low compression (new top end)

79 wr 250
Any idea how far down in the cyl. your piston should be at tdc ?
This one is at least a freakin half inch, what gives? is this why it only has 45 psi. of compression?

has to be a wrong part somewhere.

The only different 250's I can think of is the 250mag engine and the 250 standard engine(later). These have two different heights on the cylinder. I believe the 250mag engine has a shorter connecting rod.

The top of the piston should go low enough to let the side transfer ports fully open.
At the bottom it's BDC. Bottom dead center lowest point. The tdc means top dead center, the highest point of the stroke.

would have thought wrong piston or rod would have made the piston hit the crank . maybe its a bastard child .
i would think piston crank rod must familiar but not to the barrel ?

need more info, make and part # of piston.

Yes, I'm beginning to think a have a bastard child on my hands, this motor has been one thing after another!
Here is the piston info ,
Wossner
8004db
hva 250 74-84
pin:wp033-
clearance: 0.06mm

piston should be ok, i have installed that one before. whats the cylinder look like?

Here are the pics, never mind the water on the top . I just did a leak down test before I tore it down ( it passed the leak down test too)

this was at tdc... I guess I will go and reassemble again for kicks and giggles . You can also see where it was higher at one time in the cyl. if you look closely (residue stain at 8 o clock position.

maybe a screw up with wossner? does the cylinder look right for 79?

Yup it is a newer cyl. bought it off a guy on here and it measures the same height as original one. piston measures roughly 2 and 3/4 high. I don't have the orig. piston to go by would you happen to know if this is the correct height for the piston?

hmm, i have an 84 wr piston in the garage i could measure.

That would be great, thanks. I am trying to measuring my ring gap now and hope fully it will be ok.

i do not have the bike anymore (traded for my 95). but in that bike i installed a wossner..after assembling it showed 160psi on my gauge. but i do have the original mahle out of it. it wont be till later today but i can measure it. strangely i bought another 84 wr because i liked the old one so much.

Take that piston off the rod, find your old piston, slide the wrist pin in halfway to connect both pistons together, compare compression height. Sorry just now read your previous post that you don't have your old piston. I went out to garage and found a original mahle piston from my 81 250cr, MY digital calipers are at work, but I measered with metric ruller from the top of the wrist pin hole to the top of the dome was roughly 25mm or 1", hope this helps.

Ok , thanks mine measures 27 mm kinda hard to measure exact (still in bike) I would venture to say its the right piston.

So I took the piston off and measured the rod as best as I could while still in the motor and came up with a length from eye to eye of 4 and 7/8" which converts to 124mm.
Can any one confirm this rod length or what is the correct rod length to a 79 250 (or where could I find this info)
Thanks

I found my old 74 250 mag crank with worn out rod bearing and came up with 125mm center to center.

i measured my old mahle 84wr piston. these are a bit rough due to the dome, but should be within a mm. 73mm total piston height, 35mm from the dome to the center of wristpin.

Ok Justin, mine overall is 71.60 and from the dome to center of pin is 34.92mm so piston looks right . I found out through a reliable source that earlier 250 models came with the shorter rods being 120mm and 125mm in length.
So it looks like some one was needing parts at one time and thought they could mix match some pieces and parts. When I measured the wet side of the crank for a new seal it was 28mm I thought it was to be 30mm , and I found out the earlier model had a 28mm seal.
My best calculation is that someone swapped in a different crank and rod at one time but they never changed the clutch side crank seal because the old one I took out was a 30mm and I so pose thats why it smoked like a frieght train burning gear oil !!! I have no idea if it even ran because I don't have the old piston any more ( I have my doubts )

Thanks for all your help guys...
I'll keep you updated.
Bary.