If it were me I'd chuck the crankshaft into a lathe after welding the weights on and spin it up and take a dial indicator and see if there is any warp caused by the welding and you may see if it became out of balance by added weight of welding material. But heck you know that's just me.
On the 1/2 VW I believe you would have a rod knock while engine was cold then it would probably quit up some once engine warms up if only a couple thousands out. but I'm sure you'd may be changing main bearings before 100 hr. of flight. Preheating and stitch weld would be the way I go but then I would dip shaft in oil for quenching cooling and some also cover it with sand after quenching and let sit until it reaches room temperature.
I don't have any intention to repair the crank after welding. Some don't use the counterweights at all and some do. I may choose an alternative engine if welding requires too much fuss.
So my question remains: does the welding warp the crank or not?
The welded counter-weighted cranks made with OEM stock were universally supplied .010" undersized. Checking for true on a lathe, or straightening a cut 1/2 VW crank is going to be a challenge becuse one of the end centers has been cut off. The cut crank can be center drilled, before welding, on the cut end to provide a new center for checking run-out after welding, and to provide a reference for future grinding.
This new center can/will move due to the welding with respect to the original center line. Using the new center will making it seem like the bending is at the pulley end. The run out of the former center bearing will look OK due to the short distance and the small amount of Sin error at that point.
The other option is to use the original surface/center line of the gear area as the reference until the crank has been ground using the new drilled center. Unless a new crank is used the small pulley end bearing can't be trusted to be concentric with the main bearings. It tends to wear to one side due to flexing of the stock non-counter weighted crank. In extreme cases there will be a 'raised' ring on one side that can be felt with a fingernail.
To answer the original question:
Welding will warp the crank. How much depends on the amount of preheat and the welding rate. Some people have apparently been able to use a welded 1/2 VW crank with no post weld grinding or straightening. The Hummel plans I got years ago made no mention of post weld work being needed.
All of the welded cranks I have used were ground post weld.
I welded the counter weighs on the 1/2 VW that I built. Pre-heated a little and skip welded. No problem. It was a very hard job to weld. No room, had to use a short Tig holder to get it all. Still have to balance it. I would never do it again, just buy it already done by Scott at Hummel Engines.
I used #1 bearing in the place of #3 bearing along with the thrust shims. Lots of work.
It’s going to be crank to crank on if it warps bad or not. It matters where the weld is, how thick it is, material, the welder. Probably a hundred thing that can mess it up. If this needs to be in-house only, I would pick an engine that needs no help. If you can’t go that far, you have to have a machine shop that can Do things you request
Car rim makers, that weld the wheels, have a torch heating the rim 180 out from the welding. Warping is hard to control.
There are no suitable 30hp engines.
Looking at all options for affordable conversions.
The A40 was considered the engine that made private aviation possible. It was crude and light. Doesn't even have a center main bearing at all and the crank deflection wallows out the end bearings at just 250 hours. That level of crudeness is acceptable.
I would be checking for any warpage, and if there was ANY I would regrind it. I ground thousands of compressor crankshafts in the 1980s--had a big old 1950 Storm-Vulcan Model 10 crank grinder in the shop--and can tell you that if all the main and rod journals aren't parallel to each other you will have bearing failures. Bearing clearances are typically considerably less than .001" to prevent loss of oil pressure and subsequent knocking and wear and excessive oil consumption caused by too much oil being thrown into the cylinders and hydroplaning the rings. Cocked journals cause rapid and catastrophic bearing wear. Cocked rod journals can break the rod. It gets nasty.