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Discussion Starter #1
I recently picked up an ‘03 XR80R for s good price. It needs a top end, clutch, rear spokes, tires and some love.

Searching around I didn’t find much info on the 80. So we’re gonna add a little to the little that’s out there.

The plan is

CB400F piston
Bigger carb
Cam
Intake
Xr100 clutch

And eventually suspension after its rippin again. I’m 6’ and weight 160lbs, so I want to add some height and springs or shocks to handle my weight. Right now I’m thinking xr100 forks, swingarm, and wheels/tires.

I haven’t decided if I want to do exhaust or not yet for noise concerns.

After a good cleaning I brought the bike inside and started tear down. The bike had been sitting outside for a while so was filled up with water from the exhaust not being plugged. I still need to pull the engine off the frame so I can tear down further, had to leave town for a week long work conference so that got put on hold for a minute.

When I pulled the cam I wasn’t exactly pleased. The cam journals don’t look all that great. There are grooves on both that I can catch a fingernail on and material knurled out where the caps meet. Cam looks the same. If it weren’t for this situation the stock cam would likely go back in


My friend is a machinist and thinks he can get them right. Worst case I’ll have to source another head.

Doing A LOT of sleuthing all over the inter webs I found a few posts talking about cb400f pistons as a possibility for use as a big bore xr80 piston. They have a 13mm wrist pin and we’re supposed to be 51mm std bore. I liked the idea of being able to still have room and piston options if I needed a rebuild, and 92ish cc sounds better than 80.

So,I ordered some 1976 CB400F pistons off of eBay. Got all 4 with rods and wrist pins for $20 shipped. I arrived home from my work trip today to them at the door. They seem to be in good shape. I put lime green grips on everything with handle bars, so I also picked up some Oury waffle grips. anyways pics


I was seeing conflicting posts about the size of the cb400f pistons. Some said 51mm and some said 54mm. Thanks to someone on one of the posts for this Honda piston chart


It claims 51mm as well. However, I still wasn’t for sure until today. 1976 CB400F std pistons are 51mm


I need to order some rings now and figure out what tires I want/can afford. I’m not sure if I should just do the xr100r rear setup first thing instead of putting money into new spokes and a tire for the 80 setup.

Off to the machine shop Monday. More to come
 

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Discussion Starter #3
RHMotoWorx has everything to raise up your 80. look them up on facebook.
That’s you correct? I’ve read your some of your threads. On the link and cr forks too. I’d like to upgrade to that setup one day in the future.




Yesterday I picked up an xr100 roller and an engine and a half. Rear wheel needs spokes but the suspension is all there. It’s missing some stuff like random plastics and the carb, but most everything else is there.

I’m trying to decide if I should go ahead and build it with the 100 suspension, tires, & wheels. I already bought new tires and tubes for the 16/14 setup off the 80 but the rear wheel of the 80 needs spokes as well.

Will the 19/16” wheel and tire setup be that much better for me as a 6’ 160lb guy or should I continue with the 16/14 setup?

My concern would be that the bike would be too big to be fun.


Pics



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I have 5 inches and 50lbs on you. When i ride bikes too small my hips hurt. Good thing about your pile of parts is that if you run the 100 swinger, the wheels from both can swap back and forth so you can try it out. What sort of riding are you doing? It is more fun for me to roll over things easier with the larger wheels...
 

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Discussion Starter #5
I mostly used to play in the yard and close by trails on the xr70 I had. I’m imagining that’s what I’ll be doing with this. I am however thinking I will prefer the bigger tires like you since I have them available.

I already had new tires on the way for the14/16 setup from the 80. I’m going to swap the forks and swingarm and play with thatbsetup for a bit until I can recoup some funds to get the 16” rear wheel re strung and a new tire and tube for that. I haven’t even tried to see if the 19” front will hold air. The tire looks okay.


I had no good carb solution on hand so I figured I’d try out a 1984 atc200x carb. It will be here Monday along with the rebuild kit, manifold (bc I didn’t have one) and motionpro twist throttle and cable. I’m thinking this carb might be a little too big, but I’ll be running it through the stock airbox with a drilled lid. If I can’t get it to work I’ll grab an xr100 carb.

