Lets play piston identification this time!

[SamH]

Ok guys, you were on the money with the camshaft id, now, what are you like with pistons?

I mentioned in my other post that there were a bunch of pistons with this bike. I thought 4 were new and 2 were original but now I've turned my attention to them I think 2 are old, 2 are used but in good condition and 2 are new.

The old one's I'm not worried about, they can stay where they are. The two new ones I think might be standard 9.0 pistons and the ones that look like they were in use when the bike came apart... I have no idea. They look odd.

Pics!

Stock R90 9.0:1 piston? I don't know but it's got 9.0 in red ink on the top.


The odd ball pistons.




They look like the valve pockets have been machined a long way down from stock. Do they look familiar to anyone?

[SamH]

Just checked, theses barrels are also over size. The valves are 42mm and 40mm. *looks on interweb* So the valves are standard.

[Garnet]

"They look like the valve pockets have been machined a long way down from stock."

The 336 cam requires valve pocket modifications. Perhaps they are R90S pistons that have been modified. The first pics look like 9.0/1.

When Matt was planing his new engine we where looking for pics of R90S pistons vs R90 pistons. At the time I couldn't find any.

[mattcfish]

I'd say 90S pistons machines for a race cam. The dome is way more abrupt than a 90/6. I've owned both kinds, side by side the dome and deeper cut pockets are quite noticeable. When I machined the pistons for my 336 installation I didn't need to cut nearly that deep or wide. Maybe a full race cam and big valve heads were once used. They'll definitely give a good safety margin on the 336. Never seen them machined with the dimple in the center like that. Probably an attempt to preserve compression.

[SamH]

Yeah, the 336 info sheet suggests milling 0.4mm out. There's a lot more than 0.4 out of these pistons!

[George Ryals]

The squish band on the cut pistons look too wide to be stock R90 pistons. Lets see a picture with the two pistons
Side by side with each on the end of the same wrist pin.

[mattcfish]

The squish band on the cut pistons look too wide to be stock R90 pistons. Lets see a picture with the two pistons
Side by side with each on the end of the same wrist pin.

I agree, those pistons were customized for something specific. They may actually yield lower compression on stock heads.
Do you have a pair of custom heads to go with them?

[SamH]

Custom heads? I couldn't say. If someone's got a stock head that they can measure then I could check. I put the two pistons onto a single pin and yep, sure enough, the '90s' pistons are about 2mm lower at the edge.
I put a barrel and piston on and moved the crank to TDC, this had the squish band (is that the term?) sitting pretty much flush with the top of the barrel. The stock pistons would sit proud of the barrel. I don't know if that's normal or not.

[George Ryals]

I would also recommend measuring the length of the cylinders. It's not unheard of for a hot rodder to shorten cylinders to gain compression, which is something desired when running a 336 cam.

[mattcfish]

Custom heads? I couldn't say. If someone's got a stock head that they can measure then I could check. I put the two pistons onto a single pin and yep, sure enough, the '90s' pistons are about 2mm lower at the edge.
I put a barrel and piston on and moved the crank to TDC, this had the squish band (is that the term?) sitting pretty much flush with the top of the barrel. The stock pistons would sit proud of the barrel. I don't know if that's normal or not.

George has the answer. The barrels have been shortened to increase compression. the squish band was modified to keep the piston from hitting the head. Stock pistons should be close to flush, not "proud". That also explains the need for extra deep pockets. I wonder what compression that baby is set up for? Don't run stock pistons with those shortened barrels unless you have adequately shimmed the base.