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How to destroke a XR100 or stroke a XR75?

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Our race series changed cc limits outlawing 4 of the CRF100 based bikes our team built for the series. We destroked 1 to fit the cc limit & it won it's first race so we're now looking for a cheaper way to destroke the other 3.

Does anyone know if the XR75 or any other less than 43mm stroke Honda crank will drop into the XR100 engine with just a main bearing change or minor mods?

It looks like guys have been using the XR80-XR100 45mm stroke crank int eh XR75 so we're thinking the reverse of that might be do-able.

Anybody know?

We are aware of & can handle piston, connecting rod length, & timing chain issues.

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Why not use a XR80 motor then?

I don't think the XR75 cranks fit the 100 motor maybe they will fit the 80 motor.

What are you doing about piston height in the cylinder.

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I have a XR75 cylinder for the hieght issue. $20 plus $7 shipping. It's been a paper weight for a long time, so I'll need to find it.

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Maybe we weren't clear? We need to know if the XR75 or other stock Honda under 43mm stroke Honda crank will fit in the XR100 case with a main bearing change or other minor mods. That's it.

It looks like it will fit in diagrams & specs & there are guys out there that did the reverse of that so it's probably do-able, but we'd like some confirmation that it will before dropping $200+ on a new crank to see ourselves. We're shopping used cranks for checking purposes, but don't have one here yet to check & would have to rebuild it before use.

In response Minifig: The XR80 cases, top end, clutch, & some gears/shafts are too weak for this application. We already have proven, bulletproof, winning combinations that never DNFd so we will stick with the combos we have.

The main problem with the xr80 is the cases main bearing bores & cylinder skirt hole are both way too small & too poorly supported for this power & rpm level & some XR80 gears/shafts don't last more than a few laps without chunking or starting to seize on the shaft. That's why all that is bigger & better supported in the xr100 & we beef them up from there. You can definitly build a fun XR80, but it has no advantage in our application.

A benefit to destroking our existing combos is we keep the entire package except for the timing chain & crank by simply shaving the cylinder or a longer rod. A side benefit is we can also go back to big bore if the rules change again with a crank, chain change, & a fresh cylinder. Since we keep 3-4 big bore spares on the shelf the crank swap is the most cost & time effective way to do it.

Another benefit to reducing stroke & keeping our existing top end is what we lost by destroking we're nearly making up for with more top end, a better rod/stroke ratio, & a wider powerband. With more dyno time, & what is probably a lighter crank, we'll probably close the gap.

The one that we destroked won it's class & nearly won the next class up despite being very de-tuned for a rich low RPM break-in mode, so the idea definitely needs no rethinking. It works for us.

We race multiple classes & can use the XR100 cranks for seasonal rebuilds of the bigger class engines so swapping cranks is the smart thing to do.

All THAT is why all we need to know is if the XR75 crank or similar stroke Honda crank [the longer the rod the better] will drop into the XR100 cases with a bearing change or other minor mod. Anybody know?

THANKS!

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We got a XR75 crank & I guess to answer our own question here's what we found: While the size, style, & rough dimensions of the crank appear close there are serious differences that make them useless to us. It is basically identical to the CRF80 crank, but the stroke is shorter. The crank stud journals of the CRF80/XR75 are much smaller than the CRF100 so the CRF100 bearings will not just slide on to let you drop the thing in CRF100 cases & visa versa if you were to try to drop a CRF100 crank into a CRF80 or XR75. The crank bearings are smaller OD & ID. It might be possible to stick a XR75 crank in bored XR80 cases that will accept our trick top end but the expense would be more than destroking the CRF100 crank & we'd have the new problem of the weak CRF80 cases & the cylinder base flange that won't accept our big bore. There are no off the shelf bearings that would adapt the 2 that we have found yet, but we are looking. The XR75 cam chain drive gear has one less tooth & a CRF100 cam drive gear is too big for the XR75 crank stud. The XR75 connecting rod big end is much smaller than the CRF100 & the rod too short for enough dwell to let the big bore breath @ the RPM we need to see. The rod small end doesn't have enough material to be bored over to the CRF100 size safely. If we changed pistons the pin size would be too weak to support the HP these things make. So, we'd have to bore the crank for a CRF100 rod or have a custom piston made & risk blowing the thing & that would be exactly where we are now. Also, the drive gear end of the XR75 crank [like th CRF80] is smaller than the XR100 crank & we were already shearing the XR100 crank stub off, so it will not work.This near new XR75 crank is useless to us & is now for sale if anyone would like to make an offer before we run it on eBay.

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Hmmn sorry to be off the topic question, but why not re pin the 100 crank and turn down the flywheels, if its not already the case you may have the technology to eccentric turn the flywheel to maximise throw inside the big cases for good torque. I tried to de bore a 100 to a 80cc but the head flowed too much giving shit loads (that means heaps for you americans LOL) of power only in a very almost destructible high rev range. Good luck.

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I'm sorry, I understand shit loads, but not heaps. What's a heap? :worthy::banana: :banana: :banana: :banana:

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