the wrong head gasket (I installed a 750cc gasket not the 823). The noise was the pistons hitting the inner metal rings of the gasket.

Did you overbore? Did you install the correct head gasket?

And welcome to DTT!
 

teazer

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Good thought there. Would be horrible if the gaskets were RD250...
Another ghastly thought is when you ran out of oil, how long did it run like that - best guess - and how are the crank and rods? Any play in there?
 

crazypj

Split personality, I fake being smart
You didn't put the pistons in backwards?
Arrow pointing to exhaust ( and piston skirt 'windows' to intake)
Circlips were seated properly?
Small end bearings new?
Did you check the port chamfers?
Exhaust port is real easy to do but the transfers and bypass ports quite a lot more difficult
I would pull heads at least and check it isn't 250 gaskets (wouldn't be the first time the wrong ones were supplied)
Remove generator cover and try lifting crank end up and down, noisy main bearings are a 350 (and 400 after 8~10,000miles) 'standard option' although it doesn't sound like mains to me
 

Desi_RD350_TX

New Member
I had a similar sound in my newly-rebuilt engine. I over-bored a CB750 inline 4 cylinder engine to 823cc, but when I rebuilt the thing I installed the wrong head gasket (I installed a 750cc gasket not the 823). The noise was the pistons hitting the inner metal rings of the gasket.

Did you overbore? Did you install the correct head gasket?

And welcome to DTT!
Yes, The Cylinder was overbored (1.75mm to be exact). I opened the head yesterday and noticed that the pistons don't reach all the way to head gaskets so no question of pistons hitting the inner metal rings of the gasket. I installed the correct size head gasket meant for RD350. Link below:

https://www.economycycle.com/shop/y...te-gasket-set-vg-251-for-yamaha-rd350rd250r5/
 

Desi_RD350_TX

New Member
You didn't put the pistons in backwards?
Arrow pointing to exhaust ( and piston skirt 'windows' to intake)
Circlips were seated properly?
Small end bearings new?
Did you check the port chamfers?
Exhaust port is real easy to do but the transfers and bypass ports quite a lot more difficult
I would pull heads at least and check it isn't 250 gaskets (wouldn't be the first time the wrong ones were supplied)
Remove generator cover and try lifting crank end up and down, noisy main bearings are a 350 (and 400 after 8~10,000miles) 'standard option' although it doesn't sound like mains to me
I took a picture of old pistons and made sure that the new piston arrows match the old piston arrow direction. They are pointing towards exhaust. Circlips were fitted properly and small end bearings are new. Ports are chamfered and feel smooth to my fingers. I also installed the correct head gaskets. Link below:

https://www.economycycle.com/shop/y...te-gasket-set-vg-251-for-yamaha-rd350rd250r5/

Note: I will try this tomorrow -> Remove generator cover and try lifting crank end up and down.
 

crazypj

Split personality, I fake being smart
If I remember right, LC pistons have smaller windows in piston and are 2mm shorter than RD? (it's been around 35 years since I worked in Yamaha dealers)
Clearance should be around 0.002" but will work up to 0.005", above that the skirts break off from piston slap.
I didn't see bore clearance anywhere?
Usually, it's pretty safe to make bore exact millimeter size as pistons are the correct amount undersize (not ideal but always a good place to start)
If place doing bores only had experience of old Wiseco pistons with the massive clearance and expansion ,they have they may have followed incorrect instructions?
 

teazer

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I just read that other thread and the guys there are saying:
Excessive piston to bore clearance (still to be measured but machined at .002" which would be noisy but fine)
Poor/insufficient chamfer on ports
Wrong pistons. That's interesting that they believe you cannot use watercooled pistons an an aircooled motor and yet hundreds of uas have been doing that for years. The idea I think is that the taper on a watercooled piston would be different to an aircooled one and therefore it will rattle. But no one had the actual taper and cam numbers to compare and I understand the rationalization, I am a little skeptical.

If it were my motor I would remove the top end and measure actual piston to bore clearance and rod rock. I would be surprised if it's big ends. They typically have a deep knock sound not a light rattle.

if there is nothing wrong with bore clearance or port chamfers I'd just run it. If bores are too large go up to +2.0mm pistons and see how that works, but first measure clearances.
 

Hurco550

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If I remember right, LC pistons have smaller windows in piston and are 2mm shorter than RD? (it's been around 35 years since I worked in Yamaha dealers)
Clearance should be around 0.002" but will work up to 0.005", above that the skirts break off from piston slap.
I didn't see bore clearance anywhere?
Usually, it's pretty safe to make bore exact millimeter size as pistons are the correct amount undersize (not ideal but always a good place to start)
If place doing bores only had experience of old Wiseco pistons with the massive clearance and expansion ,they have they may have followed incorrect instructions?
I've got a set of LC pistons (and barrels) handy if anything needs measured. I'd be happy to.
 

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