YS96SRX Piston hitting backplate

[MadBird]

Has anyone had an issue where the piston in their 96SRX has been hitting the backplate?

Mine has and it caused failure of the piston and subsequent pulverising of the dislodged portions in the top of the head. I messaged Richard at YS for his comment just shy of 3 weeks ago and he hasn't replied as yet. I've sent him another email today as I want to fix it, but don't want to play around too much if he wants the components back.

If people have had this issue what have you done to prevent it happening again whilst still using the 91 pistons? Maybe I just need to cut my losses with my stock of 91 pistons and buy the 96 ones.

This happened to the stock engine. I had not altered it since new.

[sdunne]

More likely part of piston skirt broke away causing the subsequent damage. Part hit backplate then up through the transfer port to do the remaining damage in the bore.
Was it running lean/hot?

[MadBird]

She was running perfectly fine before the piston broke. Was tuned just on the rich side.

It was definitely piston hitting the backplate, there are marks where both sides were contacting the backplate and the piston is warped and has a hairline fracture on the side that did not break off.

Richard had previously mentioned to me when I queried him about upgrading my 91SRX to 96 that if a 91SRX is converted that the piston may contact the backplate in exactly this fashion and that some reliefs may need to be cut in. That makes me suspicious about the build tolerances of this 'OEM' engine.

I'm in the process now of converting my 91SRX, so I'll pay close attention to the clearances on that and see.

[abit late]

The backplate on my 96SRX has piston skirt cutouts. There cannot be any contact between the two.

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[MadBird]

Originally Posted by abit late

The backplate on my 96SRX has piston skirt cutouts. There cannot be any contact between the two.

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Very interesting... I wonder if mine may be a very early unit that never received this update.

Do they look cnc'ed or done by hand?

Any chance you could throw up a pic or two of it?

[Baedarlboo]

How's your rear bearing look? Rear bearing failure can cause damage like this.

[MadBird]

Originally Posted by Baedarlboo

How's your rear bearing look? Rear bearing failure can cause damage like this.

Cheers for the suggestion Baedarlboo, but the rear bearing is perfect and I know that the problem was caused by piston hitting the backplate, there is no disputing the fact.

This is how the backplate should be machined. Found the pic on a Turkish site.

Now I just need to figure out some measurements so I can do the same to mine.

[Baedarlboo]

Originally Posted by MadBird

Cheers for the suggestion Baedarlboo, but the rear bearing is perfect and I know that the problem was caused by piston hitting the backplate, there is no disputing the fact.

This is how the backplate should be machined. Found the pic on a Turkish site.

Now I just need to figure out some measurements so I can do the same to mine.

Strange, I don't think either my 91srx or 96srx have cutouts. Both of mine are steal lined like yours. The picture you posted is a none steal lined backplate. Green heads were sold to US markets, black heads were sold everywhere else. The green heads had steal lining on the back plate, while the black heads did not have the lining.

[MadBird]

Originally Posted by Baedarlboo

Strange, I don't think either my 91srx or 96srx have cutouts. Both of mine are steal lined like yours. The picture you posted is a none steal lined backplate. Green heads were sold to US markets, black heads were sold everywhere else. The green heads had steal lining on the back plate, while the black heads did not have the lining.

Yeah it is strange and my 91SRX backplate is exactly the same with no cutout, although the 91's stroke isn't as long therefore no cutout is required.
My 96 is a US one and I know YS USA is aware of the problem.

Ordinarily I'd have bought the Japanese version as I live in Australia, but I got this one put in a heli that I had built for me stateside or the RCHN funfly last year. Hopefully Richard returns my email shortly and sheds some light on a definitive remedy.

[dbcaster]

Are you sure that's a YS piston? I haven't seen a YS piston with holes in the top. OS pistons have holes like that though. Maybe the 96 is different than the 91 and 120?

[Baedarlboo]

Originally Posted by dbcaster

Are you sure that's a YS piston? I haven't seen a YS piston with holes in the top. OS pistons have holes like that though. Maybe the 96 is different than the 91 and 120?

I was thinking the same thing but I think the older SR models had oil holes on top. I'm wondering if the motor was rebuilt with different parts.

I've converted a 91srx to 96srx and the only parts required were, 96 head, 96 crank, and 96 sleeve. The piston/ring, rod, case, and everything else is the exact same.

[MadBird]

Originally Posted by dbcaster

Are you sure that's a YS piston? I haven't seen a YS piston with holes in the top. OS pistons have holes like that though. Maybe the 96 is different than the 91 and 120?

I can only assume thats it's a YS piston. That is how the engine came to me new.

[MadBird]

Originally Posted by Baedarlboo

I was thinking the same thing but I think the older SR models had oil holes on top. I'm wondering if the motor was rebuilt with different parts.

