Rebuilding 79 390WR Engine

I am working out the details on a 79 390WR engine I have picked up with the warning the center cases needed to be replaced. So far I have found defects that can be repaired with careful application of JB Weld or epoxy. The issue on the left center case in addition to the small hole punched into it is that the web over the top of the crank cheek was apparently struck with the skirt of an incorrect piston( skirt too long fr wristpin). The portion of the web went around with the auto type crank and got slammed into the top inside of the crank well near the top (rear wall).

What I need to know is if that the function of that web is important enough to restore or replace the left case. I can get a left case on eBay, but can I count on the fitment without buying both cases?

I am fairly comfortable with repairing the breaches that flood the crank well with the transmission oil as I do not see a significant reduction of lowered bottom end compression to worry about. If I am mistaken in this idea, I would like to hear any concerns. I feel the slight loss of web over the crank is not worth the detriment of properly replacing it as the heat from magnesium weld would warp the gasket plane.

I can get a left case right now but can not afford both case halves

I naturally agree with you because I have had enough experience with Husqvarna going back to the first; the 1969 stripped Sportsman I had at age 14. Would it be a detriment to performance if I chose to repair the gushers but not the web? Also would another 390 left case possibly mate properly with the repaired right case? I tend to believe so because of the stringent manufacturing consistency. I can get the left case and a reed cage from the same seller. There is another reed cage from a 77 360WR. Did the 77 360WR have a Bing or Mikuni carb?

I had the same issue with my 77 250CR, a piece of the piston broke off and got jammed.
I went on Ebay looking at cases for Yamaha's, Kawasaki's and Suzuki's. I noticed none of them had
the webbing around the crank like the Husky.
I decided to grind off the other 3.
Then I mounted the cylinder to each case half separately, to blend the transfer ports to the cases. Had a bad step there.
I had my crank disassembled to clean it and make sure the rod was still straight and then of course trued.
I have not had any issues with mine. I say run them!!
I would not recommended using a separate center case. I've heard over the years many times, they are machined as a set.

I would also say it shouldn't be that big a deal. if your worried, plug the balance holes in the crank halves (if they have them) to boost primary compression (always a good idea as new bikes have plugs as std)

The crank seems to be from an Auto as the crank cheeks are not circular. I have all the internals cleaned and packed inside ID'd zip lock freezer bags

So far the patching is going well. I have the holes that breach into the crankcase patched. I cast into place after grinding in relief in the crankwell so that pressure will blow the patches into the transmission cavity with ample support from transmission side as well. I have decided to repair the fractured web by sinking a row of support pins into the case, then casting a web out of JB Weld then final form.

Pictures soon to come

I got front and rear engine mounts from a 1978 390 OR off eBay last week so for my July 4 holiday, I cleaned up the mounts, put the engine cases together with the mounts , and bolted them into the 1978 WR frame. Bolted the cylinder and head onto the cases. I borrowed the reed cage out of the 430WR parts bin and the carb from one of the 250WRs. I wanted to test the fit of the 1979 390 exhaust in the 1978 frame so it looks pretty good after cutting off the rear mounting bracket. After I finish the internal repairs on the engine cases, I will be looking for a 2nd over piston kit.

The exhaust fit did not look so good after trying to put a tank and seat on it. I was looking at 79 frame and saw they are eerily similar to the 82 WN frame I have. The top frame tube is steeper than the 78 frame and seems to place the triple clamps higher as well. This may also explain the fork tubes on a set of 79 forks being about 4 inches longer than the tubes from the 1978 forks I have.

So the next mockup may be in a WN frame this weekend seeing I have one more frame than the regular engines on hand.

PM me I may be interested. Also let me know if the swingarm is available as well

jimspac, while rebuilding a 75 360 I took notice of the "webs" in the crankcase to see what purpose they may serve. My guess is they're used to create a high pressure area at the base of the transfer ports.

The attached pictures show how the "webs" may direct the fuel/air mix to the base of the transfer ports creating an area of increased pressure. Consequently the transfer port area would have a lower pressure. This difference would result in the fuel/air mix moving through the transfer ports at a higher velocity and when creating performance velocity is important.

ruwfo's comment about reducing port swirl would also be correct. Without the "webs" more of the fuel/air mix would get trapped under the piston creating turbulence as it tried to find its way to the transfer port. Turbulence slows down fuel/air causing the fuel particles to condense into liquid and its fuel vapor that creates the big bang!

Stuff like this makes me appreciate the efforts of the HVA engineers to create a better motorcycle back in the day. BTW the earlier 72 450 has these webs but the 68 360 and 72 400 doesn't.

I think these webs were used to hold the oil mix in the lower crank case longer so the rod bearing and outer crank bearing get oil. They use the pressure when the piston moves down before the transfer ports come into play. They use that peak time pressure to inject the oil mix into the bearings.

The old husqvarna Swedish engineers were the gurus that everyone else followed.

Right now I have the engine still setup as a stunt dummy for mockups. I was testing assembly into the target 78 ML frame first with the 79 exhaust I got with the engine. That was going to require mods to the pipe to get it to work so when I found a 77/78 390 pipe on eBay, I grabbed it. $90 shipped was the best price I could find.

After initially trying a fitting into an 82 WR frame that was spare, I thought the 79 pipe would not work there either. After reaching the decision to break down the mockup, I played with it first and got the pipe to settle into a spot that looks good enough to check the fit with the WN airbox to figure out what position to design front engine plates for.

I will be casting a web for the broken spot with pin reinforced JB weld as the webs are non-structural

I got the missing web rough cast about 2 weeks ago and got it well finished last weekend. This weekend I fit the crankcases together to check and it looks good profilewise. The opening between webs need to be adjusted but that is minor now. The cool thing is that to tap on one of the mag cast webs then on the JB Weld rib, the ping sounds very similar and both have the same resistance to deflection. Pictures to follow soon my friends!