1971 SL350 Engine work questions. Cyl #2 scuffed a bit

[MotorbikeBruno]

Hey guys! I'll say thanks in advance for any insight and the best way to go about it.

I'm helping a co-worker get his 1971 SL350 back in shape. It was sitting in a barn for several years. It ran great for a while and then I heard some cam chain noise. So I decided to check the tensioner as I've seen tons of threads showing bad news. And this did too! Found a few bits in the oil "filter" and bottom of the engine. So I've got her apart and thought I would ask a question regarding the right cylinder. #2 is looking beat up. Why is this? Anything I should be looking for? Replace them both? Best choice for parts...Wiseco? Bring the jugs in!? Sorry I've only done top end stuff, nothing with cylinders/pistons really. Approximately 21,000 miles on the engine. New tensioner already installed (lower and upper rollers!) AND I didn't lose any of those little pieces that hold that bottom roller in place! Woohoo!

Now for the trouble....

 

[VonYinzer]

I'd clean, hone, and rebuild with new rings. That said... We'd really need to see what the cylinder wall looks like to be sure.

 

[MotorbikeBruno]

I'd clean, hone, and rebuild with new rings. That said... We'd really need to see what the cylinder wall looks like to be sure.

Thanks for the reply Von. I'll let him know new rings are in his future, he was expecting that. I'll snap a picture of the cylinder walls tomorrow when I get in. (both sides just in case) to give a clearer idea of what might be going on. So the scratches on the piston aren't really a "problem" as far as the piston is concerned?

 

[AlphaDogChoppers]

Look very carefully at the cylinder bore. Is it smooth where the rings travel? A little scuffing from the piston in the lower part of the cylinder can be tolerated if the budget is tight. Lightly clean up the piston with 600 grit sandpaper where it is scuffed. You are not trying to remove the scuffing. That would remove too much metal. Just take down the high spots a little.

If this was my engine, I would probably opt for first oversize rebore. (Always both cylinders.) But, like I said, if it is a budget rebuild, as long as the area swept by the rings is smooth, just a little honing and re-ring should be adequate. Don't hone too much. Again, you are not trying to remove all traces of the scuffing. You would end up with a loose piston. Just enough to clean it up and knock down the high spots. After a little honing, you can better see the true condition of the cylinder, too.

 

[MotorbikeBruno]

Look very carefully at the cylinder bore. Is it smooth where the rings travel? A little scuffing from the piston in the lower part of the cylinder can be tolerated if the budget is tight. Lightly clean up the piston with 600 grit sandpaper where it is scuffed. You are not trying to remove the scuffing. That would remove too much metal. Just take down the high spots a little.

If this was my engine, I would probably opt for first oversize rebore. (Always both cylinders.) But, like I said, if it is a budget rebuild, as long as the area swept by the rings is smooth, just a little honing and re-ring should be adequate. Don't hone too much. Again, you are not trying to remove all traces of the scuffing. You would end up with a loose piston. Just enough to clean it up and knock down the high spots. After a little honing, you can better see the true condition of the cylinder, too.

Thanks Alpha. Yeah I don't think it's a budget build, as much as a "I want to ride it now" build. haha. But with today's parts being relatively available, I think we should take care of it correctly. I'm going to take a picture right now and post it up of the cylinders.

 

[MotorbikeBruno]

File 34 is the RIGHT cylinder that has the scuffs

File 35 is the LEFT cylinder that the piston looks good.

 

[VonYinzer]

Yup. Light hone and new rings. If it were my bike. Worst case scenario, you do that and over winter pull it apart for a rebore amd new pistons.

 

[MotorbikeBruno]

Yup. Light hone and new rings. If it were my bike. Worst case scenario, you do that and over winter pull it apart for a rebore amd new pistons.

Yeah, I think I'm going to have him just do it now. That way I don't have to do it again this winter!

http://www.ebay.com/itm/HONDA-CB350-CL350-CB350G-SL350-PISTON-SET-FIRST-OVER-025-OVER-/130923213115 Good deal?

