Dirt bike engine cylinder hone

My sons 70cc dirt bike needs reboring. I plan to do this in my lathe using a faceplate which the cylinder will be bolted to. I've never done this, I always just farmed my cylinders out over the years. I decided that me and my son could do this together so he can see a practical application to what I do in my shop.
Boring seems fairly straight forward , I understand to leave a few thousands to hone out.
I do not have a true engine cylinder hone, I have a brake hone that I use on shotgun barrels and suppressor tubes. It's not expensive.
I see engine cylinder hones in the 1.5" to 2" range costing from $25 to well over $100. Is there any practical difference in them ? He's not racing, this is just a bike he rides on the family property.

No point in doing it if the bore does not end up straight and round. Lathe will bore it ok but you need to finish it with a proper rigid hone with the correct grit stones. This is about the most important part of the job and not a place to cut corners. I use Sunnen hones and stones exclusively and recommend you do so as well.

Have you looked into a sleeve for that engine? I don't have the proper equipment to bore out a block to the proper precision but I have sleeved about a half dozen with good results. If the cylinder you now have is nickel plated (they used to call that nickosil) you are going to lose that anyway. If you sleeve it then you just have to grind out the ports to match them up with a die grinder.

No I have not, it's a Honda crf70f kids dirt bike , I doubt it's plated as it's not high performance and Honda sells oversized pistons for us as well telling me it's meant to be bored.

A proper rigid cylinder hone is needed.

Flex hones are not going to make a round true hole. They are fine for a quick deglaze/crosshatch, or brake cylinder touchup. Brake cylinders do not run at 5k rpm, with cast iron rings...

Do it once, do it right... If you can't justify buying a rigid hone, farm it out. Air cooled motors work and last best, with proper piston fit over full range of piston travel.

It's an xr70, just go for it, worst case you might need to buy another cylinder for a couple dollars.
On the other hand you can buy an 88cc top end kit complete for $125.
Father son time might be better spent installing the new parts on the bike.
It's not plated, just a cast iron sleeve and any hone will do the job.

A friend of mine who builds Briggs racing motors has a homemade hone setup. It's just a counterweighted drill that slides up and down with a cheep hone in the chuck. Has a little pump which flushes the bore with mineral spirits.


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2 or 4 stroke? .it makes a difference to what I may be able to suggest.

Doing the boring on the lathe will be fine. A brake cylinder hone will work. Just hone it to get some cross hatching so it will hold some oil. Just remember you might have file the ring gap to proper spec.

kineticmx said:

...you can buy an 88cc top end kit complete for $125...

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...This...

Limy Sami said:

2 or 4 stroke? .it makes a difference to what I may be able to suggest.

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This.

A 2 stroke will require a ball hone of proper size. If it is a two stroke dont forget to relive the exhaust port bridge(if it has one) by a few thou, or it may seize on you.

Brush Research FLEX-HONE Cylinder Hone, GB Series, Silicon Carbide Abrasive: Amazon.com: Industrial & Scientific

I have honed boat loads of four stroke non plated steel and cast iron cylinders with a standard hone without any noticeable or measurable undesirable effects. but as stated these hones will only follow an existing hole. they will not correct any problems.

Amazon.com: Lisle LI15 Engine Cylinder Hone: Automotive

Lathes are pretty good at making round things. Although i have never bored a cylinder in this manner, i don't see why it wont make a round and square hole. (I have a bar meant just for boring motorcycle cylinders so ive never needed to try.)

I have full confidence that on a 70cc honda or honda copy, that your method will work just fine. Especially considering the application.
Good luck.

It's a four stroke.

Limy Sami said:

2 or 4 stroke? .it makes a difference to what I may be able to suggest.

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its a 4 stroke

kineticmx said:

On the other hand you can buy an 88cc top end kit complete for $125.

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Click to expand...

adding more power to a beginner rider 10yo is not gonna fly with the wife, it was a shouting match to get her to allow the dirtbike in the first place. I have a lovely scar across my forehead from a bike wreck when I was a kid.

what is everyones opinions on ball hones for this applicaton. I used one on a shotgun barrel once and it was great!!

akajun said:

its a 4 stroke!!

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PURIST WARNING !!! *OLD SCHOOL ALERT* PURIST WARNING !!!

Okay, golden rule - loose is better than tight.

Single point bore the cyl in a lathe using the stoutest boring bar you can.

For a 0.020'' OS make it 2 cuts, say .008 doc ( 0.016 total) to get under the glaze and hard surface, (Honda liners tend to be hard, sharpen hone etc the tool then a final finishing cut to ''just under size'' .....so the piston just enters but is tight.

Then using 120 grit emery cloth strip on a stick and dead slow speed, work the emery back and forth to get a cross hatch finish and suitable piston clearance - easily checked by a feeler gauge between the piston and bore wall.

Clean properly and rebuild engine, .......my personal preference is to coat the piston and rings with Moly Disulphide grease (CV joint grease) on assembly, .........to prevent scuffing and possible seizure on start up.

Some of the newer stuff can be O/S by as little as .005". if that is the case here then boring to the next size may be nearly impossible if work hardened. In those cases only a Sunnen type hone will probably do the job. I think that old school Honda is a different animal. If you can bore the cylinder straight I don't see the problem, try to use the new clearance values in the book as close as possible on the plus side only. The hone on a new bore is for putting the cross hatch pattern for oil retention and proper seating of the rings. If your finished cylinder is tapered, bell mouthed or under size there can be problems, if not with the proper finish it should be as good as a new one. Just my 2 cents worth.
Dan

Limy Sami said:

my personal preference is to coat the piston and rings with Moly Disulphide grease (CV joint grease) on assembly, .........to prevent scuffing and possible seizure on start up.

