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brantc

Piston mix and match.

8 posts in this topic

If this question has been asked, please point me there.

I have a 87 XL600R that I'm doing stuff to.

I want to up the compression, but not to the 11:1 Wiesco because then I "may" have detonation problems.

So the option is to use a piston from some other thumper that will fit.

So, what is important in choosing a piston?

I would think wrist pin to crown length.

Diameter at the skirt.

And total length.

Is this correct?

The goal is to obtain a 10:1 compression ratio for my XL with the current being 9:1.:thumbsup:

Piston diameter OD at skirt 99.95-99.98 mm(3.3937 -3.9375)

Cylinder ID 100.00-100.01 mm

Any ideas??? If you know of a company that makes a piston, please let me know.

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I am fairly sure Weisco makes a 10:1 piston. Did they say that they do not make that piston?

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Looking at the piston compression question, I thought I posed a question on the type of piston I found in my bike when I tore it apart so after a quick search this is what I found. I am running an 11:1 piston and have to use premium pump gas because I start to get some "pinging" during the summer months when using regular fuel. If you are OK with using premium fuel, then the 11:1 will be fine.

http://www.thumpertalk.com/forum/showthread.php?t=474655

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I am fairly sure Weisco makes a 10:1 piston. Did they say that they do not make that piston?

They only make an 11:1 for the XL600R. I called them.

I emailed their engineers and I talked to a physicist friend of mine.

The stock piston has a concave shape on the crown. It is about 30 thou deep.

My friend said just cut the piston down half as much as the dish on the stock piston. DUH>>>>>>>:confused::ride: :ride: What an easy fix...:thumbsup: :thumbsup:

Then I talked to Wiseco and they said that the 11:1 piston has a crown thick enough to cut it down 20 thou(maybe more) without loss of strength.

The thing that I have to watch out for is, when you cut the top of a piston, the skirts may "spring" outward but it probably wont be a problem for me.

So thats what I'm going to do. get the 11:1 and cut it down to 10:1 so that I raise my stock compression by 1 point.

:D

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Another thing you can do is add another base gasket to achieve the 20 to 30 th. A perfectly built engine will have a zero deck height have you checked that yet. I don't like my pistons sitting in the bore more then .040th.

So if your at zero the added base gasket will work just fine.

If your a picky builder you will get a custom piston made .

Here's a calculator to determine compression.http://www.fordmuscle.com/calculators/compression.shtml

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The other option is to use the 11:1 wiseco with a thicker head gasket (cometic make custom thicknesses), or use an XR650L head which has lower compression.

The thicker head (or base) gasket will also affect the valve timing (advance it). This should give slightly better midrange power.

HTH

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Zero deck ht. will have less chance of detonation so a thicker head gasket would be great if you can get one. That's why it's better to be at zero or no more then .040 in the hole. Check your deck ht. like the calculator says an see where your at first.

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Did you check your deck ht?

How many c.c's is your combustion chamber?

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