Piston difference

[tarzan]

I honed my cylinders today and am preparing to assemble the top end.
I bought a new piston and in comparing the new to old there are a couple differences.The outside dimensions are the same,however the new one doesn't have a relief that is the same as the new.My concern is that the weight may not be the same but also will I need to machine the same indent in the new one?
BTW,the reason I am replacing the old one is it appears that a bolt or something was once caught in that cylinder and beat up the top of the piston.
So I would like some feedback on this and maybe hear what you guys would do.
I would use the old piston,it looks like it is still solid,but I tried to file out some of the little divots and they are too deep to get out.And I am worried about building carbon in them and possibly hot spots.Does this sound reasonable.
Here are a couple pics.



Thanks.Rick

[XSokieSPECIAL]

Rick,

I've never had one of these things apart so I am far from an expert.

However, I know that the 80 and newer 1100 has a larger intake valve. looking at the pic it seems the larger relief is at the rear of the piston, that would be the intake side.

It looks like you have a later engine in there. That piston will most likely interfere with the intake valve.

IMHO Greg

[Guy_b_g]

The new piston will also have lower compression if it is from an older ('78 '79) bike.

[XSokieSPECIAL]

LOL Guy,

I've got the same picture you have on your avitar, hanging on my toolbox at work.

[Guy_b_g]

Cool I love that pic.

[trbig]

The replacement piston does look to me to be from a later model, though it's kind of hard to tell. I have never seen an original piston that looks like yours with the valve relief coming all the way out to the edge from one of these motors. One quick check you can do, is put both pistons onto one piston pin, and check the crown. There's a pretty noticeable difference between the early pistons and the late. Here, I have the two different ones with the early model being on the right.





If your new piston has the same crown, I'd use it. Just make sure you mount it so the arrow on top is facing the front (Exhaust). You'd like to see everything the same, but if you had the money to spend, you would have done a complete rebuild with 4 new.. I assume.


Tod

[tarzan]

Yea I was kind of figuring the new one is for a later model.I didn't pay attention to the dome.I was thinking the higher compression was only from the head side.
But now remember reading about the pistons being different.
I will check the two domes this afternoon after work.
I doubt this piston will work now.Bummer,oh well I got it on ebay from meriamxs.And the price was pretty cheap so I should be able to sell it and recoup my money.
It looks like I will have to just knock the sharp edges off the divots and run the old piston.
BTW-my engine is the 78E

[tarzan]

I just went and checked the ebay listing from the piston I bought and it specifically says the piston is for a 78-79 XS1100.Not the later model.
I know meriamxs is Bob Jones old business,so they should know.
So now I have hope,I will still check the domes tonight.

[dbeardslee]

The new piston will also have lower compression if it is from an older ('78 '79) bike.

Not to nitpick, but the 78/79's had a 9.2:1 compression ratio, while the 80/81's had a 9:1 ratio. Wouldn't that suggest that an older piston would have a higher compression ratio?

[tarzan]

Not to nitpick, but the 78/79's had a 9.2:1 compression ratio, while the 80/81's had a 9:1 ratio. Wouldn't that suggest that an older piston would have a higher compression ratio?

Actually looking at TRGBIG's photo the early piston itself would give lower compression due to the lower dome.But the quench area in the head is probably smaller on the early models.

[Big Daddy Funk]

Looks strange to me

Maybe one of the guru's might have seen that valve relief cut down into the side of the old piston like that, but it looks unusual to me. Are the rest of the pistons cut that way? Rick

[trbig]

The early model pistons were lower domed but a smaller head space. Later heads had the larger valves and the head space was opened up larger to accomodate this. They made the pistons higher domed to try to gain back some of the compression lost, but didn't quite get it all back.

The Wiseco piston's dome in the 1179 and 1196 rebuild kits are in between these two pistons. This results in a better compression GAIN for the early model engines/heads vs. later model rebuilds.

I have both sets of pistons... one set from a 78 and several sets from the later models, mostly XJ. As stated, I've never seen a piston valve relief that looks like that. Could it be possible that there is a '79 specific piston?


Tod

[DGXSER]

Interesting, the Microfiche shows the same piston on 78 and 79 models. However, every year has a different part number for the different cylinders, as if piston for 1 or 2 or 3 or 4 is different than the others.

[oseaghdha]

That is a really odd relief.

It could be to unshroud the intake valve right at opening, but it's not for clearance.

[tarzan]

Heres the two side by side,on the same pin


They look the same dome wise to me.That weird relief is the only thing that puzzles me now.I will see if I can borrow a scale and weigh the two.If they are close in weight I will use the new one.