Then when those come in, I bring the jugs and these pistons in to them correct? No need to change anything with the head? (I'll take a look at the sealing surfaces, and pull the valves to have a look and clean them up) Also, what about the HEAD GASKET!? Might not fit the new pistons?

Left Bore was 2.511Inches and Right Bore (with scuffs) was 2.501 Inches (used a micrometer)

 

[cyclhed]

IMO it should be bored and new pistons installed. Those scratches probably won't hone out. You should measure the pistons and bores and check them against the numbers in the WM. It looks like it overheated probably from low oil level or oil not being changed. Those are cool bikes, I have one.

 

[MotorbikeBruno]

IMO it should be bored and new pistons installed. Those scratches probably won't hone out. You should measure the pistons and bores and check them against the numbers in the WM. It looks like it overheated probably from low oil level or oil not being changed. Those are cool bikes, I have one.

Ok that's the plan gents. New Pistons, 1st size over (have to check manual to verify it hasn't been done before) and get it bored out correctly.

The story is that he bought it brand new when he was 18. Rode it for a couple of years, sold it to the farmer down the road. Called me up because he knew I had been working on my own bikes and said he just got his original bike back from that farmer for $50. Missing the exhaust and other things. He took it to the local Honda dealer and they said they got it running, unfortunately they didn't have the exhaust on there, so maybe that did some damage? He brought it to me and I actually got it running relatively nicely after getting a set of CL pipes for it, then after a few miles the cam chain made some noise and I checked the tensioner...and here we are! haha. It has some sentimental value to him, and he went and got the frame painted etc. Looks pretty slick.

 

[AlphaDogChoppers]

I can't be sure, but it looks like the scuffed cylinder has a pretty deep scratch all the way to the top of the cylinder toward the left side of that photo. Re-bore is your best way to do it. It'll only take a few extra days to do the job right.

 

[MotorbikeBruno]

I can't be sure, but it looks like the scuffed cylinder has a pretty deep scratch all the way to the top of the cylinder toward the left side of that photo. Re-bore is your best way to do it. It'll only take a few extra days to do the job right.

Agreed. And you are correct, that is a "deep" scratch that goes up the whole bore. I'm getting quotes and prices from a few local guys and Bore-tech who sells a lot of racing parts etc for the 350's

As always, your help and info has been MUCH appreciated. One last question is about the head gasket. If I bore over a little, can I still use the stock one? Or do I send the new head gasket with the boring and they bore that to make it work?

 

[AlphaDogChoppers]

Head gasket should no require any modification with a simple oversize bore. It isn't a "big bore" job, just going 20 thousandths over.

 

[MotorbikeBruno]

Head gasket should no require any modification with a simple oversize bore. It isn't a "big bore" job, just going 20 thousandths over.

Awesome. That puts my mind at ease, since I already have it! I'll report back when all is said and done. Got quotes coming from a few different guys. This thing is going to be awesome! Should I be PM'ing PJ to ask about oil modifications, or does this bike not have the same tendency to eat it's own head like the 360 does?

 

[Maritime]

Awesome. That puts my mind at ease, since I already have it! I'll report back when all is said and done. Got quotes coming from a few different guys. This thing is going to be awesome! Should I be PM'ing PJ to ask about oil modifications, or does this bike not have the same tendency to eat it's own head like the 360 does?

Might want to give PJ a couple more days to come down off the morphine so the answer makes sense. I am pretty sure the 350 doesn't have the same oil issues but someone else will verify. DR J is pretty knowledgable on the 350

 

[MotorbikeBruno]

Might want to give PJ a couple more days to come down off the morphine so the answer makes sense. I am pretty sure the 350 doesn't have the same oil issues but someone else will verify. DR J is pretty knowledgable on the 350

Yeah I saw he just had surgery, I hope it's for the best this time, as he's had some rough back pain for a while it seems.