WR390 rebuild/questions

[jones390]

Well i pulled the head of my 1980 wr390 because after i re jetted the carb, packed the silencer and changed the air filter i got piston slap. It sucks that i have to start this now because after the work i just did the bike was running awesome. I was going to wait to do the rebuild till winter but i think I'll just go ahead and start it now. I don't know if any work had been done to the bike other than cosmetic before i got it (the person i bought it from restored it to look good so that it could be a display bike in his dealership) so i am planning on just doing both the top and bottom to be safe.

After pulling the head i noticed a couple of things that seemed odd. First, and its probably a dumb question but is there really no head gasket between the head and the cylinder? There is the one between the crankcase and the cylinder but not the head. I also noticed that it looks like someone ported it unless this is what they look like coming out of the factory,





From the piston markings it still has the standard bore size however,



This is the best picture that shows just how much play that the piston had in the cylinder that i could take,

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In that and the other pictures you can see that there is a decent amount of scuffing but some of the crosshatch is still showing. I'm having trouble getting a definite measurements with my snap gauge and caliper of the bore to see just how much it will have to be bored over so i will have to wait till i can take the head to a shop or get my hands on some better calipers, hopefully it isn't to far out of round and i can just go to the first step up.

A few months back i did a compression test and it had decent compression but i wasn't thinking today and pulled the head without rechecking to see what it was at now. Honestly after getting this far i'm actually surprised the bike was running as good as it was with the amount of blow by i was getting past the piston judging by the amount of gas it had under it.

As of right now i am planning on just going with the smallest overbore i can but i would like to do what ever i can to get some extra power out of it while i'm at it, I'm kicking around the idea of doing the 420 conversion but im not sure if i want to go that route. I'll document the rebuild as i go and ask questions on here as i come up with them. If anyone has any advice or suggestions i would appreciate them.

 

[justintendo]

none of the air cooled 2 strokes had head gaskets, it is a lap fit. during a rebuild the head will need lapped to the cylinder liner to form a good seal.

having a lot of gas in the crank has nothing to do with blow by, your fuel and air charge on most any two stroke passes through or under the piston into the crankcase after it leaves the carb/reeds.

using up the oversizes that are left would not be smart in my opinion, for a small return. have you ridden a well tuned 390? they rip! have you tried finding new sleeves for that bike? not easy or cheap. i bet the next size piston will work fine, theres not alot of damage there. if you really do want more power porting will make it run even better, sometimes at the cost of durability.
and yes, thats the factory husky port cleanup

 

[justintendo]

does this bike still have the original spark arrestor with the springs on the endcap? if it does a modern spark arrestor muffler or a longer straight core would do wonders for it, and not be some thing that could not be reversed.

 

[troy deck]

JUSTINS RIGHT A GOOD RUNNING 390 IS PLENTY GET IT ALL TITE N RITE AND HOLD ON THE SAME WAY IF YOU WANT MORE LATER WE WILL BE HERE TO GUIDE YOU ONE MOD AT A TIME TILL IT RIPS ONE OF YOUR ARMS OFF

 

[jones390]

I did not realize that none of them had head gaskets, that's pretty cool. By trade I'm a 3rd assistant engineer in the merchant marine so the majority of my experience with 2 strokes is actually on very large 2 stroke diesels, the only previous experience working on 2 stroke dirt bikes was pretty much all liquid cooled newer Hondas and i never hung onto one long enough to get into doing much work on them.

I shouldn't have used blow by for to describe what i meant for the amount of fuel below the piston, i just wasn't sure how to say it. There was probably a pop can full that was sitting bellow the piston and im not sure how much was in the crankcase which seems excessive for what would be there from scavenging.

I don't plan on going over what is needed if i use my current cylinder, i tried briefly to look for sleeves and realized just what a pain that would end up being. I was thinking the 420 conversion because i thought that it used the 420 auto piston and head but i looking through some post i had remembered wrong and it uses my cylinder to do it il wait until i go through a couple oversizes for that. Im hoping that i can just go one up, i just grabbed a better set of calipers from a buddy so il head out and take some measurements here in a little bit.
That make sense that it was factory it really didn't look like enough porting was done to be someone looking for power. I have some experience doing porting (only on car engines) how much porting is safe to do on these if i decide to? I wouldn't do much if i do but i touch it id like to see what people have done to be on the safe side.
It does still have the factory pipe and silencer, as far as i can tell everything on the bike is original but the fenders and some small parts like grips (all the parts that have the serial numbers match). I am planning on getting a different exhaust set up for it, but finding a set is tough and im not sure where at the moment would be the best to go through to have one made. I was planning on striping the bike down and having the paint redone and going though the engine to bring it to show quality because its in such good shape so any thing i do for performance it would be nice to be able to take it back to looking stock if i decide to show it.

