R90/6 Texas Dirtbike rebuild

[DiabloADV]

So I picked up this dirtbike. Will be doing a frame-off refresh, and change the styling a bit more toward scrambler. Figured I'd start a thread so I can ask all my questions and give progress reports in one spot. The bike starts and I rode it 20 miles home, but it's really rough. Won't idle and the exhaust pressure between the two sides was WAAAAAY different. It needs some love. Previous owner got it running after it was sitting for 5 years, but that's all he did.



Folks will want to see the rear end modifications. Extended swingarm, custom shock mounts and subframe. Cable-activated brake from custom brake lever.






Front is XR600 forks with CR500 hub. Nice fork brace down low.




I pulled the top end today and it's a sticky gooey mess. Wet carbon everywhere. Sealant half clogging up the stud holes. I'll take the heads, cylinders and pistons to somebody that knows more than me to have them assessed for next step. But...good news is no valve recession and the conrods seem perfect.

Anyhoo...more as it happens....
Rick

[DiabloADV]

Both exhaust nuts were frozen in an earlier time. Pushed on the wrench, used heat, etc...until the fins bent. Cut a slot, broke 'em loose and moving on to my spare set.

[DiabloADV]

Some strangeness...

Pistons measure 93.3mm. Cylinders 93.7mm.

Book says R90/6 is 90mm. 100's pistons are 93.96mm (or bigger for overbores...).

So...what the heck do I have here...?

Any help....?

[Beater]

Originally Posted by DiabloADV

Some strangeness...

Pistons measure 93.3mm. Cylinders 93.7mm.

Book says R90/6 is 90mm. 100's pistons are 93.96mm (or bigger for overbores...).

So...what the heck do I have here...?

Any help....?

You had a 0.2mm gap all the way around? That seems high to me ... Both cylinders?

Not to be a jerk ... but are you sure you're reading your caliper right? If it is correct, then it's a WAY overbore on an R90 ... but I'll bet that it's a R100.set though.

Just tryin to help ...

[DiabloADV]

My caliper is accurate to withing +/- 0.1mm. Not good enough for machine work, but good enough to know these aren't stock R90/6 cylinders and pistons. Measuring at the middle ring, I get 93.4 on the piston. The bore is 93.7 in several spots. That's too much gap...

The rings all have end gaps in spec. Seems that if the bore was too big, the end gaps would be wider. Piston sides and skirts show no wear other than very slight marks up/down. The original machining marks are still on the piston sides.

But -- these pistons/bores are too big to be 90/6 overbores, and too small to match any of the 100 specs. I'm gonna have to take them in to a machine shop for proper measurement. The piston/bore gap should be ~0.04, not 0.15.

Off to see Ted with these in hand.

OK, EDIT...

Here's pics of the markings on the cyl and piston:



I scrubbed a bit on the top of the piston...



That would be the spec for an R100 piston. I must not be getting a good measurement. Will have them checked out.

[Beater]

That would be the spec for an R100 piston. I must not be getting a good measurement. Will have them checked out.

There you go ... an R100 set. Yeah, I'd get those calipers checked out ... or better yet, buy a new set ... they can be pretty cheap these days. I actually have my grandfathers set ... it's a 'slide rule' type ... I can get 0.001 accuracy (with enough practice )

Those the iron bores? Might want to think about a set of Nikasil's ... Should be plug and play, and you'll never have this probelm again.

[DiabloADV]

Originally Posted by Beater

Might want to think about a set of Nikasil's ...

Those are cheap, right? Coupla bucks a pair...

[Beater]

Originally Posted by DiabloADV

Those are cheap, right? Coupla bucks a pair...

Something like that

Post something in the Airhead Flea and see what heppens ... I had a real kind inmate send me his after he went big on a Seibenrock setup. You never know what might happen.

This place is cool like that.

[DiabloADV]

Took 'em down to an engine shop for proper measurement. The guy measures at the wide spot of the skirt (perpendicular to the pin). He used mics and then feelers for the final test.

I've got the bare minimum of clearance -- 0.03mm on each side of the skirt.

My end gaps of the rings are at their outside limit, so I'll replace those.

Now...on to the heads!!!

[Kai Ju]

"Measuring at the middle ring, I get 93.4 on the piston. The bore is 93.7 in several spots. "

Measuring at the ring lands is why you got the measurements you did.
If you put a straight edge up against the piston you'll see how far the ring land area is recessed from the rest of the skirt.
Pistons are at their smallest at the ring lands.
Your machinist picked the right spot.

[DiabloADV]

Originally Posted by Kai Ju

Your machinist picked the right spot.

I'm learning new stuff all the time!

[DiabloADV]

It's nice to be expecting the worst and opening it up and finding this. Unretouched!

[DiabloADV]

Getting farther along in the teardown (got sidetracked solving a bad cylinder problem). Anyhoo...here's the head tube setup.

The steer tube is ~25.8mm (+/- 0.2mm) at the upper and lower bearing. Isn't the stock airhead bearing 28mm ID...? My micrometer isn't perfect, but it's not that far off.

Any thoughts on what's been done here?

EDIT: I found the specs for an XR600 bearing -- 26mm, top and bottom. So, that's what I have here. Rather than an airhead steer tube pressed into the triple, I have a stock XR600 triple, with the head tube on my frame having been modified to fit. Good to know, should I ever need to replace the triples or the bearings...

[DiabloADV]

This is the wiring nest. Before I pull this apart, am I correct in observing that this is the regular wiring board, etc., just in this box instead of a headlight shell? If so, I should be able to reconnect it all with a std wiring diagram...?

[DiabloADV]

Got the driveshaft apart. Here's a look at how the lengthening was done. It's about 3".