HELP! Stroker crank

[Shane¹]

I have a brainstorm going on for a cheap stroker crank. If there is enough material could you offset grind the pin hole and use a bigger bearing and pin and then press it all back together? Would going a mm or 2 bigger make the crank too far out of balance? Taking any thoughts and input thanks

 

[Ketchup]

Never done it, but here’s some thoughts.

A pin that’s 2mm bigger only gains 1mm of stroke, right?

The big end of the rod is probably the limiting factor for pin diameter.

At some point the crank weights won’t be balanced enough.

You might be able to use a rod from one model while using the weights and shaft from another.

Machining a full circle crank may not be much harder.

 

[Bigmac]

Your theory is correct, I have a cr500 crank that was offset bored and the hole bushed so the original crank pin could be used, the bushing was tig’ed in place. Having precise machine work and a well trued crank the vibes would still increase but could be minimal. There are other things that can help, lighting the piston, another consideration depending on the model is the piston hitting the counter weight on the down stroke!

Have done a lot of mx and atv strokers

 

[drf256]

You also have the problem of case capacity. Saw cases have little space around the crank lobes and large rod end, so stroking one generally requires thinning the case.

 

[Shane¹]

Never done it, but here’s some thoughts.

A pin that’s 2mm bigger only gains 1mm of stroke, right?

The big end of the rod is probably the limiting factor for pin diameter.

At some point the crank weights won’t be balanced enough.

You might be able to use a rod from one model while using the weights and shaft from another.

Machining a full circle crank may not be much harder.

I have no idea if I am right but old pin is 14.4mm if I offset it to a 16mm pin. It would move the center line .8mm and change the stroke by 1.6mm

 

[Ketchup]

Right. My thinking wasn’t totally sound there. The center line moves half the distance but you get that distance top and bottom.

Dr Al is right about case clearance. The rod itself is usually the tightest clearance point on a stroked saw. On an offset the rod could stick out wider than the lobes but also hit the rim of the case at around 45 ATDC and BTDC.

 

[Shane¹]

Right. My thinking wasn’t totally sound there. The center line moves half the distance but you get that distance top and bottom.

Dr Al is right about case clearance. The rod itself is usually the tightest clearance point on a stroked saw. On an offset the rod could stick out wider than the lobes but also hit the rim of the case at around 45 ATDC and BTDC.

I would definitely do some more measurements I was just curious what peoples thoughts were about if it would be too unbalanced if everything else cleared the case and piston

 

[mgr1]

There is unbalance if you use a bigger big end pin.
Easiest is with offset boring and use welded bushings in place with standard big end pin.

Balancing, look some videos on yt.

What chainsaw are you going to stroke?

 

[srcarr52]

Are you planning to use the same rod? Because increasing the rod big end diameter and the stroke you'll likely be larger than the case can fit even after some clearance work.

 

[Shane¹]

Are you planning to use the same rod? Because increasing the rod big end diameter and the stroke you'll likely be larger than the case can fit even after some clearance work.

Thinking the same length rod but a different one I haven't made any actual measurements yet was more curious if just the offset would shake to much or not

 

[Bigmac]

Here is a pic, stock
Stroked,
Have seen guy fill the pin hole with weld too.

And depending on how beefy the Rod is, I’ve seen some people machine some off of the rod surface to help clear the cases, that’s how they do the Honda Trx250r 4mm cranks

 

[Shane¹]

Here is a pic, stockView attachment 408645
Stroked, View attachment 408644
Have seen guy fill the pin hole with weld too.

And depending on how beefy the Rod is, I’ve seen some people machine some off of the rod surface to help clear the cases, that’s how they do the Honda Trx250r 4mm cranks

Does the offset change the balance at all? If not I think I may try it sometime. Only reason I was thinking about doing the bigger pin is I have to have some one machine it for me and I can get a connecting rod thats the same length just bigger pin I think it would be fairly easy. But for now I put it back together stock maybe the next one I will try it

 

[Bigmac]

Like the guys have been saying, a bigger pin a rod are going to create clearance issues, plus finding a rod at a similar length and bigger pin could be difficult. Going to a 2mm bigger pin will effectively double your clearance issue, your basically moving the outside edge of the rod 2mm out vs 1mm for a 2mm stroker.
With the bushings, you can still use your stock rod, pin and bearing.

It’s going to effect balance, but the larger pin would be even worse

 

[Rich Fife]

Are these cranks balanced? I'd love to see the nitty gritty details from the engineering side of it... Mainly the details on the rotational mass, percentage, etc... I know the counter weight / shaft alignment is typically within .0002" - .0003"...
Someone should add heavy metal to the crank and see how it responds.

 

[Bigmac]

I personally don’t think it will be a major issue, big bore piston are usually heavier and will vibrate more, but it’s usually tolerable and usually smooth at rpm, vibration is usually worse at idle.

In a stroker crank I would be more worried with poor machine work, and assembly. A misalignment in the pin hole or out of true, would cause bad vibrations and maybe catastrophic failure.

I’m sure the cranks aren’t precision balanced, Have also heard of people doing extreme lightning to the Piston and having excessive vibration, so it’s still something to consider.

 

[drf256]

I don’t think that single piston engines can truly be balanced. It just becomes harmonics and at what rpm the most vibrations occur. You just don’t want the vibes in the working rpm.

 

[Rich Fife]

My buddy has a balancing machine that has a single cylinder motorcycle standard that shows 61% rotational mass... idk if that was for a honda or what... sounds like a little R&D needs to take place. Lol

 

[Dolkitafreak]

Balance factor is important with cranks, they can’t be 100% balanced from what I understand but balanced in your desired rpm range can matter. Just stroking I doubt balance is altered a ton, should be fine. Adding heavy metals etc has been done as well as offset pins etc. Boring cases is pretty standard for stroker builds. Usually need a longer rod for piston to clear crank, there’s threads here covering all this if you dig for them!

 

[srcarr52]

The balance factor of a single cylinder crank is pretty easy to measure. These are usually between 50-60% balanced.

The problem with a single cylinder engines that lack a balance shaft is if you 100% balanced the crank for piston and rod weight you'd now have a large imbalance perpendicular to the cylinder (for/aft for a vertical mounted cylinder).

To the OP's original question, yes stroking the crank will effect the balance but probably not enough for you to notice.

 

[Ketchup]

Usually need a longer rod for piston to clear crank.

That’s a valuable point.

Are crank weights typically just mild steel?