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Sami, I really like you, and you are usually spot on with your advice.

But, that last bit I quoted is just about the worst engine rebuilding advice I have ever seen.
Cylinder pressure, and friction, are the two elements required to get a proper piston-ring to cylinder wall seal.
Grease those parts, and you will never, ever achieve a proper "break-in".

Here is how I break in a fresh engine:
(assuming single cylinder dirt-bike/quad mill)
In a small 4-stroke with iron cylinder, I use straight 30wt for the initial fill.
Start the engine, and get it up to operating temp.
Go through the gears. Don't pussy foot around! Open the damn throttle!
Let cool. Completely!
Repeat this process a few times.
Dump that first oil, and refill with the oil you plan to always run in the engine.
Repeat the process a couple more times.
Dump and fill the oil again.

Now beat the snot out of it.

Seriously, cylinder pressure is what is needed to seat the ring/s. And the only way to build cylinder pressure is open the throttle!

All these people that tell you to break shit in slow, and tell people "keep it under half throttle for two tanks of fuel" type bullsit?
It is just that, bullshit!

I have 40 years of dirt-bike experience. In my younger years, I had trashed my fair share of engines. Seized my fair share of fresh engines.
Reading the comment about relieving the exhaust port bridge in a 2-stroke made me chuckle. I learned that the hard way!
About 20 years ago, I was personally taught the proper way to break in a fresh top-end by a guy most "in the know" consider the godfather.
I have had not one single failure of a fresh engine since adopting his knowledge.
And, engines sure run better when the ring/s are sealing properly. They also run cooler, and last much longer.

wheelieking71 said:

Sami, I really like you, and you are usually spot on with your advice.

But, that last bit I quoted is just about the worst engine rebuilding advice I have ever seen.
Cylinder pressure, and friction, are the two elements to get a proper piston-ring to cylinder wall seal.
Grease those parts, and you will never, ever achieve a proper "break-in".

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I thought that would come up, and I know it is a bone of contention.
In practice the Moly doesn't stay there long enough to cause break or bed in problems, just enough to prevent initial problems, .........in the early 70's I rebored a good few Villers, Bultaco and Montesa 2 stroke trials motors, and without graphite or moly grease a start up seizure was more than a possibility, especially on the Spanish engines!

The ''respected'' engine builder who taught me, that and many other twists, always reckoned ;- it loaded the dice in his favour.

P.S. That was one of the reasons I put the purist old school warning at the head of my post

Limy Sami said:

I thought that would come up, and I know it is a bone of contention.
In practice the Moly doesn't stay there long enough to cause break or bed in problems, just enough to prevent initial problems, .........in the early 70's I rebored a good few Villers, Bultaco and Montesa 2 stroke trials motors, and without graphite or moly grease a start up seizure was more than a possibility, especially on the Spanish engines!

The ''respected'' engine builder who taught me, that and many other twists, always reckoned ;- it loaded the dice in his favour.

P.S. That was one of the reasons I put the purist old school warning at the head of my post

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Fair enough!

Bye-the-way, I have not seen anybody mention a Villers in a looooong time!

Limy Sami said:

I thought that would come up, and I know it is a bone of contention.
In practice the Moly doesn't stay there long enough to cause break or bed in problems, just enough to prevent initial problems, .........in the early 70's I rebored a good few Villers, Bultaco and Montesa 2 stroke trials motors, and without graphite or moly grease a start up seizure was more than a possibility, especially on the Spanish engines!

The ''respected'' engine builder who taught me, that and many other twists, always reckoned ;- it loaded the dice in his favour.

P.S. That was one of the reasons I put the purist old school warning at the head of my post

Click to expand...

Back in the day I built quite a few Bultaco Astro's. Won lots of races. 70's.....damn, I'm old! Proper piston clearance, a bit of Castrol R30 on the skirt and 100 octane race gas / 30-1 Castrol R30 in the tank. It won't stick, and it will smell good too!

Moonlight, I do not doubt that your Astros won a lot of races in the early 70's, but I am almost certain that in 76 or 77 when the AMA in there infinite wisdom went and changed the 360cc rule to 250's your winning streak was sharply curtailed.
My first race bike was the 360 Astro, fast and almost bullet proof. Then the dreaded 250 kit made that bullet a grenade. LOL!!!
I too used Castrol R. When running it sure smelt sweet

After that little trip down memory lane. Akajun, after many years of building both 2 strokes and 4 stroke engines, yes you can very easily do the rebores in a lathe.
I have done a fair amount of Honda XR 500's out to 600cc by putting the barrel in a fixture in the lathe and boring the barrel and fitting a sleeve, then doing the sleeve to fit the piston.
If you choose to go the route of a kit, I am sure the added cc's will not make the bike any more lethal than it was stock. Just my humble opinion though!!
Dave

One thing not mentioned yet is holding the cylinder in the lathe. DO NOT USE a chuck directly on the cylinder. It will distort the bore. You must make a fixture that the cylinder mounts in and bolts to. It is the fixture that will be held by the chuck.