390 on a 420auto

Hello Auto Guys
Can you put a 390 top end on a set of 420 auto cases i see the stroke on both is 71mm would there be any advantage or problems by doing this? thanks for the help.
Tom

I know that the 1980 390 has the same bolt pattern on the cylinder studs so it should be a bolt on , I have one if you want to try it , as you know I'm in Greenville ,mi

Advantages compared to what? I have no 390 stuff to compare to. It appears to me the 420 cylinder is the same casting as the 430 air cooled casting with some planed off the top and bottom. Not all of them appear to be done the same way. I doubt it is any advantage over cutting down a 430 cylinder and then the head will have more fins. The 420 is basically all ball bearings where the manual for the 390 shows lots of needle and thrust bearings. The 420 has the engagement that lasted to the end while the 360 and 390 have an earlier design. The 420 is about the most oversquare set up so doing that would make the bore stroke ratio a bit more sensible, with the less stress on the starting system. I think whether the cylinder studs are set the same is on this site somewhere but I do not know off hand.

The 420 piston might be easier to get as it was the same as the air cooled 430 and a more modern 430 piston is lighter with the ring a bit higher. If you like offset piston pins then that would be an advantage as that is what woosner is.

Thanks Al & Frank for the info what happened was i bought a 420 auto on ebay a part at a time i got the whole motor for under 5 bills but the top end is a 390 it fits on the cases so unless they mixed it up with a 390 some one put the 390 on the 420 bottom and when they took the bike apart to sell parts they must have assumed it was a 420 because thats what the case tag says it is.I was wondering if anyone had ever tried it before.The cylinder is on the original bore and it looks good it looks like a hone job and a new ring and it would be fine.Say Al did you go to Hayes farm?
Tom

A 420 is simply an over bored '80 390, they are the same engine. What probably happened is the 420 got to its last bore and instead of getting a sleeve they just put a 390 cylinder on it. There is very little difference in power between a 390 and 420.

no didn't make it to Hayes , loaded and ready to go,, the wind storm on Friday took out our power for two days had to man the generator, really wanted to see Terry C rip it up on the automatic
Al

Bummer Al sounds like it was a good time.We were lucky up hear we never lost power.Well i guess i'm going to have me a 390 auto i think i'll put it in a 78" OR frame I've got with a set of 40mm front forks that should make a good ride.

I like my 390 auto the best out of the three I have , but the 250 is the most reliable out of all of them . You will have to do a enduro with one of those , nothing is better in vintage enduro than a auto with the 40mm forks, here is a link to pictures from hayes farm https://www.facebook.com/groups/425...658945194168&notif_t=group_activity&ref=notif

Thats the plan Al i bought that 420 auto down in Kalamazoo the one you thought of getting but you said your next one was going to be LC. I went thru the entire tranny and new top end now the frame, but we are putting an addition on the house and on the garage this summer so i dont know if i'm going to have it done by fall mite have to wait till next yr. The wife thinks the garage addition is for her car but really its for my 7 bikes i need more room

So the 390 & 420 cylinders are the same except for the bore size. What about the port timing and porting. Is the 420 as wild as the 390cr is? Or did they lay the port timing on the mellow side on the 420.?

Beat's me Bill but i'm going to find out i'm restoring an 81"420 and iv'e got this 420 auto bottom with a 390 top end that i'm putting in a 78 OR frame so i will be comparing.............someday.

If the 420 is like the 390 only more hang on when you wickit. The 390’s have fun written all over it.

The 80 390, 420 and 430 cylinders are all basically identical except for bore and very minor port timing differences. The 420 and 430 make a bit more power everywhere than the 390 but its not a huge difference.

I had a 430wr 83 that had tractor like torque. The 400 cross country was like steer it and wickit and she would put you there. I had a 430cr before but never had it running.

This basically answers the question in post 1. It should be noted that the 430 you speak of is the air cooled version with the reed valve set up of the those. The water cooled 430 with the later reed valve set up I would expect to have quite an advantage in any actual race where extra power matters.

The next step would he for folks to measure down to the top of the exhaust port and post.

Yes, I was speaking of the air cooled 430.