89 crank in 86 motor?

[CoDean45]

I have an 89 r motor with a stock crank and a spacer plate and an 85-86 piston in it. I know the 87-89 250r's had a longer connecting rod in them. Is there any way to make this crank work with my 86 motor. I was hoping I could use it to get the same effect as a stroker crank. I want to use my 86 cylinder and head also. It is an almost new 265cc national cylinder. There is nothing wrong with my 86 motor. I was just hoping it would help with performance to put the longer rod crank in it. If it would work would it be worth my time or should I leave it alone? What kind of power gains would I get from it if any? Thanks for any help guys.

[DPR250R]

Kind of hard to follow what your asking but an 89 crank will work just fine in an 86 motor.

Just make sure you use the correct piston/spacer plate combo....

Long Rod/No Spacer/87-89 Piston
Long Rod/Spacer/86 Piston
Short Rod/No Spacer/86 Piston

The only combo that does NOT work would be a short crank with an 87-89 piston....

[CoDean45]

Thanks for your reply.

Pretty much I'm just asking what would it take to make an 89 crank work in an 86 motor. If it would work then would there be any power gains from it since the 89 has a longer rod than the 86.

[darren250r]

DPR250R explained the combinations of what will work perfectly. You will gain absolutely no performance by doing the switch. The only benefit to the long rod, and long rod w/spacer & 86 piston is reduced rod angle. This lessons the tendancy for the crank to try to "shove" the piston out of the ports. You might get a little better throttle response, but they will run basically identical.

A stroker CRANK is what will make a performance difference. The rod only connects the crank to the piston, while a stroker crank is essentially a larger lever arm spinning the crank which will add torque. A stroker crank moves the spot where the rod connects to the crank away from the center like using a longer wrench to loosen a tough bolt, more torque makes it easier to turn.

Darren

[Aughtx]

Originally posted by darren250r
A stroker CRANK is what will make a performance difference. The rod only connects the crank to the piston, while a stroker crank is essentially a larger lever arm spinning the crank which will add torque. A stroker crank moves the spot where the rod connects to the crank away from the center like using a longer wrench to loosen a tough bolt, more torque makes it easier to turn.

Darren

I guess I need a refresher on this too. I always thought the 86 motors were the stokers. From your post it sounds like a long rod with a 86 piston and no spacer would be a stroker.

[darren250r]

Originally posted by Aughtx
I guess I need a refresher on this too. I always thought the 86 motors were the stokers. From your post it sounds like a long rod with a 86 piston and no spacer would be a stroker.

All 250r motors come stock with the same stroke. The 86 has a shorter rod, but the wristpin is lower on the 86 piston (basically the piston is a bit taller to make up for the shorter rod). The 86 short rod motor has slightly more rod angle. You are only talking about a 1/4" difference between the two rods.

Darren

[Aughtx]

Originally posted by darren250r
All 250r motors come stock with the same stroke. The 86 has a shorter rod, but the wristpin is lower on the 86 piston (basically the piston is a bit taller to make up for the shorter rod). The 86 short rod motor has slightly more rod angle. You are only talking about a 1/4" difference between the two rods.

Darren

Ok that is what I am recalling. What is considered a "stroker" then?

[wilkin250r]

The distance the piston travels is the "stroke". On both the 86 and the 89 cranks, the stroke is exactly the same distance, it's just in a slightly different spot. It's like having a glass of beer. Just because you put a 5mm spacer beneath it, that doesn't give you more beer, it just puts your beer in a different spot.

Picture a crank in your mind, and the piston attached to it. Imagine it actually going through the stroke. What determines the distance that the piston travels? It's not the length of the rod, it's the distance from the center of the crank to the big end of the rod. This distance is the exact same on the 86 or 89 cranks.

In order to get a "stroker" crank, you need to move the rod outwards on the crank. This is not an easy modification, it requires very precise machining and fixtures to keep the crank true and aligned. It's expensive. And, on 2-strokes, it changes all your port timings, which changes your power characteristics. Generally, when you do a stroker, you need to design your entire engine and porting to go along with the stroker.

[Aughtx]

Originally posted by wilkin250r
The distance the piston travels is the "stroke". On both the 86 and the 89 cranks, the stroke is exactly the same distance, it's just in a slightly different spot. It's like having a glass of beer. Just because you put a 5mm spacer beneath it, that doesn't give you more beer, it just puts your beer in a different spot.

Picture a crank in your mind, and the piston attached to it. Imagine it actually going through the stroke. What determines the distance that the piston travels? It's not the length of the rod, it's the distance from the center of the crank to the big end of the rod. This distance is the exact same on the 86 or 89 cranks.

In order to get a "stroker" crank, you need to move the rod outwards on the crank. This is not an easy modification, it requires very precise machining and fixtures to keep the crank true and aligned. It's expensive. And, on 2-strokes, it changes all your port timings, which changes your power characteristics. Generally, when you do a stroker, you need to design your entire engine and porting to go along with the stroker.

So a "stroker" cannot be achieved with stock cranks / rods / piston combos? The crank itself has to "mount" to the rod lower or "outward" on the crank. So if the crank was bottom dead center you would lower that pivot point and insert a longer rod to make a stroker? Thus modifying the crank to accept the new pivot.

[wilkin250r]

Go through the math.

The connecting rod is 36mm from the center of the crank. That means the piston travels upwards 36mm, and then downwards 36mm for a total of 72mm.

The rod length doesn't change this. On the short rod motors, the center of the piston (the wristpin) starts 89.3mm from the crank at BDC, and ends 161.3mm from the crank at TDC. If you subtract them, you'll get a total distance of 72mm traveled from BDC to TDC.

On the long rod motors, the wristpin starts in a different spot. It starts 94.3mm from the center of the crank, and ends 166.3mm away. But if you subtract them, you still get a total of 72mm distance traveled. It's in a slightly different spot, but it's still the exact same stroke.

[250R-Dee]

Wilkin explained that perfectly!

"Stroker" = changing the actual stroke distance so changing to a long rod = "rodder":D

[EXtreme-]

great thread...good stuff!

(we all'd like to think we have a long rod :D )

[KY250r]

so i got a 86 motor.....i just had a long rod pu tin n i run a 86 piston. so if i go with a 87-89 piston i can do thqt without a spacer plate?

[250R-Dee]

The long with it's corresponding '87-'89 piston do not need a spacer plate. Spacer plate is needed when running a long rod with an '86 piston.

DPR250R already explained the setups.

[Rich250RRacer]

My turn. Wilkin is dead on. There will be no power gain, but the long rod motors tend to be a little smoother, but I feel you would have to spend considerable time on each in order to notice a difference.

[parttime]

who sells the stroker cranks, how much and does it make a considerable amout of power for the money that it costs.