Cylinder scoring after rebuild

[richclare]

Hi everyone,

I have just finished rebuilding a 4l0. I had the cylinders rebored to +1.00 with new mitaka pistons. I have run it through about 4 heat cycles (maybe 45 minutes running in total), trying to sort a small water leak.
This morning I took the cylinders off to check the gasket face in detail. On both cylinders and pistons there is light scoring, on the inlet and exhaust sides. Is this normal as the engine hasn't yet been put under any stress (unlike me😣)?

I would appreciate your thoughts.

I will post some photos tonight when I get home.

Cheers,

Rich

[midlifecrisisrd]

Check the ring gap

It has been known that the piston clearance is ok but the ring gap is too tight so when they heat up the have no where to expand so damage the bore

Steve

[richclare]

Sept 25, 2017 11:16:42 GMT 1 midlifecrisisrd said:

Check the ring gap

It has been known that the piston clearance is ok but the ring gap is too tight so when they heat up the have no where to expand so damage the bore

Steve

Thanks Steve, I will check tonight.

Rich

[richclare]

Hello again,

OK so I first checked the ring gap which is 0.012", so just within the tolerance 0.012-0.018".
The photos below show the scoring. What I don't understand is the scoring doesn't go all the way to the top of the bore ie. where the ring upper limit is. If the bore is too big (or even too small) for the piston would it slap around and cause this scoring pattern? Can I measure piston/bore clearance with feeler gauges?

Lots of questions, but perhaps someone has had the same.

Cheers

Rich

[midlifecrisisrd]

Don't like the lack of chamferring on the ports

Even the pistons are only marked from the ports down

Steve

[midlifecrisisrd]

Should have said that's not tight rings

Are they just rebored and to what size

Steve

[richclare]

Hi Steve,

For the port chamfer, can I remedy this myself? I have a Dremel or is it better with a file? What kind of chamfer - is it just taking the 'edge' off?

Both cylinders were rebored to +1.00. When you say it's not tight, should it be less?

Thanks for replying so quickly.

Rich

[midlifecrisisrd]

Just take the edges off at a 45 degree angle

If you can get in with a Dremel that's fine

The gap as long as in the spec is fine

Steve

[stusco]

I couldn’t do mine with a dremmel ,I used needle files the cylinders are hard so it takes a few and time ,I gave mine about half a mil I hope this is enough

[Tobyjugs]

It could be some contaminant has gone through the barrels

Difficult to say without measuring everything. You can still see hone pattern under the scuffing (so not deep). I would leave it. You could give it a very very light hone and inspect the piston rings for damage (magnifying glass). The pistons are ok and then put it all back together.

Also solve your water leak Did the engine get too hot?

Now Steve has mentioned it i think he's right.

[richclare]

Cheers guys, I appreciate your advice.

Steve & Stusco - I will try the dremel and if that doesn't work then some good old elbow grease with files. I am assuming ALL ports need to be done.

Tobyjugs - Thanks for your optimistic reply.... I was pretty anal with the cleaning of everything and as both cylinders were the same I thought it was something other than foreign matter. I am still trying to solve the water leak. I don't believe it overheated as I was so careful when I started it and let it heat up (feeling the barrels and water hoses to check it heated evenly). The water leak was the reason I discovered the scuffing, so I'm thankful for that. 

Thanks again. Really appreciate your expertise.

Rich

[midlifecrisisrd]

I'd also put a ring in the bore at different points to check for a taper

Use the piston skirt to push it down square in the bore from the top and check every 10mm or so

Steve

[richclare]

Sept 25, 2017 20:26:49 GMT 1 midlifecrisisrd said:

I'd also put a ring in the bore at different points to check for a taper

Use the piston skirt to push it down square in the bore from the top and check every 10mm or so

Steve

Thanks Steve - will do. I think I will be busy this week😉

[richclare]

Sept 25, 2017 20:32:58 GMT 1 richclare said:

Sept 25, 2017 20:26:49 GMT 1 midlifecrisisrd said:

I'd also put a ring in the bore at different points to check for a taper

Use the piston skirt to push it down square in the bore from the top and check every 10mm or so

Steve

Thanks Steve - will do. I think I will be busy this week😉

Steve, just a question - how can a rebore end up tapered.... I thought it was done on a kind of lathe, so everything was parallel.

[stusco]

Machines can run off who did your rebore?they can be done on a lathe but there are machines just fore boring cylinders

[richclare]

Sept 25, 2017 20:52:36 GMT 1 stusco said:

Machines can run off who did your rebore?they can be done on a lathe but there are machines just fore boring cylinders

It was a company in France, close to where I live. They just do automotive and bike engineering - skimming, valve work, rebores etc. They were recommended, but you never know. They didn't chamfer the ports!

[stusco]

I had a bad rebore last summer just like you leaking head gasket whipped the head off big scores needed another rebore ! Yours will be ok if it’s the right size mine was overbored

[midlifecrisisrd]

Depends how they hone it or if they make an arse of the bore

Steve

[richclare]

My list of 'checks' is getting longer😉

[4l04ever]

As the boring bar goes up or down the cylinder, the cutting tool can get hotter as it goes and grow slightly, so the bore is smaller at the start of the bore and a little larger at the end. I had a cylinder done many years ago, which had 1 thou clearance at the top, but 5 though at the bottom of the bore.....

[jackjabba]

Will the piston drop through the barrel with the rings off, if it doesn't its too tight.

[Tobyjugs]

I Think the best check is what Steve advised it's simple and fast and if the ring gap changes i would look deeper into it.

[richclare]

So I did the ring test, as suggested by Steve (midlifecrisisrd). The gap was constant all the way down the bore - sigh of relief!

Next step is to fettle the port openings with a wee chamfer. The more I look at the bore's and pistons (as the scuffing is the same across both barrels) it seems to indicate this is the culprit.

Thanks again for all your opinions and comments.

Rich

[steeley]

Hi , over the years of owning 2 strokes I have built up a bit of a collection of various files . I put a chamfer on the top and bottom of the ports and take the edge off the sides. when its all done I mix up a bucket of hot soapy water and soak and scrub them out .

[richclare]

Sept 26, 2017 20:13:23 GMT 1 steeley said:

Hi , over the years of owning 2 strokes I have built up a bit of a collection of various files . I put a chamfer on the top and bottom of the ports and take the edge off the sides. when its all done I mix up a bucket of hot soapy water and soak and scrub them out .

Thanks for the tips steeley. I have yet to find my files. All the ones so far have been too long! You wouldn't have a nice photo of your files, just to tease me😉. Joking aside it may help my search.

Cheers Rich

[arrow]

I've got a set of diamond coated needle files from Aldi. They are superb.

[steeley]

Hi, if its any help I could put a small video up for you of the files I use . Photobucket has gone to pot and not sorted any other out yet . As arrow said diamond coated needle files are good I picked 2 sets up years ago at a discount store . Greedy I know Lol.

[richclare]

Thanks guys for your shopping tips .
Steeley, if you could post a video that would be very useful.

Rich

[steeley]

Rich hi, hope this helps you out.

[richclare]

Hi Steeley,

You're a star, thank you. Particularly like Poppy's 15 seconds of fame!!
Just a question which you or someone else may be able to answer. Do I need to chamfer the sides of the ports, in particularly of the transfer ports. I have managed to do the tops and bottoms, but the sides are hard to get at (unless I buy more files).

Thanks again,

Rich