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Final sanity check: engine combo
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Sigurd
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PostPosted: Mon Nov 30, 2009 2:13 pm    Post subject: Final sanity check: engine combo Reply with quote

Alright, I'm getting a few final details ironed out. My last issue is, how much stroke variation is allowable on a given motor? I have verified that two of my crank throws are 0.007" longer than the other two. This amounts to a 0.2 CR point difference, so two cylinders will be at 8.9 and two will be at 9.1 for an average of 9. I would have liked it to be closer, but that's what you get with a 4340 crank now. How am I doing?
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modok
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PostPosted: Mon Nov 30, 2009 7:05 pm    Post subject: Reply with quote

Well, to make the best of it, which two are longer?
There are things that can be done.
That is close enough if you ask me though.
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Sigurd
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PostPosted: Mon Nov 30, 2009 7:35 pm    Post subject: Reply with quote

It's not two on the same side, if that's what you're asking. I think it's 1 and 4 that are longer. My notes are down in the shop right now.
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DarthWeber
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PostPosted: Mon Nov 30, 2009 7:57 pm    Post subject: Re: Final sanity check: engine combo Reply with quote

Sigurd wrote:
I would have liked it to be closer, but that's what you get with a 4340 crank now. How am I doing?


With all due respect, that's why you should have paid a little more and got a DPR crank. Jose would give you a crank that's perfect in all respects.
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stoneloco808
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PostPosted: Mon Nov 30, 2009 8:26 pm    Post subject: Reply with quote

Well IMO if its not going in a race car then I wouldn't really stress on it. But for me, knowing such details tend to irritate me later knowing there are ways to correct it. I would end up just trimming the tops of the 2 pistons.
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modok
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PostPosted: Mon Nov 30, 2009 8:34 pm    Post subject: Reply with quote

Well if that is the case it may be the crank.
They screwed it up when they ground it.
Call china to complain.
Then run it.
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Sigurd
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PostPosted: Mon Nov 30, 2009 9:41 pm    Post subject: Reply with quote

I'm absolutely certain it is the crank. We threw it in the lathe and miked it. I've swapped pistons, cylinders, and rods and I get the same numbers. Also, the tops of the cylinders are even right now, so I can't cut them. NSRacing decked my case offset to correct the offset crank main bore. It definitely is the crank. Funny thing, I was just on DPR's website today looking at cranks. Rolling Eyes I can't do it. I already have this thing balanced and everything. I just wanted some thought on if it's too far out for a street car.
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stoneloco808
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PostPosted: Mon Nov 30, 2009 10:09 pm    Post subject: Reply with quote

Like I mentioned in an earlier post, just run it as is. If it bothers you enough, trim down the 2 pistons with the longer throw, then rebalance. This is a perfect example of why I like to balance everything once engine mock-up is satisfactory. I have ran into these problems before, but left it up to the customer to decide what to do, since the parts will be sent to the machine shop for balancing anyway.
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Sigurd
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PostPosted: Tue Dec 01, 2009 6:50 am    Post subject: Reply with quote

Yeah, well, the things you learn. Rolling Eyes I assumed it was bad decking, and proceeded on assembly. Welllll, the thing got back from decking and I still got the same numbers, albeit slightly more consistant. But now my pistons will run parallel to the cylinders. Maybe I'll just run it.
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jfats808
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PostPosted: Tue Dec 01, 2009 7:01 am    Post subject: Reply with quote

Sig,
I had the same problem but it was my 2 of my pistons that were off. One was .003 the other was .005 from the other two that were equal. Sent em in to the machinist with exact measurements. I get em back and he shaved all 4... all equal. He later told me that that little difference isnt going to make a difference especially on a street motor. 8 months now the engine is running fine and dandy. To this day when i checked deck height, reading your post... i dont know if it was actually the crank ( scat 82) or the pistons. I guess as long as your cr readings check out near to what you want and the clay clears the valves, your in the clear. Regarding to the specifics though, i feel your pain. Been there.-Jon
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Sigurd
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PostPosted: Tue Dec 01, 2009 7:56 am    Post subject: Reply with quote

Thanks. Even if CR had come up short, I probably would have been happy. But I have been chasing funky deck number gremlins for awhile now. I would have been more happy if the numbers were all equal, but CR was lower than I wanted.
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evolks13
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PostPosted: Tue Dec 01, 2009 11:27 am    Post subject: Reply with quote

Close to a year ago I built a 76x87 1809cc for a desert race car. I had 1 & 4 that were .005" longer and was looking at options. I spoke with DPR and from what I remember, I was told that .005" is splitting hairs and he couldn't guarantee a crank to have zero variation without a lot of extra time put into it. Then I spoke to a couple machinists who also shared the same opinion. Think about it - Look at a .007" feeler gauge, it's not as dramatic as the math is when you calculate the CR's. I ended up shaving the tops of 2 pistons for peace of mind. Think about this... are all 4 of your cylinders flowing the same amount of air/fuel with no variation? That would have more affect on the engine than .007".
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Sigurd
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PostPosted: Tue Dec 01, 2009 12:18 pm    Post subject: Reply with quote

I just got off the phone with Jose, and actually placed an order. Then I called him back and asked him about this very issue. He too thought I was splitting hairs.

"Do you think this is a problem?"
"No, man. You trying to kill mosquitos with a cannon!"
"Thanks Jose. Can I cancel this? I'm sorry."
"Yeah man, you can cancel. You don't need any of this stuff!"

And that was that. I felt bad for ordering and cancelling within 10 minutes, but he laughed it off and said that balancing and making sure the oil holes on the bearings and crank lined up were far more important.

So I'm gonna run it. Twisted Evil
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