Welding Crank Pin

[rustyATV]

A motor with a CT330 kit I pulled apart a while back had a crank with a welded big end pin.

I was curious if this was something that really needed to be done and, if so, how easily rebuilt welded cranks are.

[hondamancbr03]

My machinist who welds and trues my Banshee cranks has a long history with all different types of two strokes....I'm in the middle of a 250R build and installing a new crank so i asked that very question just last week, his answer was no, it is not required being there is no where for the crank to seperate like twin or larger style cranks.

I would check with whomever does your machine work just to make sure...If you hear different i would like to know what another machinist has to say.

[rsss396]

singles are much less likely to twist.

but when a crank is rebuilt the crank ends never hold the pin as well as the first time so on rebuilt cranks I think it a good idea even though many are not done even after mulitiple rebuilds.
The other time would would be for the Kart racers since they turn so many rpms


But the cans should be welded on all hotrod and wiseco cranks
I seen a can let go on a hotrod crank and it took out both brand new crank halfs with only 20 hours at most on the crank.

I would not have the crank pin welded in place on a 250R engine. Find a really good builder who specializes in 250R builds. Back in the late 80’s and early 90’s, the ATC250R’s and later the TRX250R’s built by the very best SoCal shops did not have the crankpin welded after crankshaft rebuilds.

Today, everyone wants to weld the crank pin in place, mostly this is due to not truly knowing 250R engines and as an un-needed safety feature that actually causes more harm than good on Honda 250R’s. I still have 2 cranks fully re-built by Selvy’s Performance from the late 80’s, early 90’s that run awesome with pins that were not welded when re-assembled.

Now and days, in SoCal, the only shop that I have found that re-builds cranks correctly is BDT Motorsports, not by coincidence either; the owner of BDT-M was a major 250R builder in the 80’s-90’s…

[rustyATV]

That's about what I thought. Couldn't figure out how that area would get twisted like that unless the crank was so worn from rebuilds that it was losing its press fit.

Thanks!

[rsss396]

whether you believe it or not there is twisting forces on both sides of the crank.
Twisting happens everytime you rev the motor up and dump the clutch.
The inertia from the flywheel is trying to keep the crank going forward and the PTO end is trying to stop it.
Now these forces are not as great as a twin cylinder motor but they are real.
Do you have to weld it? no like I said but something to consider in some cases

[rustyATV]

Originally posted by rsss396
whether you believe it or not there is twisting forces on both sides of the crank.
Twisting happens everytime you rev the motor up and dump the clutch.
The inertia from the flywheel is trying to keep the crank going forward and the PTO end is trying to stop it.
Now these forces are not as great as a twin cylinder motor but they are real.
Do you have to weld it? no like I said but something to consider in some cases

Well, while I agree that there are torque loads on each side of the crank, I disagree that significant torque is transmitted through the crank pin on the 250R for a few reasons.

First, the only loads on the stator side of the crank are from the magnetic drag of the stator (hysteresis?) and the mass of the crank half and flywheel. That is not that great.

Next, the crankcase holds the main bearings fairly well in line, so there is not any movement between the two crank halves. I think if there were, then there would be larger issues than the crank pin.

Also, The piston and rod do not put a meaningful torque on the crank pin, since the line of action of the rod is always straight through the center line of the pin. There might be some torque due to drag of the big end bearing and viscosity of the lubricants, but these would be minimal.

Finally, I don't think the CR500 crank is welded, and that has a lot more power than a 330. Though it could be a larger pin, I don't know.

[latheboy]

For what its worth,

I asked Arlan about this in the early spring when he was building my motor, and he said to go ahead and weld it. I had him weld it right away. He said he would if it was his motor. This is in a 350pv.

[rsss396]

The twisting does not happen from the push of the piston.

It is the inertia of the crank half's mass spinning at say 6000rpm and when you dump the clutch you have the PTO side instantly loaded and changing the rpm at least 2000 rpm in a instant.

