SL70 compatible cylinder/heads?

[RadRacer203]

Now I should say I'm not planning on building this anytime soon, but I'm starting to gather parts for a 4 speed motor build. I have an SL70 bottom end that's going to be my starting point. Kinda rough but it shifts and turns. I'm wondering what cylinder and heads are compatible and make a pretty hot motor. I'd like to make a period correct build like would have been done in the 70's or 80's. I'd like to scrounge and use as many Honda parts as possible, just wondering what I should be looking for for maximum power. I know I'm probably going to have to port and polish the head, etc, but what would be the best starting point?

No idea what this motor is going to go in, but I'm thinking maybe something like a c90, that frame style

 

[fatcaaat]

If you want to do it period correct, you are going to be in for a much larger spend than doing it with modern aftermarket.

Period correct (because I had mapped this out previously) looks as follows:
Head - mild port and polish. Can't get agressive because you can't get a big enough cam or piston for it.
Cylinder - max safe boring on a stock cylinder is 50mm. You can find pistons for it.
Crank - you can offset the pin in the crank to 52mm if you can get a machinist to do this work for you. It involves relocating the pin and heat pressing the connecting rod either 4mm (period correct piston) or 2mm (using an xr50 deck piston)
Cam - send the cam out to webcamshafts for a number 3 or 57 grind. you'll need some new springs too.
Carb - aim for a 20mm carb..mikuni VM20's were available during that time and still are
Exhaust - custom bend a 1" ID pipe to a supertrapp.
All of this will run you at least 1000 or more, depending on the hourly machining rate.

Alternative bolting on an 106 kit from trailbikes will produce more power and cost you about 600.

 

[RadRacer203]

I think the one thing I might go modern with is the cylinder, just cause it doesn't have 40 years of wear and tear. But I want to use mostly old parts for a period build even though it'll cost more. Are all the heads the same? Like what are the differences between the heads on say a z50, ct70, sl70, etc? The Supertrapp muffler sounds great, definitely going with that if I can find one, same with that cam and a Mikuni carb. I wonder if any aftermarket stroker cranks were made back then? Or rods?

 

[Robert thran]

A true sl70 or cl70, xl70 head will have an extra cooling fin right above the round cam sprocket cover on the top of the head.. if you look on my sl70 build thread there’s pics of the motor out of the frame…z50 heads have a smaller combustion chamber and valves than a ct70 head.

 

[Gary]

. I wonder if any aftermarket stroker cranks were made back then? Or rods?

Powroll was the one back in the day. Seems like someone has a facebook page for them as well,but the original company is out of business and the fb page has no activity.
https://powrollpast.home.blog/

 

[fatcaaat]

CT70, xl70, sl70, cl70, s65, atc70, C70, and trx70 from 69-79 are all directly swappable. I believe in 1975 they stopped having the heavy steel combustion chambers and instead are just part of the aluminum casting. I believe it was somewhere in early 80's that the valve sizes switched from 25/22 to 24/21 and eventually to 23/20. So my suggestion, if building something up for a period racer, would be to find a honda SL or XL head or an early CT70 head that uses the steel combustion chamber.

You will never find a supertrapp made for the CT70...they are long gone and simply unobtanium. I happen to have one. However they are not difficult to make from parts if you have a bender and can weld or if you buy some mandrel bends. The vintage cans, used, fetch premium dollars but you can actually still purchase new...cans, discs, and packing will run you about 250 for the muffler piece.

A mikuni VM20 is the best option. make sure it is the real VM20 with the main jet sticking out of the bowl and is hot swappable. That's the one you want. Buy yourself an ATC125M intake manifold...best choice IMO. ATC110 and 90 will not work...must be 125M.

For the barrel, there's no issue using an old steel barrel unless you want to be bigger than 50mm. As long as all the fins are there you can take to any machinist and they can bore it to match a 50mm piston (which you can get at dratv.com along with your carb). He also happens to have great options for cams...so instead of paying 200 for webcamshafts to grind you one, pick the one he has on his site that has the bearing on the one side...That's the biggest and most aggressive you can get, they work fantastic, and run you 30 bucks.

Honestly, though, the best thing you can do is stroke it. TB, takegawa, kitaco, and even dratv sell stroker cranks. Go with a 51 or 52 depending on what you match up with on the top end. It makes a much bigger difference than the piston.

True story here. A buddy and I were in an arms race about 2001 with CT70 builds. I had a NOS ATC125M engine shoehorned in my bike and we were heading out for an all day ride...he showed up, with what he claimed was a stock engine with a cam, carb (VM20) and exhaust. I was extremely surprised when he was able to keep up with me and even pull away from me. After half a day, and with my suspicions at feverpitch, he finally admitted that he ad a 52mm stroker crank in it, but still had a stock piston. After having built several dozens of engines from mild to wild, if you pit an 88cc bore up kit with stock crank, against a stoke bore with 52mm crank (90cc) all things being equal, the stroker crank will walk all over the 88 kit.

 

[kirrbby]

CT70, xl70, sl70, cl70, s65, atc70, C70, and trx70 from 69-79 are all directly swappable. I believe in 1975 they stopped having the heavy steel combustion chambers and instead are just part of the aluminum casting. I believe it was somewhere in early 80's that the valve sizes switched from 25/22 to 24/21 and eventually to 23/20. So my suggestion, if building something up for a period racer, would be to find a honda SL or XL head or an early CT70 head that uses the steel combustion chamber.

You will never find a supertrapp made for the CT70...they are long gone and simply unobtanium. I happen to have one. However they are not difficult to make from parts if you have a bender and can weld or if you buy some mandrel bends. The vintage cans, used, fetch premium dollars but you can actually still purchase new...cans, discs, and packing will run you about 250 for the muffler piece.

