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Project Bike Update: Crankcase Repair Tips

If you’ve ever experienced a ride or race where your crankcases got damaged in some way, whether from a rock or engine failure you know just how painful dealing with the aftermath of damaged crankcases can be. For those of you who haven’t had to deal with crankcase issues, consider yourselves lucky and know that if you stay in motorsports long enough, your time will come. Late last year I picked up a 2005 Honda CRF250 project bike (click here to read about it) which came with crankcase damage. The crankcases had the following issues:

  • Stripped right main bearing retainer bolt hole
    stripped bearing retainer
     
  • Broken off right front shift shaft boss/bore
    broken shift shaft boss
     
  • Partially broken off cam chain guide
    broken cam guide

I chose to repair the crankcases because it can be a much more cost-effective solution than buying brand new cases. In this post, I’m going to cover crankcase repair options and tips that may help you save your next set of injured cases when the need arises. While you may or may not be equipped with a welder or feel comfortable carrying out the repairs, the information provided will be equally helpful when discussing repair options with your local welder or fab shop.

Strength Considerations
First off, since the crankcases house the heart of the engine, consideration should be made as to how structural the repair area is to the functionality of the engine. Secondly, the likelihood of making a repair that results in a joint that is equally strong to the original in critical areas should be considered. If the criticality of the joint is deemed too high or the repair is too difficult, then it is advisable to pursue a new set of cases instead of risking a mediocre repair that could jeopardize future operation.

Many people often question whether or not a welded joint or the weld material is as strong as the base material used to manufacture the crankcases? The answer to this is, it depends. The specific casting process used to make the crankcases has a significant effect on how strong the cases are and whether or not the manufacturer chooses to heat treat the cases after casting them.

The strength of the welded repair is only as good as the weld itself. Cleanliness, depth of penetration, and skill all play into how strong the repair will be. Most of these variables are relatively controllable; however, skill and experience are primary drivers.

Since engineering data that outlines what materials and processes manufacturers’ use to make their crankcases isn’t available, it is impossible to say with any certainty that a weld repair will be as strong as the original material. However, it could also be stronger or weaker. We simply don’t know without testing or analysis. Personally, based on my knowledge of casting processes and how I believe most engine cases are made I don’t believe the alloys used to make weld repairs on crankcases differ significantly in properties from the base material.

Repair Methods
Outside of welding, there are a couple of other repair methods worth considering depending on the issue at hand. With proper preparation, epoxies (think JB Weld or equivalent) can be used to fix minor issues. For stripped threads, heli-coils or increasing the tapped size can be good options.

Welder Types
Both MIG and TIG welders can be set up to weld aluminum; however, in my experience, it is preferable to use a TIG welder because they offer significantly more control throughout the welding process. Due to the thick walls found in many crankcases using a welder with enough amperage is imperative. For most dirt bike applications a welder capable of at least 150 amps will be necessary. 

Welding Best Practices

Preparing the Damaged Area - Since untreated aluminum oxidizes when exposed to oxygen, the area being repaired should be prepped by grinding away the surface layer in and around the joint. The material can be removed with a die grinder and an appropriately sized bur. Depending on the area and geometry of the repair it may be necessary to groove out the joint so that additional surface area and weld depth can be achieved. 

grinding prep

Check Flatness - Before welding, take the opportunity to assess the flatness of all the crankcase mating surfaces. This initial assessment will serve as a benchmark to compare against once all welding is complete. 

Cleanliness - The cleanliness of the area to be repaired is critical to the success of the job and to keep frustration levels low. Carefully clean the crankcases using isopropyl alcohol, acetone, or other suitable cleaners. Be sure to verify whatever cleaner you use will not emit harmful potentially life-threatening chemicals should you inadvertently expose them to high temperatures and argon gas. Brake cleaners are typically a no-no since they may contain Tetrachloroethylene which can become fumes of phosgene when exposed to high temps. Phosgene, even in small doses, can be lethal.

