HK0 head and cylinder options.

[Tripod]

Im in the process of rebuilding my HK0 motor. The head had a stripped spark plug, and now its also got a stump of a screw and an easy out broken off in one of the fin cover holes.

The cylinder itself looks to be ok, but will probably need to be one size over. It has some staining inside it, but nothing that fails the fingernail test.

Ive been watching from a distance all the possible engine combinations but am not sure where to take this. The four speed crank doesnt have any slop in it and is useable. Im switching from the mitsu stator over to a hitachi. I'd like to keep the crank, and the original look of the carb and air filter combo.

Will an 12v big bearing honda head from a 91+ ct 70 fit?

Kinda would like to hear my options. Not trying to set the world on fire with speed, but i'd like to avoid chinese junk.

 

[OLD CT]

Any head from 69 to 1981 will work. A 91 CT head/engine uses a long rod crank and a different piston, ''12v style''. Buy a period correct E9 head on e bay and call it a day. Someone funked up the intake threads also
You would be best served just buying a replacement cylinder as it will most likely cost more to have it bored.
I never used this kit but looks ok. ebay # 252983055321 and back it up with a old dome rebuilt E9 head...
I know you said you didn't want cheap Chinese but the cylinder should work in this kit. It comes with gaskets and the correct piston for a HK0. Worst case scenario, you use an oem style Art piston and rings down the road.

 

[kirrbby]

The stock H has a short rod. You just need a piston for a short rod crank, and a new 70 dome. New 70 dome means it will work with the new 70 head...big bearing head.

Dratv has the piston(s)
http://dratv.com/kitpiston.html

I'm not sure how much you have to gain with the newer head tho... More choices of cams, and...it's a newer part and easy to find in good tic.
If I we're building that engine back to 72cc, I'd probably bore the stock cylinder, and have the stock head properly rebuilt.
A stock H engine is the MOST widely accepted and appreciated engine on a H bike, IMO. Some folks might shy away from a, even slightly, modified engine when shopping for a H.

You could probably still do a little porting on the head, add a bigger cam, and run a fancy carb.

Something else to consider...I was told that a stock CT cylinder could be sleeved to take it out to 51mm...88cc sleeper..?

 

[Tripod]

New 70 dome...big bearing head like 91+ ct70, xr, crf? The larger dome means i'd need a piston with more dome on top to keep the compression ratio the same?

Most of the k0 heads im finding on ebay are pretty crusty with pitting in the dome.

I dont have any plans on selling this bike. Plenty of others to go before this one. I need to go out to the garage and do some measurements on the cylinder. See how much wear it has on it.

Sure would be nice to have a chart with possible combinations and parts required. Some of you guys know them backwards and forwards.

 

[kirrbby]

http://dratv.com/whdoi.html

http://dratv.com/infoonfitment.html

New 70 dome would be 90s CT, and XR, CRF 70. Big bearing was what you called it, but I would call it a ball bearing cam.

New 70 dome is a smaller dome than the old dome. Lol...it's sounds like a lot of confusing BS, but it really isn't that bad. The 2 pages I linked above are good info.

 

[Tripod]

I went out to the garage and got some measurements.

Piston 1.8480

Cylinder:
1.8517
1.8520

I measured the cylinder with both measurements perpendicular about 10mm deep. Doesn't seem to be out of spec.

I did dig out an old z50 engine I bought years ago. Has a busted motor mount so its just been sitting. Early k1 case and as usual I dont throw out old parts. Still seems to have some good internals. The motor has what seemed to be a 70 head on it. I just tore the top end off.

As far as the pics go, I'd like to know if this is a Chinese head or Honda head. Has E1 stamped on it. Will need to be reuilt. I poured some mineral spirits in and the exhaust valve leaked.

The picture of the piston is at the top of its stroke. Not sure how well a 49cc cylinder and a 70cc head would run but oh well i didn't buid it.

Pictures explained:

LEFT COLUMN
HK0 cylinder
donor case
donor at top of stroke
donor head right cam bearing

RIGHT COLUMN
donor head
donor head dome
donor head left side cam bearing
Hk0 head / donor head

 

[69ST]

That cylinder really doesn't look bad, at all, in terms of doing an overbore. I didn't see the broken screw you mentioned. That could be the most difficult thing to correct, if there's an EZ-out broken-off in it. It's still not impossible. As I recall, our own faatcaaat posted about dissolving a broken tap (essentially the same as a tap) using alum dissolved in water. A machine shop could do this with little fanfare. Stripped holes are usually no big deal. Helicoils and/or Timeserts have a long history of success. The threads will be a lot stronger afterward.

While I prefer the later "12v" style head because of the ball bearing cam, there's not a lot to be gained, otherwise. I agree with OLD CT that sticking with a 6v-era CT70 head will make your life easier. If you were going with a "12v" style crank, then the small dome/"12v" head might become the easier option...but...then you'd open another can of worms - the alternator assembly.

 

[Tripod]

After reading your posts, looks like i'll get a DRATV "old dome/short rod piston and rings kit, with the three piece oil ring, refresh the "E1" head i just pulled off another engine, and clean up the bore on the cylinder with a light hone.

I appreciate all the input.

On the z50 motor, i tore it down after pulling the head off. Really nice bore on the cylinder although it had a tiny aftermarket piston in it. Inside the case was clean and the three speed crankshaft was in super shape. Will be transferring the guts and just put the case up on a shelf..

 

[Tripod]

I pulled the vales out of the donor head. Measured the intake and exhaust. 26/22
Old head had the standard 25/22.

 

[69ST]

I suggest deglazing the cylinder first. Find out if a uniformly crosshatched surface can be restored without going beyond the wear limit for std bore and without any score lines/pits remaining. If it passes muster, then order the piston & rings.

 

[OLD CT]

That doesn't look like it's going to buff out...

Might be 2nd or 3rd over time.

 

[69ST]

That's a really good photo in #11. Pat is on-the-money. While it's uncommon to find a cylinder that's tapered enough to require more than one O/S beyond its current diameter, rust pitting is a whole other thing. Rust pitting usually requires going out 2 O/S, sometimes 3, to reach clean, uniformly, solid metal. The only other surprise is how many of these engines have already been overbored at least once, as-received.

None of this is a big deal, really. There's plenty of material thickness. But...this illustrates why it's best to check a cylinder before ordering a new piston/rings.