A mikuni VM20 is the best option. make sure it is the real VM20 with the main jet sticking out of the bowl and is hot swappable. That's the one you want. Buy yourself an ATC125M intake manifold...best choice IMO. ATC110 and 90 will not work...must be 125M.

For the barrel, there's no issue using an old steel barrel unless you want to be bigger than 50mm. As long as all the fins are there you can take to any machinist and they can bore it to match a 50mm piston (which you can get at dratv.com along with your carb). He also happens to have great options for cams...so instead of paying 200 for webcamshafts to grind you one, pick the one he has on his site that has the bearing on the one side...That's the biggest and most aggressive you can get, they work fantastic, and run you 30 bucks.

Honestly, though, the best thing you can do is stroke it. TB, takegawa, kitaco, and even dratv sell stroker cranks. Go with a 51 or 52 depending on what you match up with on the top end. It makes a much bigger difference than the piston.

True story here. A buddy and I were in an arms race about 2001 with CT70 builds. I had a NOS ATC125M engine shoehorned in my bike and we were heading out for an all day ride...he showed up, with what he claimed was a stock engine with a cam, carb (VM20) and exhaust. I was extremely surprised when he was able to keep up with me and even pull away from me. After half a day, and with my suspicions at feverpitch, he finally admitted that he ad a 52mm stroker crank in it, but still had a stock piston. After having built several dozens of engines from mild to wild, if you pit an 88cc bore up kit with stock crank, against a stoke bore with 52mm crank (90cc) all things being equal, the stroker crank will walk all over the 88 kit.

This thread is getting good!

I think it was Terry who told me that it was possible to sleeve a stock cylinder to run a 51mm piston..? Is that a option?

Could you sleeve a 49cc Z50 cylinder for a big piston too?? They come stock with a much smaller spigot than the 72cc cylinders I think...but not sure it that matters at all.

 

[kirrbby]

Typical of me...I have 10 more questions. But I'll start with this one.

I'm ignorant about cams...somewhat ignorant.
What changes do you get from the higher lift cams. What from the "longer duration" cams??
That bearing cam from the doctor...the "most radical" cam...what does it do that's better than a milder cam, or STOCK cam.

I'm talking mainly about how will the engine ACT...in use...rev quicker, for a quicker bike???maybe just move more air for a faster bike???
What ALL does the cam affect??
Again, I'm ignorant...cams, for dummies. Don't leave anything out

 

[Robert thran]

My machinist has bored those cylinders so far with big sleeves that he had to wield in between the cooling fins..can’t remember the exact size but it wouldn’t kick start …you had to push start it from high compression where he wielded a dome on the top of the piston.. that’s old school ways of doing things before you could buy tons of aftermarket parts… all there was back then was power roll an P.K. I have an sl70 motor I built about 15 years ago that run good for the day with a power roll 88cc piston and a power roll cam and valve springs and ported head ,bassanni pipe ….I ran the crap out of that thing and never could kill it!!! Still have the complete motor .

 

[PoppaBear]

CT70, xl70, sl70, cl70, s65, atc70, C70, and trx70 from 69-79 are all directly swappable. I believe in 1975 they stopped having the heavy steel combustion chambers and instead are just part of the aluminum casting. I believe it was somewhere in early 80's that the valve sizes switched from 25/22 to 24/21 and eventually to 23/20. So my suggestion, if building something up for a period racer, would be to find a honda SL or XL head or an early CT70 head that uses the steel combustion chamber.

You will never find a supertrapp made for the CT70...they are long gone and simply unobtanium. I happen to have one. However they are not difficult to make from parts if you have a bender and can weld or if you buy some mandrel bends. The vintage cans, used, fetch premium dollars but you can actually still purchase new...cans, discs, and packing will run you about 250 for the muffler piece.

A mikuni VM20 is the best option. make sure it is the real VM20 with the main jet sticking out of the bowl and is hot swappable. That's the one you want. Buy yourself an ATC125M intake manifold...best choice IMO. ATC110 and 90 will not work...must be 125M.

For the barrel, there's no issue using an old steel barrel unless you want to be bigger than 50mm. As long as all the fins are there you can take to any machinist and they can bore it to match a 50mm piston (which you can get at dratv.com along with your carb). He also happens to have great options for cams...so instead of paying 200 for webcamshafts to grind you one, pick the one he has on his site that has the bearing on the one side...That's the biggest and most aggressive you can get, they work fantastic, and run you 30 bucks.

Honestly, though, the best thing you can do is stroke it. TB, takegawa, kitaco, and even dratv sell stroker cranks. Go with a 51 or 52 depending on what you match up with on the top end. It makes a much bigger difference than the piston.

True story here. A buddy and I were in an arms race about 2001 with CT70 builds. I had a NOS ATC125M engine shoehorned in my bike and we were heading out for an all day ride...he showed up, with what he claimed was a stock engine with a cam, carb (VM20) and exhaust. I was extremely surprised when he was able to keep up with me and even pull away from me. After half a day, and with my suspicions at feverpitch, he finally admitted that he ad a 52mm stroker crank in it, but still had a stock piston. After having built several dozens of engines from mild to wild, if you pit an 88cc bore up kit with stock crank, against a stoke bore with 52mm crank (90cc) all things being equal, the stroker crank will walk all over the 88 kit.

Nicely done fatcaaat. Your story is a testament to torque, and the last sentence sums it up perfectly. The hot rod analogy is taking a SBC 350ci, and stroking it out to a 383ci. You can do the longest burnouts in the world....