If the crankcases are very porous, an initial cleaning can be performed to remove most of the oil. The cases can then be baked at around 250°F (121°C) for 15 minutes, which will help draw out oil from beneath the surface. The cases can be cleaned again. 

damaged cases

Filler Rod Choice - 4043 filler rod should work well for most castings and is what I used to carry out the repairs on my CRF250 crankcases. Alternatives to consider include 4047 and 4943. 4047 filler rod offers improved resistance to porosity over 4043 due to its higher silicon content but offers slightly less strength. 4943 filler rod was designed to be an improvement over 4043 and may offer additional strength while retaining similar flow characteristics.

Preheat - Heating the crankcase in the oven and raising its temperature before welding will help reduce distortion post welding and make welding the part easier. Oven temps up to 300°F (149°C) and bake times up to 30 minutes can be used to preheat the crankcases.

Limit Distortion - Distortion is a risk anytime something is welded and due to the criticality of the location of bearing bores, shafts, and other features found within the cases everything possible should be done to limit distortion throughout the welding process. What can be done will be situation dependent, but the following are recommendations that can be implemented when practical:

  • Preheat the crankcases
  • Make weld repairs with the case halves bolted together
  • Clamp a single case half to a flat surface if the repair permits
  • Leave bearings in their bores

Check Flatness - Upon completion of all repairs, check mating surface flatness before returning the case to service. If mating surfaces are not flat corrections can be made on a surface plate or via milling in extreme cases.

checking flatness

CRF250 Case Welding and Repair - My CRF250 cases welded well, and I was able to get proper fill with minimal porosity. The weld layers built nicely as I filled in the cam chain guide, shift shaft boss, and bearing retainer hole.

shift shaft borecam guide weldretainer weld

Post welding I carefully machined in a new bearing retainer bore, and shift shaft bore.

machined bores

My case did not distort due to welding but had some knicks and dings on their mating surfaces, which I addressed using my surface plate and sandpaper. Upon completion of all the work, I inserted the dowel pins and shift shaft back in the cases and checked fitment of the shift shaft in the new bore. Everything lined up well, and the case halves and shaft mated perfectly.

sanded case half

Wrap-up
I hope you enjoyed this write-up on welding crankcases and that you consider it as an option the next time your cases incur life-threatening damage. I’m looking forward to moving my CRF250 project forward and taking the next steps to get my engine running. If you want more engine building knowledge at your fingertips, check out my books on two and four-stroke engine building. The dirt bike engine building handbooks are nearly 300 pages apiece and share a wealth of knowledge, such as repairing crankcases, which you won’t find in your service manual when it comes time to rebuild your engine. Check them out on our website or on Amazon.

Two and Four Stroke dirt bike engine building handbooks

Thanks for reading and have a great week!

- Paul

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Excellent write up, we have welded over a dozen cases, all of which have held up great and have never failed.

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Great article! Many years ago I was able to repair a RC51 engine with broken case areas without disassembling the engine. I TIG welded the missing areas back up, machined it all in a milling machine then re-did the threaded holes. Totally saved the engine and the bike has been working fine since.

Here's the link to the thread on speedzilla:

www.speedzilla.com/forums/honda-rc51/63366-24-feet-welding-rod.html

RC51_Case_Repair.jpg

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Thank You  Mr. Olesen for that Very informative read w/pictures. When I worked at the Belk Tyler Honda Shop in the mid 70's, poor maintenance and an improperly routed battery vent tube over the drive chain on the CB750K3-K6 would deteriorate the chain and break under stressed conditions (AKA = drag racing, TSB from HMC came eventually). We saw damage from cracks to holes as large as kitchen match boxes. If the major sizeable parts were found/left, we would have the case front sprocket area Heli-arch (TIG) repaired. These cases split horizontally and were sold as "matched cases" only. Replacement wasn't cheap. We were sometimes also able to repair/patch/fabricate case material with grey Marine-Tex to save the damaged cases. You could drill, tap beat with hammer, it was very durable if proper methods of use were followed correcty. I have saved many a rotor/point/magneto cover on Z-50, SL/XL-70, XR/XL-75/80/100's, & Atc-70/90/110's with this method also 

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