Front cylinder low compression + blow-by in rocker cover + pushrod tubes

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Ratedspeed
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Re: Front cylinder low compression + blow-by in rocker cover + pushrod tubes

#91

Post by Ratedspeed »

awander wrote: Tue Dec 27, 2022 12:13 am Rod length has nothing to do with stroke length, guys.
Just to clarify,
Installing a shorter rod will not change the effective stroke, but it will change where the stroke takes place within the cylinder.
Installing a later 7.440" rod with a OE style piston will lower the effective stroke .030" in the cylinder which in turn moves the piston crown .030" below the squish band resulting in lower compression, which was one of the OP's original concerns.
Conversely, installing a 8.500" rod wouldn't change the effective stroke either, but the sound you'd hear at TDC would. :D

Mike
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Re: Front cylinder low compression + blow-by in rocker cover + pushrod tubes

#92

Post by panama86 »

Well I am back with an update.

I spoke to the bloke who built this motor originally and mentioned the endplay I had on my flywheel assembly as well as my valve timing issues. He instructed me to pull the motor down and bring in just the cases with rods sticking out and he would split them and take a look. Turns my right side case bearing race was chewed up on the face. That would explain both the metal shavings I had found in my oil filter as well as that end play on the assembly. He sorted it out, added new thrust washers and put it back together for me. He was unsure of what may have caused it as he said everything was fine with the assembly. We discussed the valve timing some more and he said he couldn't see any issues with the crank side of things. He said only thing could be cam timing or maybe the fact that the H grind came had different valve timing for front cylinder. I had a sifton 412 so I've put that in.

Yesterday I reassembled everything, including new rings. While setting my timing I was rolling the motor over by hand with my thumb over front cylinder waiting for compression. There was barely any, rear had plenty. Obviously with new rings this could be partially attributed to them needing to seat. I decided to check my valve timing again after pushrod adjustment to see where they were at TDC on compression. Same issue. Maybe even more so. Front cylinder only starts building compression right at the end of compression stroke. Rear starts a lot earlier. I am losing my mind with this thing to the point where I am considering selling the motor off as parts. I suspect this has been an issue with this motor since day one as I have always had issues with the front cylinder, both in compression and carbon build up.

Is there any thing else that I could be missing here. Everything is timed correctly in the cam chest. Only things I can think of are potentially are that my pinion gear may be manufactured incorrectly and the timing mark is not where it should be throwing the whole thing out of whack. I've compared the gear with images of other stock gears though and it seems to be setup correctly. I considered maybe an issue with my lifters/pushrods but they are in good shape and logically I can't see how they could cause this as long as they are adjusted correctly. I have the solid alloy lifters with the longer of the 4 on the front exhaust. I also cannot think of any way that the heads could be causing this, I did have the heads rebuilt not long ago with new guides, springs, valves etc. Is there any way that the heads could be causing this issue?

I'm at a total loss.
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Re: Front cylinder low compression + blow-by in rocker cover + pushrod tubes

#93

Post by Ratedspeed »

After you installed the cylinders did you happen to rotate the motor over and check the piston height of both at TDC and did the pistons look identical when they were separated from the rods ?

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Re: Front cylinder low compression + blow-by in rocker cover + pushrod tubes

#94

Post by panama86 »

I didn't because when I pulled it down to take it out to have the bottom end done I had only recently replace head gaskets. I pulled the cylinders and heads as one and re-installed them as one. Both cylinders still had the crosshatch from previous hone visible as I'd done the top end not long ago. The pistons were the same, I purchased and installed them myself last time. They were a set of v-twin cast pistons.
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Re: Front cylinder low compression + blow-by in rocker cover + pushrod tubes

#95

Post by RooDog »

Just a thought:
Are the rods a matched pair. Early, 1972 & older, and later rods vary in length by about thirty thou inches. Would this affect compression between the two cylinders if one rod was shorter than the other?
In theory, yes; In the real world, I don't know....
....RooDog...
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