This is actually the first husky that i have owned, i have always lusted after them and finally was in a situation where i could get one and was able to find the 390 for a steal so i jumped on it. It was used as a display bike in a shop so the previous owner has brought it to good cosmetic shape and never really rode it ever so it wasn't really tuned tune well at all (i think the carb that was on it was off a different bike and just on it because he had a spare) and i finally just got it tune and running good and then the top went. I have road big bore 2 strokes that my friends have and it defiantly is up there in power in comparison, it even scared my buddy who has a cr500 but that might have had something to do with me telling him what would happen if he laid it down and hurt the tank.

this is the bike



And this is the type of two strokes i normally play with



Think we could squeeze one of these pistons in a husky

 

[oldbikedude]

The cylinder looks pretty good to me, normal wear I'd say.......shouldn't need to go much bigger. I'd be interested in hearing the clearance reading. Good luck with the rebuild. Nice bike!

 

[justintendo]

you do not want to port it yourself..you can easily destroy the cylinder removing metal in the wrong place. you must have a thorough understanding of port timing and 2 stroke understanding. you must also have a small 90 degree grinder to work on the boost ports, these are the intake to the chamber, not what is behind the reeds.

 

[justintendo]

you do not want to port it yourself..you can easily destroy the cylinder removing metal in the wrong place. you must have a thorough understanding of port timing and 2 stroke understanding. you must also have a small 90 degree grinder to work on the boost ports, these are the intake to the chamber, not what is behind the reeds.

it is much easier to ruin the broad powerband than to improve what husky designed, even back then. that being said, a pro with experience can do some good work on one of these..the 390 has a great band and if you want to ride faster money would be better spent getting the suspension dialed.
if you would like to do some grinding tho, there are a few areas.
with the cases split, you can match the crankcase to the cylinder one side at a time. also make sure the base gasket does not protrude at all. most all need a bit of trimming. also you can clean up where the exhaust spout mounts to the jug, these are usually off a bit..

 

[suprize]

Im with Justindo here, I tidied up the transfers and cleaned the intake passage on the 400 to effectively blueprint it. I had a mate finish it off for me and he took a bit off the top of the exhaust " to liven her up" he said. it did but it now has a bit of a hole before it climbs onto a stronger top end. so leave the port dimensions alone and just match all gasket edges and smooth any sharp curves a bit.

 

[jones390]

Ok good to know about the porting I will stay away from messing with them other than maybe il do some of the clean up if I end up splitting the cases. I know there is a lot to port timing because i was doing a lot of research on possibly making my own expansion chamber, but i didn't even think about it in relation to porting and that is something i don't want to get into.
Im actually trying to get a plan set for the suspension once i get the engine all taken care of. So far what i am thinking is getting a set of triple clamps, and forks from a 87 or 88 to get the 40mm disc break, once i get a set before i put them on I am going to get the raceteck emulators and piston rings. As far as the rear shocks go i am not sure what i want to yet. As it sits though the bike handles really well and as far as i can tell none of the suspension parts seem to be even close to worn out, honestly i don't think the bike was ever really ridden much. I will be also replacing all of the bushings and bearings through out the bike this winter when i strip it down for powder coating. If you all have any suggestions let me know.
I took all the measurements today and i was actually pretty surprised. For the cylinder the diameter the gap at the top of the cylinder where the clearance space is from front to back it measured 82.92mm and where the piston travels it was 82.87mm for a 0.05mm difference and from side to side it was 82.92mm and 82.89mm for a 0.03 difference at the skirt the biggest feeler gauge i could fit was 0.2mm so there is a little more wear front to back but overall it is far from the maximum difference required for a overbore. The end gap on the piston ring was 0.89mm so it is way out of spec and i took that reading just below the clearance space and not the bottom. As far as bearings go the rod bearing on the piston end doesn't seem to have any play and from what i can tell with out setting it up with a dial indicator the crank side has no play also. So with those readings I plan on putting a piston kit in, but i am not sure if i really need to have the cylinder overboard to the next size or if i can get away with having it honed and using a standard size piston. I cleaned the cylinder up and i can really see a lot of the old crosshatch through out the cylinder and the scuffing seems to be little enough that it possibly would be taken care of with the honing. The one thing that kind of worries me is the difference in diameter from side to side and front to back but with it only being 0.02mm i don't thing it would make a difference. I'm asking around to see where the best place to go around here for cylinder work is, and once i decide who i want to go to i will have them take measurement also to see how close i am with mine and get their opinion but with what i have I'm not sure what i have to do. If i can get away with out having to bore to the next piston size it would be nice to save that for the next time the top end needs work so i can get as much time out of that cylinder as possible.
As far as pistons go who do you all recommend, i have been getting all my parts a husqvarna-parts.com and they are great so at the moment that is my plan. I would like to get a high performance piston if possible but im not having any luck finding any other than just different bore stock pistons, i looked on wiseco's site but for 1980 they don't have anything for the 390. Is there anything out there that i am over looking?