The flywheel side and all its inertia gets slowed down by the means of the crank pin.
All bearings have clearances in order to spin, these cases have some flex also even though its not great its still there.
Add up flex and clearances and it my be more than the crank likes.
I have seen many flywheel keys sheared from not being properly torqued. These flywheel nuts where tight but since there can be major acceleration and decelerations it must be very tight.

how much torque would you think it takes to slow down a crank half and flywheel that is spinning 6000 rpm to 4000 rpms in fractions of a second? And just think how small that pins o.d. is and the small amount of surface area there is on that press fit.

Again; the crank pin does NOT need to be welded if the crank is properly built by a competent builder.

Find a top builder and he would not suggest you weld the crank pin on 250R engines. Welding the crank pin allows the builder to cut corners that take more time and skill to complete. Not everything that is made or produced is done the best way possible. Most item are manufactured to avoid high levels of skill and time. The best builders I have had the opportunity to see never welded or weld the crank pin on 250R engines.

[rsss396]

Originally posted by acecarlos
Again; the crank pin does NOT need to be welded if the crank is properly built by a competent builder.

Find a top builder and he would not suggest you weld the crank pin on 250R engines. Welding the crank pin allows the builder to cut corners that take more time and skill to complete. Not everything that is made or produced is done the best way possible. Most item are manufactured to avoid high levels of skill and time. The best builders I have had the opportunity to see never welded or weld the crank pin on 250R engines.

Not everyone does the same out there and I am explaining my reasons why I feel it should be done on a mechancal level.

Save your super builder stories for the weak minded please.

On a "mechanical level" the crank pin does not need to be welded if the person rebuilding the crank does it correctly, obviously you are the simple minded one, and have no understanding of what you are talking about.

[rsss396]

well since you put it that way it makes perfect sense.

I don't why I never thought of it like that.

Its great to have such great knowledgeable people on this board that can explain things in great detail.

I have seen many race 250R's over the last 25 years without welded crank pins after crank rebuilds...I used to think you needed a welded pin because most builders push that theory, after talking to Rob Selvy in the late 80's and Honda HRC Engineer in the 90's, and BDT Motorsports in 2010, they all laughted at the though or suggestion of welding the crank pin after rebuilds.

When I hear it from these folks, and see it work for +20 years, I believe it, you may not and keep having your crankpins welded.

Sorry for not living up to your detail standards...

[C-LEIGH RACING]

How many of you seen what a TRX250R crank web look like with the tins removed ??.

If you did, did you take notice how thin the metal is between the crank pin & the outside of the web.

Just a general rode all around yard 250R crank might not need welding, but a racing one, one your depending on staying together, specialy a stroked one, you better weld it.
Neil

Neil:
Looks like 2 completely opposing camps on this question. Your point on a stroker crank is well made and always much appreciated. Other top builders have built and continue to build stroker cranks without welding the crankpin in place, at least up to +4 MM. I am not sure if Selvy’s Performance back in the late 80’s or BDT Motorsports today weld crank pins on stroker cranks above +4MM.

I know my Selvy built 4MM stroker crank that Rob built for me in 86-87 does not have the crank pin welded in place. This particular crank had been raced in the late 80’s and early 90’s extensively, I know personally the TRX250R it was in. I used this crank in the late 90’s on my 250R for a few years without issue, had the crankpin, bearing and rod replaced and continue to use it to this day. Last year I had BDT Motorsports build me a +4MM stroker crank of an OEM without the crank pin welded in place. I have been using this one in my 330R BDT Motorsports built for me since last December without any issues.

One last note: BDT Motorsports (under a different name, same owner) was the shop Selvy’s used back in the 80’s-90’s to perform most if not all the machine work on 250R race engines Rob built including my +4 MM stroker crank originally built in 86 (without a welded crank pin)…interesting bit of 250R history 