Engine identification


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Today I made a tread about what size rear sprocket to use for my bike. The problem is that my engine isn't a stock xs650 anymore

A little info:
I got the bike in pieces from someone who sadly passed away. Only the engine was fully rebuild and tuned. The problem is that I don't really know what he exactly did to the engine, these details died with him. Maybe it's kind of weird but I don't want to take the engine apart since I want to respect his efforts and it did run great in the engine stand.

He also stroked the engine with spacer plates, and i hope you guys could help me identify what he modifications he possibly has made to the engine. Maybe it's to much to ask, we will see.

Picture of spacer plates:

With gaskets the plate is 8mm thick, I guess thats 5/16 of an inch.

I'm really curious how much cc the engine is now and what it does to performance. And what will this mean to my rear sprocket.

Hope you guys here can help me out.
the clutch has also been changed to a pull mechanism:

the cylinder with harley wiring:

it also has a large oil cooler system at the front of the frame
Hi BJ,
without tearing the engine down and looking the best I can suggest is:-
Check for engine re-phasing by poking a thin wooden rod down each spark plug hole to see if the pistons rise and fall together.
Check for engine capacity by breaking the ceramic out of an old spark plug to make an adapter to connect to a hose, pour a cylinder full of engine oil at bottom dead centre, rotate the engine 180º and measure how much oil comes out.
Do a compression test to see if it's got decent compression.
And WTF with those spacers?
Longer rods because the engine has been over-stroked or that's all the builder had available?
Compression ratio dropped to about 4 to 1 so it'd run on kerosene?
Highly unlikely, same as a "big bang" conversion, though I've heard of someone doing that.
Best you can do on determining the power is to run the engine on a dyno.
Looking at the engine would say it was from a sidecar motocross outfit, pull clutches are very common on them, if you are in europe it's probably taken out to either 84 or 86mm may even be 88mm as it has 88 stamped on the side, and stroked taking it up to around the 1000cc mark maybe more.
Rear sprocket would depend on if it has had the primary gears altered, and what you are using it for, plus what size front sprocket you have bearing in mind this isn't a standard engine, I run 18/33 on mine which is 750 a friend with an 840 runs 18/32
I did the test with the wooden rods, it seems that they rise and fall together.. (little dissapointed right now:p haha)

i could identify the pistons partnumber by shining a light in the chamber; 4045p8 XC. It turns up to be a Wiseco piston, so i called them and they gave me this:

bore: 87
height: 73

They also said it belongs to a XT500, which has a round piston deck of 6,5. But looking at the bore it seems that the bike is atleast 860cc, is that correct?

I will do the compression test later on today when a friend will return my compression gauge

I found this engine on the forum, and it has almost the same setup as I have such as two 40 dellorto's so I think the engine is based on sidecar cross:
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