Willard's vintage Hotsaw Builds

[Willard]

I measured the sleeve thickness in the CR250 and if I bored it out completely I would end up with a 72mm aluminum bore.
From what I can see with the sleeve removed the ports would basically retain their same dimensions.
So can I get the aluminum cylinder bore plated with say nikisel?
Then I would have 300cc with a square 72mm×72mm engine?

 

[Bigmac]

I measured the sleeve thickness in the CR250 and if I bored it out completely I would end up with a 72mm aluminum bore.
From what I can see with the sleeve removed the ports would basically retain their same dimensions.
So can I get the aluminum cylinder bore plated with say nikisel?
Then I would have 300cc with a square 72mm×72mm engine?

That’s surprising compared to the TRX, and the base sleeve thickness max bore on this is 69.5

 

[Bigmac]

I have always wanted to do a 69mm nikasil bore over the iron, but if I remember correctly your base was 76-ish mm so the differential metals in the bore would be bad, I am surprised they tapered the stock sleeve that much, is it really 72mm on top?

 

[Willard]

I have always wanted to do a 69mm nikasil bore over the iron, but if I remember correctly your base was 76-ish mm so the differential metals in the bore would be bad, I am surprised they tapered the stock sleeve that much, is it really 72mm on top?

It's hard to take a good picture on top of the cylinder. One side the sleeve outline is clear and opposite side almost blends into the aluminum.
But yes it's 72mm approximately on top and 78 mm on the base.

 

[Willard]

Here's the pics 78mm base and 72 approximately on top.

 

[Bigmac]

Ok, ya that’s clear 69mm would be max! A strait sleeve would allow a 72mm bore, and probably 73.5 max. If you think you want to big bore in the future, the 68-69mm bore would be fun to test with first!

 

[Bigmac]

I do like the look of that cylinder, and the port layout, it’s a lot like a trx. They used to to aluminum sleeves for the trx and nikasil bore, for max effort stuff, but it is not huge! The other factor, especially for what you’re doing is you want a little bit of heat to build up quickly for the type of use you’re doing, I have seen many sleeved 01 dirtbike motors, they warm up and clean out a lot faster than the stock plated cylinder did, the iron bore could be an advantage!

 

[Willard]

You can see the overbore lines in the sleeve from inside going larger out.
It's at about 66.75mm overbore and when I measured to the furthest outside line it was 68mm bore. So 69-70mm is doable.

If I bored the sleeve out at 72 mm can the remaining aluminum bore be plated?

 

[Willard]

I do like the look of that cylinder, and the port layout, it’s a lot like a trx. They used to to aluminum sleeves for the trx and nikasil bore, for max effort stuff, but it is not huge! The other factor, especially for what you’re doing is you want a little bit of heat to build up quickly for the type of use you’re doing, I have seen many sleeved 01 dirtbike motors, they warm up and clean out a lot faster than the stock plated cylinder did, the iron bore could be an advantage!

I can see your point on the iron heat retention especially running methonal

 

[Bigmac]

You can see the overbore lines in the sleeve from inside going larger out.
It's at about 66.75mm overbore and when I measured to the furthest outside line it was 68mm bore. So 69-70mm is doable.

If I bored the sleeve out at 72 mm can the remaining aluminum bore be plated?View attachment 213918

I wouldn’t go 72mm and plate, the iron base skirt is relying on the bore for strength if you essentially cut it off I believe the base sleeve would fail. There is also a great deal of debate over the 69 bore versus the 72 bore in a stock cylinder, the lighter weight of the 69 bore allows for an advantage in RPM

 

[Willard]

Ok gotcha makes sense.

 

[Willard]

I think a 67 or 68mm would be my next move.
I needed to take a break in the fabrication building the fuel tanks and dwell on the cylinder for a while.
I just narrowed the engine by almost a inch by cutting off the outside edge of the cr250"s mag housing like I did on the 125.
So narrower fuel tanks and top handle width.
It's 1:40 am here gotta hit the hay . Thanks for your help .

