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Old 06-26-2015, 06:15 PM   #1
Desiderata OP
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Joined: Jan 2015
Oddometer: 14
'79 RD400 Daytona top end questions???

I have a '79 Yamaha Daytona RD400 with only 4000 miles on it. I bought it in a largely disassembled state though. The motor is in very good shape, except it appears as if the PO bored it once over and replaced the pistons. I took the cylinders off and inspected them. Left piston has some heavy scoring, right piston seems basically ok. Left cylinder has some aluminum metal transfer, but otherwise seems ok. Right seems ok but with slight dark bands running lengthwise (see pics). Here's what I measured:

Left piston (heavily scored): 2.534" or 64.36mm (at skirt)
Right piston (moderate scuffing): 2.537" or 64.44mm (at skirt)

The bores on both jugs are right at about 2.538" dia or 64.47mm

As you can see, the clearance between the right piston especially and jugs is very tight. And sure enough, I tried sliding the pistons in and out of the bores and they were tight, with almost no discernible free play (without the rings on them). I actually had a mildly hard time getting them to slide back out once pushed into the bores. So I think I found the cause of the seizure - the bores are too small for these 0.50 over pistons. Does anyone know what the clearance should be for a stock cast piston and the bore? I ran the part number I saw on the underside of the pistons and they appear to be RZ350 pistons (from what google says).

I'm hoping I can open these bores up a thousandth or two with a honing stone and not have to rebore. I think I'll keep the better piston and probably replace the scored piston and maybe the rings depending on how they gap after the hone.

My question now is what to do? I'm pretty sure this bike was bored at only 4000 miles and then not run long at all before being seized. So I'm wondering if I might get away from a hone on the cylinders and maybe replace one or probably both rings. No real experience judging wear/damage though, other than common sense assessment. Would really like to get the opinion of some experienced builders/tuners. Not looking to build a superbike. Just a good clean running stockish bike. Maybe slap the Moto Carrera chambers I have on it along with a set of rejetted 76-78 RD400 carbs.

Any and all help appreciated...









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Old 06-27-2015, 01:35 AM   #2
stainlesscycle
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cast clearance on 64mm is .0016" to .0018"

you might be able to hone that, but probably not that bad one.

if you go to forged pistons, you get a bigger gap, which will leave some room for whoever bores it to take some material off. but there's probably not a .030" forged piston made anyways.

either way no one is really gonna want to bore .001" - .002" off of the original cylinders, that means boring bar is only taking .0005" - .001" off.. so you probably can't get away with .020" forged in there and have more bores left.


do you have a local cylinder guy? if so drag the parts there and see what they recommend. you're most likely going to 3rd over (if there are 3rd over cast oem available anymore?) otherwise you're probably stuck with going to either .040" forged or cast.


if it was mine, i'd just go to the 3rd cast (if i was concerned about getting an extra bore out of it) or 4th forged....
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Old 06-27-2015, 07:03 PM   #3
ericm
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Yamaha used to make .75mm over RZ350 pistons. I'd prefer cast Yamaha pistons to forged- they cast them from high silicon alloy which does not expand or soften as much under heat as the alloys used for forged pistons.

I was never into RDs so I don't know what's different between RZ and RD 400 pistons, if anything
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