Hoping tomorrow evening I can get the cylinder ready to go to the machine shop. I’d like to get it to them this week. Still need to order piston rings. I want to see if the engine guy likes my used pistons first before I shell out for some nos rings. I think 2 of the 3 look great. One looks greatish, and one looks good. I also think they’re all useable. We’ll see what he thinks. Pics to come when parts start rolling in.
 

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Discussion Starter #6
Btw, I had that 70 just a few years back. Same size I am now. We geared it down 1 or 2 teeth up front and drilled the airbox. Made for much Breyer wheelies and it would haul the mail. I know it’s not super accurate but we were doing 47 next to my buddy’s new camaro at the time. I always loved it because of the automatic clutch. I could ride around with a drink.
 

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I have just one question for you, have you measured the deck heights of the XR80 and the CB400 pistons to make sure the 400 will not be too short or tall in the cylinder?
 

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Discussion Starter #8
I have just one question for you, have you measured the deck heights of the XR80 and the CB400 pistons to make sure the 400 will not be too short or tall in the cylinder?
I do see now that it is .5mm shorter than stock. Hmmmmm... that’s a good bit but not too crazy. At least it’s in the right direction. It will certainly pull my compression down some, but I don’t think by too much. Fast50 has a 51mm piston with 4 oversized bores that claim 12:1 compression. If these cb400 pistons don’t make enough compression that’ll most likely be the route I go. My hypothesis is that it won’t be noticeable. Im thinking that amount could be made up with a thinner head gasket I’m right?

Do you have experience with trying this setup or just asking?


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I have no experience with the 80, but StorminNorman, a respected builder here has stated several times over the years the 80 uses a very odd deck height.
 

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Discussion Starter #10
The heart line measurements are in mm
XR80 - 23+3
Cb400f - 22.5+3

Edit: That’s from wrist pin center to top of piston per the spec sheet linked in post #1
 

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Discussion Starter #12
Tires and tubes came in today

IRC motocross gs-45z1 for the front. It seems that’s the only 16” tire on the world.
Kinda Millville 2 for the rear in 90/100-14 flavor



I’m hoping to get some time on lunch Friday to go to the machine shop
 

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Discussion Starter #13
I pulled the front and rear wheels and the engine last night. The piston is pretty well frozen to the cylinder and the dome of the head was garbage. Good thing I have the xr100 head.

It’s soaking in atf and acetone right now. Hopefully I can get it popped loose tonight otherwise my Friday schedule isn’t going to work out.
 

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Discussion Starter #14
I worked on the cylinder for a couple hours last night to no avail. Heated it up with a handheld torch and poured some more atf on top to soak some more.

I did get the xr100 forks swapped over last night. I’m going to keep the 80 swingarm on until I get the engine broke free. I might have to put the bike back together and try to use the drivetrain to break the piston free. Plans extended.
 

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I used a piston from a cb750four, I had my xr80 barrel sleeved with a xr100 sleeve then bored to fit. Hogged out the case to fit the barrel. Search go my post.
 

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Discussion Starter #17
Thanks y’all. Great stuff.

The 80 is still soaking. Gonna give it another shot tomorrow.

I put the 100 engine back together today and kicked it over. It seems to have compression. All 3 of my compression tester adapters were too big to fit the head. I think Sunday I’m going to try and put the 100 engine on and see if it will fire off.

I wasn’t going to, but I stripped the frame down all the way today minus the triple tree and cleaned and scuffed it for paint tomorrow. Might as well since it’s only a few extra things to take off. I still need to get the 100 swingarm pulled from the 100 frame and prepped for paint. Going back silver.


The last thing I did today was fix a bent and cracked frame bar. The rear left bar under the rear fender was bent down about 1.5” and cracked where it meets the frame. Bent it back up, ground it out, and welded.

That’ll do



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Have you tried tapping on the top of the piston with a block of wood while someone holds the engine a few inches off of the ground?

Just an FYI, I've found that Dupli-Color Engine Enamel with ceramic to be a very close match to the Honda silver metallic color. It is Cast Coat Aluminum, #DE1650
 

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Discussion Starter #20
I picked up some duplicator metalcast anodized coating in silver. The rustoleum wheel paint had a shot ton of metal flake. Too much for my liking. So hopefully this will match better. Will post results tomorrow.


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