 

[Willard]

One more question Nathan if I bore out the complete sleeve at 78mm and epoxied in the cylinders water jackets for more strength...could the cylinder then be plated?
Ports too shallow I guess .

 

[Bigmac]

It is possible, I think, could have some casting issues to and need to bigger still, but the biggest issue is little support for the piston at bdc. It may be doable but it would be uncharted territory, most people build an oversized aluminum sleeve, that allows skirt support and a consistent aluminum finish for the plating. Porting would be critical to performance at any major overbore, 72mm bore stock trx cylinders are duds, gain some torque but they don’t rev at all, most of the time running worse and making less power than a bone stocks cylinder. I feel 68-69mm is the sweet spot for the oem cylinder, that said I couldn’t bring myself to do that with my next oem cylinder build, it’s going to be 67mm and 76mm stroke. It was stock 66mm and I just didn’t feel like maxing it out. For me it will be a 268cc at 67mm vs 284cc at 69mm

 

[Del Eaves]

Good evening men, been following this thread since the start, i got a question. Why is the Honda the preferred engine, just asking, i like the 95 cr 250 but also like a few others, is it because of hardware mounting or just the design, thank you.

 

[Bigmac]

Good evening men, been following this thread since the start, i got a question. Why is the Honda the preferred engine, just asking, i like the 95 cr 250 but also like a few others, is it because of hardware mounting or just the design, thank you.

Currently the fastest timber Sports hot saw is Honda big bore powered, it is mainly the ability to use the big bore trx cylinder on the cr bottom end, and the tuning experience with Trx250r development over the years. Esr has a 325cc kit for the Yamaha, it would make an excellent choice as well. The no powervalve is also preferable in these because it simplifies things, the big bore can have a powervalve, but the are diagram operation vs mechanical.
With the multiple trx cylinder options, a guy could build near 100hp with a 500cc single with. A Sabertooth cylinder, or 85hp with a puma, There are no cylinder options for the other makes Suzuki Kawasaki and Yamaha they can rival that

 

[Del Eaves]

Thank you sir, was just wondering, well answered.

 

[Willard]

It is possible, I think, could have some casting issues to and need to bigger still, but the biggest issue is little support for the piston at bdc. It may be doable but it would be uncharted territory, most people build an oversized aluminum sleeve, that allows skirt support and a consistent aluminum finish for the plating. Porting would be critical to performance at any major overbore, 72mm bore stock trx cylinders are duds, gain some torque but they don’t rev at all, most of the time running worse and making less power than a bone stocks cylinder. I feel 68-69mm is the sweet spot for the oem cylinder, that said I couldn’t bring myself to do that with my next oem cylinder build, it’s going to be 67mm and 76mm stroke. It was stock 66mm and I just didn’t feel like maxing it out. For me it will be a 268cc at 67mm vs 284cc at 69mm

Nathan, that right there is some very valuable information.
Thankyou!

 

[Willard]

Got both saw's fuel tanks with built in oil tanks ready for tig welding. Will weld on the side plates later this week when the weld on bungs with fittings come in.

CR250R has 17oz fuel capacity.
6oz oil.
YZ125 has 14oz fuel capacity.
5oz oil.
Only one major problem, I can't weld the cap bungs underneath the top of the tanks for a 5/16" lower profile.
So it's back to the drawing board with new 6061 2"×2"×1/8" channel and 6061 2"×1/8" flat rectangle.

View attachment 213683

Well so much for quick product research and development.
Put together my second fuel/oil tank attempt. Don't like it, overbuilt heavy and unnecessary seam to weld up front.

So it's back to to my 2"×4" tubing aluminum stache and start over with a one piece build with the dual cap top. Build the internal oil tank in the upper half and weld top and sideplates on.

Should be looking like chainsaws by Christmas, Lol. But the daughters got me committed this weekend to skiing at our local Mystery Mountain. More like big steep hills then a mountain Haha.

 

[Willard]

https://www.cbc.ca/news/canada/manitoba/mystery-mountain-ski-thompson-1.5004567?_vfz=medium=sharebar