 

[troy deck]

no hit with a ball hone n slap a ring in it id worry alot more about leaking crack seals more than anything if they are original

 

[suprize]

my thoughts exactly, the huskys are strong until seriously worked way past their wear limits then they may crack a piston skirt with the inevitable results.

I would look at a case split with new seals and main bearings then wack a piston kit in after a quick hone. strong cheap repair.. ideal.

pipe should be fine, get an alloy muffler and jet it to suit.

I stripped the 400 motor and put new seals and bearings thru the bottom end and left the rod (felt fine) and the piston and ring it had in it. I didn't measure it, took a punt on it as it looked pretty good. that was in 2011. it needs a ring and piston now... but I've had a heap of riding from it for nix.

unless you do the seals, you risk burning a hole in your new top end if it does a seal on a top gear blast . well worth doing them. as its a split, do the main bearings as well. cheap insurance...

don't worry too much about performance... if your holding it flat in every gear and screaming in your helmet "come on you slow bucket of puss " then you might need to hot her up.....

 

[jones390]

Ok sounds like i have a pretty good direction going now, thanks for the excellent advice. I actually probably should split the case anyways to check it out inside just for better piece of mind.

I also forgot to post ask about my clips, do they look broken to you?

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They are not symmetrical and the end without the bend on them look kind of ragged and i'm kinda thinking they are broke. If they are in fact broken what worry's me is where the other part is.

For splitting the case are there any husky specific tools that i will have to have to do the job? Is there anywhere that seals the seals as a whole kit? What is the best piston kit out there for the money? So far the best place i have found to get parts is Husqvarna-parts.com, his piston kit seems reasonable and more complete than others, but the seals all have to be bought individually and id like to just get them as a set so i don't accidentally miss one or get something i dont need. Is there anything in these husky motors that i should give a look while i have it apart other than what we have discuses already?

I have to admit you are both right as far as needing more power the bike defiantly has never left me needing more, i just have a problem where when i get into motor work i seem to always end up going for more power even if it is far from necessary

Sorry for all the questions, i just dont want to take any chances on doing something wrong because i love this bike and am not very familiar with huskys yet.

 

[justintendo]

there are many clips that look like that. some have no ear at all, some have both. the tusk universal splitter and tusk crank puller will do the job for teardown and reassembly. if it were me, i would run a wossner forged piston. phil sells these in addition to the nos pistons. the nos pistons are mahle which are also good but cast. do a search for the tools on here, they have been brought up before.

 

[Bigbill]

I didn't port any of my 390's they rip anyway in there stock form. My first 390 had gas in the crankcase because they flooded it when they couldn't start it and they left it. This rots the base gasket.

I designed and made my disassembly tools and assembly tools. These crankshafts have a slight press fit to the crank bearings.

 

[troy deck]

your piston will have new cir clips just make sure the opening is at the 12 or 6 oclock position there are only 5 seals in the whole works maybe 6 not sure if that one has a counter shaft seal or not its been said if you use a sealed bearing [ on one side] on the clutch arminstead of a seal it will improve the clutch action

 

[jones390]

Its not really the shape of the clips that made me worried because i have seen the c clips like that, i was just worried that the one is longer than the other, they have pretty ragged ends, and in all the husky diagrams they show the type ones with the bends on both ends. It seems like this motor is original inside, did husqvarna ever use these types of clips? Either way i have the motor out and il know soon if there is anything in the case.

After getting the motor out and taking the side covers off i am glad that i am opening it up, so far i have noticed a few things that i know are wrong, for example there was no snap ring to hold on the chain sprocket and no distance piece. I know the bike was only for display purposes in a show room before i got it but that is pretty bad, i should have checked it out when i got it but the PO had just put a new chain and sprocket on so i didn't take off the cover seeing how the chain is still in great shape and i have been working my way through other things that needed attention. Luckily as far as the cover and anywhere the chain could have rubbed on if the sprocket shifted on the shaft are fine. Im defiantly going to be looking at every part of this bike carefully to make sure there isnt anything else done wrong.

A couple things that i am not sure about are first that there is below the piston on both sides of the case there is a grove that looks like it is there for piston clearance but it also looks like it could have been smacked by the piston because of the grove and i want to be sure. Second bot the shaft for the kick start and shifter have a pretty large amount of play in and out and i feel like they shouldn't.



I should have some more work done tomorrow, thanks for all the help.

 

[troy deck]

some silicone sealer will make sure that sprocket snap ring stays put remember sharp side out not so sure on those marks bad rod bearing at one time? are there marks on the piston?

 

[suprize]

that's an interesting mark.....

 

[jones390]

Yea i thought that is was odd to, and its on both sides, but as far as i can tell the piston doesnt look like it ever hit there. So far today i everything but the flywheel off, im looking to see if i have to get the puller or if i can make one.