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
Broken off right front shift shaft boss/bore
Partially broken off cam chain 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.
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.
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.
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.
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.
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.
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.
Post welding I carefully machined in a new bearing retainer bore, and shift shaft bore.
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.
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.
Thanks for reading and have a great week!