Author Topic: '71 cb500 w/ Art CB77 Pistons (Read 3763 times)

jreich · « **on:** January 18, 2009, 06:26:45 PM »

Just picked up a couple of new cb500 projects over the weekend.

One of them has a set of Art cb77 pistons within.

I was wondering if this is/was a common practice to use these in the 500/550 series?

Here are a few pictures....you can see that this piston was holed.

bwaller · « **Reply #1 on:** January 18, 2009, 07:09:12 PM »

JD, did that engine turn over? Next question is, what did the valves look like?

jreich · « **Reply #2 on:** January 19, 2009, 06:59:19 AM »

Hey there Brent,
Unfortunately, the head was off when I bought it. The cylinder walls look good. I don't have the valves out yet; the seats are pretty carbonized, but I'm not seeing anything else out of the ordinary.
Have a good one!
JD

Quote from: bwaller on January 18, 2009, 07:09:12 PM

JD, did that engine turn over? Next question is, what did the valves look like?

jreich · « **Reply #3 on:** January 20, 2009, 09:41:02 AM »

Still curious if anyone has seen this setup on any other 500's or 550's?
When comparing a stock 500 piston to the cb77, the cb77 obviously is domed, but it looks like it might work..

Anybody tried this route? If a piston fits without topping out on the valves, how much dome is too much on these engines?

Cheers
JD

moham · « **Reply #4 on:** January 20, 2009, 10:56:00 AM »

Whats the bore of those cb77's? Stock 550 is 58.5. I think the 500 was 56-ish. I know awhile back there was much discussion about high CR pistons and what options there were for 500/550s. I don't think those pictured came up.

Old Scrambler · « **Reply #5 on:** January 21, 2009, 07:08:22 PM »

The standard bore and stroke for the CB77 motor was 60mm x 54mm. The stock pistons were rather flat. Many of the ART pistons were made for over-bores and numerous 'high-compression' versions were available. I may have a few sets. For more info look on Yahoo! Groups under honda305

turboguzzi · « **Reply #6 on:** January 22, 2009, 12:11:59 AM »

main issue with this switch is that the included valve angle of the CB77 is MUCH bigger than in the 550.

So with a high comp piston as that shown, the valve pockets might need serious reworking even with a stock cam.

Just as well, the way the piston crowns are shaped, you'll get plenty of compression but no squish, not the best setup.... much more prone to detonation.... maybe that's part of the reason that the piston was holed?

TG

jreich · « **Reply #7 on:** January 22, 2009, 05:17:47 AM »

TG; Thanks for the explanation...I'm not yet that knowledgeable about the effects of crown pitch, height, etc...on compression and detonation, but i bet you are exactly spot on as to why one of those pistons was holed.

Thanks for the info on the piston size Old Scrambler.

Have a good one
JD

turboguzzi · « **Reply #8 on:** January 22, 2009, 12:35:29 PM »

JD

that said, just out of curiosity, it'd be interesting to see how much CR these pistons create in a 550.

you can simply put one back on a rod, put crank on TDC, quickly mount the cyl block and head and measure combustion chamber volume with some thick oil like 90W.

I'd say that for a street bike, you dont want to go higher than 11:1. Wouldn't be surprised if with those pistons CR was in the 12:1 range....

another easy test would be to do the same but with some plasticine blobs at different points on the piston and turning the engine over TDC once, the squashed heights of the blobs will give you a good idea of what's going on inside

TG

bwaller · « **Reply #9 on:** January 22, 2009, 04:32:45 PM »

I know it's tough to tell by a picture but that doesn't look like damage by detonation, is that a crack leading off from the wound?

If you reassemble with clay as TG suggests when you turn it over, don't force it if it gets tight.....That's definitely the best test.

It would be interesting to see that CB77 and a 550 piston side by side on the same pin.

jreich · « **Reply #10 on:** January 22, 2009, 04:59:50 PM »

There appears to me a 1-2 mm crack stemming from each side of the hole. I don't know what a detonation crack looks like, but did clean it up a bit so you could get a better picture, although it was hard to get a good picture of the inside.

I do not have a stock 550 piston handy, but here is a 500 next to one of the 77's still on the rod. Is that what you meant by side by side on the same pin Brent? If not, I'll get another shot

I will most certainly try the clay and let you know what I end up with.

Cheers
JD

moham · « **Reply #11 on:** January 22, 2009, 05:22:05 PM »

He means to put the wrist pin halfway into each piston so they are next to each other on the same pin. From there you can see how the deck height compares.

Like thus:

bwaller · « **Reply #12 on:** January 22, 2009, 05:56:02 PM »

It almost looks like the relief cut is so deep that there is barely any material left.

jreich · « **Reply #13 on:** January 22, 2009, 06:06:33 PM »

Gotcha...thanks for clarifying Moham

I have a couple fuses propping up the 500 piston to level things out.

bwaller · « **Reply #14 on:** January 22, 2009, 06:32:47 PM »

As Turboguzzi said the compression would be way too high with no squish, not the best choice of piston.

JD, does the piston crown seem very thin were the hole is?

Got your pm.

bryanj · « **Reply #15 on:** January 23, 2009, 04:48:26 AM »

I agree, it looks like the valve pocket has had to be machined so much that there was not enough metal left---looks like it got a bit hot and "sagged" till compression blew a hole

jreich · « **Reply #16 on:** January 23, 2009, 06:35:22 AM »

Quote from: bryanj on January 23, 2009, 04:48:26 AM

I agree, it looks like the valve pocket has had to be machined so much that there was not enough metal left---looks like it got a bit hot and "sagged" till compression blew a hole

Yes, it is very thin, and the other side looks to be equally as thin.Wish I knew how many miles the person got out of them first.
Bryan, I think you are spot on...based on what I see, it looks like it did sag a bit and finally give way.

turboguzzi · « **Reply #17 on:** January 23, 2009, 12:27:18 PM »

well, now we all know why nobody runs CB77's pistons in 550's!

Look like the builder actually did the right thing, checked valve piston clerance and machined the pockets accordingly....

And that's with a standard cam? With anything hotter he might have gone through the piston while machining it.

Interesting side note, went to visit today a club memeber who's a well known tuner here, his CB500 four racer bike won third place in the italian championship in 2007 behind a TZ350 and a very special honda 500 twin.

in any case, he lent me a modified XL125 piston like he uses in his racers and it looks like this (next to the modified stock piston I used in the first season). The original XL 125 piston doesn't have a very high crown but creates too high a CR and he told me that he had to remove quite a lot of material from the dome (and machine deeper pockets of course).

you can see from the pic (03.jpg is stock XL) that the he had to machine into the full curved dome a step (04.jpg), he said that otherwise you really risk touching the head.

So basically, know you know, it's tight up there in those 500-550's and before you remove material form the piston, check that you have enough thickness to do it!

TG

moham · « **Reply #18 on:** January 23, 2009, 01:07:47 PM »

TG: what did you modify on those 500 pistons you pictured there?

turboguzzi · « **Reply #19 on:** January 23, 2009, 01:34:55 PM »

did a 1.2mm X 6mm chamfer to match the 15 degs. tapered area in the head. the stock piston is simply flat.

then milled the block 1.2mm and used a thinner copper gasket (-0.4mm) to get a -1.6mm overall height reduction = 12:1 CR

also shortened them noticeably for less weight and less friction.

TG

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Author Topic: '71 cb500 w/ Art CB77 Pistons (Read 3763 times)