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Post subject: Fitting YPVS piston in a 350 lc
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Gillybobs  Suburban Commuter Joined: 25 Oct 2007 Total posts: 63 Location: Sheffield,South Yorkshire |
Rebuilding the top end on my 350 lc and im fitting YPVS pistons.
Heard something about removing the tang at the bottom of the piston.
Just want to know why people have done this?? What problem does it cause.
The bike is in standard 350 lc tune . No mods.
Cheers
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Posted: Wed Jul 23, 2008 11:06 am
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NORBO RD/RZ Jedi Master Joined: 03 Aug 2003 Total posts: 5503 Location: Nottingham |
some people do and some people dont. I do
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Posted: Wed Jul 23, 2008 11:11 am
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Billy One-handed Stoppie! Joined: 12 Oct 2007 Total posts: 2216 Location: Sweden arise! |
Lc's doesn't have a bridged intake, which means that the tang on the piston could - if you're really unlucky - catch and break the piston.
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Posted: Wed Jul 23, 2008 11:17 am
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| Rich |
Hi, the theory goes something like this. The YPVS cylinder has a bridge down the middle of the inlet port (like a double window) so it can run a larger port and still have support for the piston skirt. The tab on the YPVS piston is in the middle of the lower rear skirt. Therefore, in a YPVS barrel this tab runs up and down quite happily against the central bridge and is nicely supported.
The LC inlet port doesn't have a bridged inlet port - it is basically one large hole without a central bridge. So, the thrust on the piston skirt will cause the piston to rock slightly and the tab will tend to 'catch' the bottom of the inlet port. There are mixed opinions about whether or not this will cause a problem - some people say it will be fine but if it was me, for peace of mind I would file off the tab.
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Posted: Wed Jul 23, 2008 11:26 am
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1982RD350LC Regular Racer Joined: 31 May 2008 Total posts: 354 Location: Vancouver, Canada |
About 2 months ago I was in your same situation and could not locate pistons for my 1982 RD350 so I settled on the ones that you are looking at. I didn't hack off the tang and rode for about 1000km with no problems. But having nothing better to do one night (and being flogged by these guys here  ) I took off the cylinders just to peek at my workmanship. What I found was that there was enough excessive wear to the point where the beveling around the skirt of the pistons near the tang was now a sharp edge. I dremeled off the tang and recreated that bevel on the skirt where the tang once was and just for the heck of it through both pistons on a really accurate scale and balanced them up to each other (but not sure if that's criticle (half a gram off). The job was easy enough to do in a night.
ron
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Posted: Wed Jul 23, 2008 3:40 pm
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| Rich |
| 1982RD350LC wrote: |
About 2 months ago I was in your same situation and could not locate pistons for my 1982 RD350 so I settled on the ones that you are looking at. I didn't hack off the tang and rode for about 1000km with no problems. But having nothing better to do one night (and being flogged by these guys here ) I took off the cylinders just to peek at my workmanship. What I found was that there was enough excessive wear to the point where the beveling around the skirt of the pistons near the tang was now a sharp edge. I dremeled off the tang and recreated that bevel on the skirt where the tang once was and just for the heck of it through both pistons on a really accurate scale and balanced them up to each other (but not sure if that's criticle (half a gram off). The job was easy enough to do in a night.
ron |
Hi Ron, thanks for that info. Did you take any pics of the wear on the piston while it was apart?
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Posted: Wed Jul 23, 2008 3:47 pm
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GillybobsSuburban Commuter Joined: 25 Oct 2007 Total posts: 63 Location: Sheffield,South Yorkshire |
Cheers for the info guys 
Was just trying to understand why the tang needed removing.
Think for peace of mind im going to remove the tang.
I'll keep you posted how i get on
Cheers
Gillybobs
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Posted: Wed Jul 23, 2008 4:33 pm
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BillyOne-handed Stoppie! Joined: 12 Oct 2007 Total posts: 2216 Location: Sweden arise! |
To be honest, you don't actually have to remove the whole tang. Filing it smooth where it meets the bottom of the intake ports is probably enough. But it looks better when removing it completely. 
Oh btw, when you're at it. You could smoothen out the edges at every port inside the cylinders. I always do that so the rings doesn't catch anything as easy. Don't touch the ports at the vertical edges, you want as much as possible left there if you're using the bike on the road. Only do the horisontal edges of the ports.
They are smoothened out originally, but only roughly, and it is the roughnes you want to make smooth. Especially the exhaust ports!!
I use fine hand files and sandpapers to do the rough parts, (the material is pretty hard) and then i finish it off using sandpaper. And then fine sandpaper like 800 or so.
On some parts you don't need anything else but sandpaper since they probably used a death grinder to "smoothen the ports out". Sadly, they did this on the exhausts aswell and you will see it on a cylinder that has never been bored, so you want to take away as little material as possible, only making a smooth edge.
I think - and now i'm only guessing - you'll lose a tiny bit of engine power by doing this, but imo it's better because you eliminate another risk of seizure.  I've seen it soooo many times, rings catching the exhaust ports, (Even on bridged exhausts!) trashing all hell up and ruin the bore. 
If you can't be arsed to do every port, then just focus on the exhaust ports. That's the most important part of the cylinder. Both upper edge and the lower edge since the rings travel over the whole ports.
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Posted: Wed Jul 23, 2008 8:08 pm
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