Rebore Required??

[jimblob]

Hi Guys,

I'm hoping a resident expert can advise me if i should stick with the hone and new rings approach or play safe and go for a rebore!!

I'm rebuilding a rd350r (minus the horrible broken fairing  ) and its done 13,000 miles.

The bike shop guy said it only needed a hone but I can still feel slight scoring on the barrel with my finger and there's a mark from liquid seeping into the barrel due to me not washing out the barrel properly after the honing was done.

I thought I'd best check whilst its still in bits.

Please see photos below......


IMG_20220330_182423_413~2 by James Smith, on Flickr


IMG_20220330_182611_131 by James Smith, on Flickr


IMG_20220330_182808_546~2 by James Smith, on Flickr

All advice gratefully received.

Cheers

Jim

[shaunthe2nd]

If it was me, and my beloved bike it would be getting a rebore unless on last size already.

[midlifecrisisrd]

At 13k I'd it would be needing a top end refresh so deffo rings

I personally wouldn't worry about the water marking

The bit that worries me is the score. Is that where the ring gap sits suggesting the end gap is too tight

What bore size is it on?

13k would suggest standard but if it's ring ends causing the problem that suggests tight which wouldn't happen on a standard 13k bore

Now here's a curve ball

Its a 350r, they don't have an expander ring under the bottom ring. Has one been fitted from aftermarket rings as this would make it too tight

Does the score correspond with the lower ring end gap?

Steve

[yamman1066]

hi,may be a light hone will sort it

[chrisg]

I would get the bores measured properly using a bore gauge and then make a decision. Can you get a feeler gauge between the piston skirt and the bore to give you an idea? Whilst it has 13k on the clock, this may not be correct, many of my mates used to take their speedo cables off to keep the miles off the bikes. 

[midlifecrisisrd]

Depends what life it's had too

My lc2 back in the day had 17k on it When I bought it and was on 0.5 over

I've also had a 350r with 63k on 0.5 over

If it's been abused or seized it will need bored regardless of miles

Had a neighbour when I was a teenager who bought a brand new ts185

They never told him you had to put 2 stroke oil in it...

Steve

[bare]

Could simply fit a fresh set of rings and see how that does?? Cheapish and easy. Do as suggested set ring end gaps though .
You can always pay for a rebore and pistons/rings ... when and if... needed :-)
Don't do it's until Proven necessary IMO.
These old engines can run well, happily ... even when in far from ideal/perfect condition.

[shaunthe2nd]

It's probably going to cost you £50 for the hone, plus £30 for rings and £25 for gaskets plus oil and coolant, say £120.
It will cost you £100 for rebore, plus £100 for pistons/rings, and £25 for gaskets plus oil and coolant, say £240.

To me its a no brainer, in the old days when we were strapped I would have risked the hone, but not now.
If the hone doesn't produce the results you are back in for a further £240 and twice the workload/hassle.

As you have top end stripped already, I'd just do it the once and do it justice from the off, run it in and ride all summer with confidence.
That's what I'd do anyway.

[steve63]

So it's already had a hone? New pistons and rings?

Personally I'd stick it together and ride it. The difference between doing that and spending hundreds on new parts and a re-bore will be negligible. I'm pretty sure there will be no difference in feel or even on a dyno so why bother?

It's up to you if you want to spend a couple of hundred quid for 0.25hp

I was told my Fizzy needed a re-bore. It was slow and the barrel had a lot of scores. I had it re-bored and ran it in. Guess what? No difference I could tell in speed or sound, nothing  .

Isn't the scoring next to the exhaust port?

Aren't the ring gaps at the back either side of the inlet port?  

Most important thing to do is make sure whatever caused the issue is fixed.

[midlifecrisisrd]

Not if the piston is in backwards 🤣🤣🤣

Steve

[jimblob]

Thanks for all the input guys.

I’ve been and had another look and found my Rusty feeler gauges!!

The pistons/rings are currently 0.25 oversize. I’ve checked the ring gap position and it isn’t near the location of the scoring. I don’t think there was a spacer fitted behind the bottom ring but I removed them a few months ago!! The score mark is more of a very faint ripple when felt with a finger nail and can’t really be felt with a finger (honest). The honing has already been done at this stage and I’ve got the new piston rings.

I’ve measured the piston ring end gap approx 20mm from top of cylinder and it’s approx .45mm!!! Does that sound about right?

I was quite happy when I was told it just needed a hone but some of the other jobs they’ve done since have been less than perfect and I thought I’d better get a 2nd opinion before I put it all back together.

I suppose for peace of mind I’d be best to find another engineering place and see what they think.

Jim

[abar121]

0.45mm is ok. I'd give it a go personally.

[jimblob]

Thanks for all your responses guys.

A local engineer has recommended a guy that’s a whiz on engines. I’m gonna take it to him to see what he says.🤞

[4l04ever]

I would not rebore unless it was well scuffed. You only get so many rebores before you need replacement cylinders or a re-sleeve job.

[dougw]

Back when young and skint I`ve rebuilt, raced and ridden on the road with far more wear and damage than that with no problems.( Well , no more than the usual problems caused by running them really lean for racing and over-revving them to oblivion)

I`m in the not rebore (yet) camp.