rearenginekarts.com

I just picked up a nice Mc9 with a very straight/round cylinder at standard bore (2.165"). Negligble taper at less than .001" and no ovality (sp?) as measured with a Fowler cylinder bore gauge.

It has the slightest bit of surface corrosion (mostly a stain, not pitted) and I'd like to make a couple light passes with a Flex-Hone to clean it up.

My question is this, of the two (2) nearest available sizes, which do I choose; 2.125" (.040" undersize) or 2.250" (.085" oversize)?

I'm also leaning towards 180 grit; is that too coarse?

I'm also hoping that Vince Hughes (or other) can supply a new piston and ring set, as after two-plus weeks of soaking in Kroil, the top ring cannot be coaxed free of its groove. Pity, as there's virtually no wear on the skirt.

Thanks for any insight on this. It has been almost exactly forty (40) years since I've had to rebuild one of these, but I'm hoping for a fairly shallow (re)learning curve.

I have saved frozen ring pistons by soaking them in something more aggressive, like acetone.
It will cut old dried up castor oil with time.
Get an old jar with a screw top,, add a 50/50 mix of acetone & your favorite penetrating oil.
Drop the piston in, screw the cover on, and let it sit for a week.

Sometimes, I will have to damage the ring to pull it loose, but they are cheaper than a whole piston.

Personally, I would use the hone just enough to knock off anything above the actual cylinder surface,
and save as much bore size as I could.

I would not do this on a modern engine, that I was using to chase points in a series.
I would go with what ever it took to get a perfect bore.
But are you really after the last bit of speed,
at what ever cost ?
Probably not.
Save the bore size, and enjoy vintage karting.

Randy,
If the bore is nice and round, why not just a few passes with a glaze breaker??
Flex hone is probably better but you don't want it too smooth either to make sure you get a good ring seat. As long as the bore is round and straight, I always just use my trusty small engine glaze breaker I got from Tulsa engine warehouse. Also, like what has already been said, I suspect you are not trying to build a super fast "A" stock race winner (can you tell I raced in the 60's). So as long as the rings seat well, what is a thou or two over.
Just my opinion. I like to have fun with these old engines anyway so I'm not looking for all out power. Parts are too hard to find these days for me to go blowin' one of "my babies" up like I used to back in the day when a new crank cost $20.
Sounds like you have a nice engine there, good luck with it.
Scott

Thanks guys.

Yep, just trying to keep the life in this one, the last .001/HP will have to be squeezed out by someone else.

Aside from being dirty in general, this engine does look pretty good. It's never had so much as a file drug across any of the ports. There's a bit of "smeared" metal on the PTO end of the crankshaft, but the taper, keyway and threads are all still good. I just wouldn't recommend an inboard clutch is all.

I'll give the acetone/Kroil cocktail a try and see what happens. In the meantime, you have a PM.

Randy

Use a heat gun on high on the ring area if you want to break them loose. Same on the wrist pin bearings if they are froze also. Never use a ball hone deglazer or the spring loaded 3 stone hones in a 2-cycle unless you really want to mess that cylinder up. The Ball Hone deglazer will snag the ports and mess them up. I use a Carbide Coated Brush hone to finish up with after honing.

Bill Johnson wrote:Use a heat gun on high on the ring area if you want to break them loose. Same on the wrist pin bearings if they are froze also. Never use a ball hone deglazer or the spring loaded 3 stone hones in a 2-cycle unless you really want to mess that cylinder up.

I'll give the heat gun a shot on the top ring. The wristpin bearings are fine.

Okay, no ball hone and no spring loaded 3-stone hones; what's left? I used a bit of fine-ish Scotchbrite by hand to remove a bit of surface crud, now there's only the stain in the pores. Should I leave it up to the next set of rings to scrape it away?

Thanks!

Randy... get a Ammco 3950 hone. They arent perfect but get the job done. You can find new ones on ebay for under $150.00 If you buy from Ammco or dealers you will pay $500.

Here is used one.... http://cgi.ebay.com/AMMCO-MODEL-3950-CY ... 286.c0.m14

(Bill, just put the ebay item number in your post, highlight it with mouse and hit the "ebay" button and it will display as above)

Here are a bunch of hones, the only home I would buy would be a "line hone", like the Ammoco hone previously recommended. These hone true the bore, spring hones and flex hones only cross hatch (deglaze) the surface and will leave an untrue bore untrue. Obtaining a true bore and fitting new correct size rings is the key to top engine performance. I have tried all those spring and flex hones, get the line hone first. I bought mine brand new on ebay for $25, and it is a brand that was much better quality than the very good ammoco hone. Usually a bore the looks good to begin with, usually isn't, once you hit it with the line hone the high spots show up immediatey, but maybe only a 1/2 a thousanth is needed to true up a decent block.

Good engine builders like to finish the last 1/2 thou with a ball stone hone to get a nice even slight chamfer thru the port belt after using a Sunnen hone to size the bore.