I've converted a 91srx to 96srx and the only parts required were, 96 head, 96 crank, and 96 sleeve. The piston/ring, rod, case, and everything else is the exact same.

The three components you mentioned are indeed different however the backplate it also different too, check the part numbers in the manual. I'd suggest removing your backplate and seeing if the piston is making contact with it slightly. Look at my pic of the backplate and you can see a slight mark where the piston was just hitting.

[Baedarlboo]

Originally Posted by MadBird

The three components you mentioned are indeed different however the backplate it also different too, check the part numbers in the manual. I'd suggest removing your backplate and seeing if the piston is making contact with it slightly. Look at my pic of the backplate and you can see a slight mark where the piston was just hitting.

Part numbers are going to be different because they don't distinguish a none steal lined vs lined back plate. Mine has been running for a few gallons and have been taken apart twice since the conversion. I don't have marks or busted piston.

[MadBird]

Ok so I've spoken to Richard on the phone and he has cleared a few things up. He was very helpful and a great guy to talk with.

Facts: The steel ringed backplate on the US version does not have any grooves like the Japanese version. Which we all know... Also the 96 pistons sold by YS USA are modified to prevent them from touching the backplate.

What does this mean? If you have a US version 96SRX you cannot use the standard replacement 91SRX piston (R7006) or there may be a risk that the piston will make contact with the backplate.

Further Info: Upon inspection he said there have been instances of the piston 'lightly' touching the backplate, but no reported catastrophic failures like mine that have been attributed to the touching components.
To get around the issue of touching the backplate, the pistons on the US version are different to those of the Japanese version and have a slight amount of material taken from them to prevent contact.
From our conversation we have figured it is likely that the piston in my particular engine was not modified to give clearance on the backplate helping cause the failure.
As pointed out earlier my piston looks a little weird for a YS one with 6 oil holes on top rather than just one like normal, something Richard is going to query the factor about.

Richard has offered to replace the damaged components. I have turned this down as I am only after answers so that I can prevent it from happening again.

Going forward: As I'm not from the US I don't normally purchase my parts from there as the shipping costs to Aus are ridiculous, so I need to find another remedy. Also for that matter I already have a supply of 91 pistons etc in my spare parts box.
I am left with two or three options;
1. Modify each piston separately as I need to replace them.
2. Modify the backplate by cutting some reliefs into it
3. Purchase a Japanese version backplate that already has reliefs cut into it

At this stage I’ll modify a piston and see how it goes as thats probably the quicker and easiest ‘one off’ fix. I may purchase a new backplate in the future when I next place a spares order, but that may not be for months or even a year if I don’t go blowing too many engines up.

Thanks for everyones input.

Cheers Matt

[MadBird]

Originally Posted by Baedarlboo

Part numbers are going to be different because they don't distinguish a none steal lined vs lined back plate. Mine has been running for a few gallons and have been taken apart twice since the conversion. I don't have marks or busted piston.

The steel lined backplate is just the 91SRX stock backplate (S9216) that YS USA had included on the US version of the 96SRX. The backplate on the original 96 (Black head) is R8016.

Just something for the US customers to be wary of if buying parts from overseas. For full compatibility, spare parts for the US version need to come from YS USA to be sure you are getting the correct ones. See my post above for more info.

[Baedarlboo]

That's strange for sure. You would think with all the 96srx floating around in the US (including converted units) there would be more reports of this issue. I honestly think that because you seem to have a different piston than the rest I've seen, there was a miss match and you had a motor built with the wrong piston. It seems to be the only logical answer at this point. Keep us updated on your rebuild with pictures of the replacement parts and old parts.

[rctintin]

I would say you must mod the back plate not the piston. If you mod the piston, you change its weight. If you change it's weight, you also have to change the crank counterbalance weight. Far better is to mod the back plate.

Remove piston and liner, and put back plate back into the case. Now use a sharpie down through the bore and scribe an arc on the backplate, and Dremel a relief groove on that line.

[Thestructured]

Mine has a steel(american model) backplate and a piston from RC Japan. There are no oil holes on the top of the piston, I've never seen that on any YS heli engine..strange.

There are no marks on the backplate from the piston hitting either and I've run a gallon through it so far. I feel that someone at the factory put the wrong piston in yours or something. I've never seen holes on top like that though it's definitely a YS piston because it has the pin mark on it. YS 91 sr didn't have them, 91 srx, 120srx, 120sr, or 96srx here..

[smwht]

Be careful with that setup- I had the same in mine and wound up cracking the piston skirt about a gallon and a half in. Very faint witness marks on the backplate from where it touched, definite cracks in the skirt.