Head ID

I picked up an xl70 engine and it had this head on it. The casting number doesn't match up with anything I can find. The seller didn't remember what the head was off of. He put what he thinks is an xr50 Takegawa 88cc cylinder kit on the engine. He said it didn't run well. I assume the piston dome isn't playing nice with this combustion chamber.

Here is the piston.

Probably a mid-80s C70 head...not my area of expertise. The design looks very much like a typical 6v CT70 head, but with the transitional single cam bearing setup.

I like the piston design...clean dome with a flat top and decent squish band around the perimeter. That compensates nicely for the tall dome, which hinders flame propagation. As long as this assembly delivers 150+ psi cranking compression and there's no valve-to-piston contact, it should run well.

The intake port is black, so the valves aren't sealing very well.

OLD CT said:

The intake port is black, so the valves aren't sealing very well.

Click to expand...

You've got eagle vision. I can't even see the inside of the port, on this monitor.

It did have an oily sludge in the chamber and on top of the piston that wiped right off ass opposed to carbon build up. The dementions match a stock XL70 head I have. The bearing cam looks a lot like a race cam DrAtv sells. I still feel like the piston dome might not play nice with the combustion chamber.

Dry-fit the head...IOW, install the cam & cam drive, bolt the assembly in place using the old head gasket (or no head gasket) then crank the engine over, slowly, by hand via the flywheel or clutch. If there's piston-to-valve contact, you'll feel it...and stop before anything is damaged. If you really want to know what's going on inside the combustion chamber, a little modeling clay, on the piston, will leave witness marks that will let you see how close the valves get to the piston.

I don't know if the cam profile is the same as the Dratv race cam you mentioned. I suspect it's not. However, you could swap-in the race cam with the convenience of having the needle bearing already in the head. You will need the "HD" valve springs...they're really not much stiffer than stock, but the coil spacing is wider...to prevent coil bind with the increased lift.

Take a look inside the intake port. If it's blackened as Pat pointed out (he's usually right), this head needs a valve job.

The doc gives dimensions of that bearing race cam. You could compare yours. The race cam has 6mm of lift...that should be enough to know if yours is the same.


That piston sure LOOKS familiar.
If you could find it, maybe on the TB, or dratv website, you can see what they recommend it for. I believe you have a "big dome" type head there.

I agree with it being a big dome head. I just wonder who made it. It was supposed to be a running engine, so the leaking valves might have been his problem all along. Thanks for the help fella’s.

Unsure about the marking. That said, this looks like an `82-era OEM Honda head.

I have tried the dratv race cam that the dr says works for a 82, in my 82 CT. It is not a good choice, the bike ran like crap with a 88 kit. I took it right out after 5 mins of run time. If this was my bike I would use the TB race head like the piston's box says.

Heads are not called ''big dome'' there is ''old'' dome head and ''new'' dome head.

OLD CT said:

I have tried the dratv race cam that the dr says works for a 82, in my 82 CT. It is not a good choice, the bike ran like crap with a 88 kit. I took it right out after 5 mins of run time. If this was my bike I would use the TB race head like the piston's box says.

Heads are not called ''big dome'' there is ''old'' dome head and ''new'' dome head.

Click to expand...

True, that IS the nomenclature and I've probably added to the confusion. The 6v head does have a larger combustion chamber/dome...and a larger piston dome is needed (compared to 12v) to maintain decent CR. The TB head is nothing special...and that's a good thing, actually; it's basically just a 12v Honda design. 12v cams are ball-bearing, a much better setup and there are more choices.

I know that dratv cam left a really bad taste in your mouth. My experiences with have been different. First build was an 88cc tune that went into a Z50 hardtail...on request. It had no low end but up top the thing ripped...~60mph...total deathtrap, imo. It'd be better with a long-stroke, undersquare, tune...where it would balance-out the high compression.

dratv sells two of these type race cams and specs them for older heads vs newer heads...i'm not sure the exact difference seeing that you can mount them interchangeably, but there's probably some spec difference. In the 95cc I recently built, i used one of these cams and it is very powerful and strong above 5k and crazy up on the 7000+ for a 95cc...but has virtually nothing below 3500.

I don't believe that they changed any of the valve angles like the subtle differences they did between the 90/110 and 125m where changing a cam could result in a horrible running engine or bent valves (happened to me personally)

That said, I'm going to say that head is 81-84 cub based on: 23/20 valves that I can see and the cam. That aligns with what bob said. However, on closer look, it appears that it's designed for normal 25/22 valves...so maybe part of the issue is that the wrong valves are installed here. Those are def look 23/20

Exactly Bob, the bike had no low end grunt in my case. The bearing cam that walked was a HK0 engine. Poor acceleration must have had something to do with the small 1982 valves combined with the flat top 1982 88cc piston with a 6mm cam. It was a dog.