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Does shorting the rods = flycutting the heads?

Hmmm, it you change to ARP bolts and refit your rods then its likely they will be slightly shorter since some material may come off the big end clamping surfaces (assuming you don't offset drill the small end). At the same time if the heads are shaved an equivalent bit to flatten them then should that compensate for the shorter rods and the clearances should just work out right?

I guess the head shaving will change the relative position of the cam to the crank which may be a problem How much can be shaved before it becomes a problem, i.e. a few thousands, ten thou?

My notes from the class say it has to be within 20 thousands...

Cheers,

Mike
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Old 02-16-2006, 08:29 AM
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0.020" might be ideal, but I suspect many engines have more material removed than that.

E.G.
If crankcase spigots are machined for bigger bore, than the cylinder mounting spigot is also machined. Many shops remachine this area too to ensure parallalism with the crank axis.

Cylinder heights might be machined to verify they're all the same height or if adjusting deck height (distance betw. piston crown and top of cylinder).

Cylinder head mounting surface machined for use with later cylinders and/or to clean up gasket surface and/or raise compression.

Yes, the "cylinder stack" becomes shorter following these operations and affects the distance between cam and crank. This will change the centerline of the cam and seal in the chain box and may also result in excessive chain slack, causing the idler arm and chain path to be too close to the chain box wall.

The solution to re-centering the cam is to machine the chain box mounting surface an amount equivalent to that removed from the cylinder stack. In order to move the chain path further away from the chain box, installing an oversize idler wheel.

Hope this helps,
Sherwood
Old 02-16-2006, 10:03 AM
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The best machine shop IMO (German Precision) does offset bore the small end to make the rods back into the original length. Even if this isn't done you'r talking about a very small change in rod length. I think you'll find in most rebuilds that much more material is taken off of the heads, cylinders, and spigots than the big end of the rod.

-Andy
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Old 02-16-2006, 11:21 AM
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Re: Does shorting the rods = flycutting the heads?

Quote:
Originally posted by Mike Juzenas
Hmmm, it you change to ARP bolts and refit your rods then its likely they will be slightly shorter since some material may come off the big end clamping surfaces (assuming you don't offset drill the small end). At the same time if the heads are shaved an equivalent bit to flatten them then should that compensate for the shorter rods and the clearances should just work out right?
I don't rebuild my own rods but I've never heard of that operation (big end resizing) making rods shorter ('cept on purpose).

My understanding was that when the big end is resized during a rod rebuilding it should end up at the correct length i.e. it won't get shorter. I thought they took a little off the big end mating surfaces and rebored the hole in the right place at the original center. Somebody clue me in please.
-Chris
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Old 02-16-2006, 11:30 AM
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But if you move the cap up on the rod then rebore at the orginal center you would not bore anything out of the rod beam, only the cap?
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Old 02-16-2006, 11:48 AM
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Quote:
Originally posted by cgarr
But if you move the cap up on the rod then rebore at the orginal center you would not bore anything out of the rod beam, only the cap?
Ah, OK, that makes sense.

AFIK, I don't get shorter rods back from the machinist so he must be resizing the big end and then changing the little end to compensate.

I'm a little embarassed to admit, I don't check the length of the rods when I get them back. I don't have a good way to measure them.
-Chris
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Old 02-16-2006, 01:06 PM
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To measure my rods I

a -Measured the inside diameter of the big end
b -Measured the inside diameter of the small end
c - Measured the distance from the bottom of the big end to the bottom of the small end (i.e. rod length)

So the center to center distance for the rod is (c) + ((a + b)/2). On my rods I am about 3 thousands off spec which is close but slightly shortened.

My case has not been machined and my cylinders as well. The heads have about 4 thousands taken off them to flatten them.

I think in my case the piston stroke is shortened by 3 thou and the heads are closer to the pistons by 4 thous so the total clearance of head/piston is 1 thou smaller than before.

Of course this is all on paper and there might be accumulation errors (especially with the 3 measurements for the rods). I will check the clearances between the piston/head using the solder method and see what that is. I think people are aiming for about 60 thou ? there so my lousy 1 thou makes no difference.

Of course this assumes I know how to use my calipers and mirometers properly... ;-)

Cheers,

Mike
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Old 02-16-2006, 02:19 PM
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"I think in my case the piston stroke is shortened by 3 thou...."]

Breathe easy. Your engine's stroke is not determined by the length of the connecting rod but by the crankarm length of the crankshaft (displacement = bore x stroke x no. of cylinders). What you have changed, however, is the rod operating angle, very slightly it would seem, but of little consequence, and the compression ratio, slightly.

Since your deck height is now greater due to the reduced length of the con rod, you can either leave it be or shim/trim the cylinders get to spec. It all depends on the other variables.

Sherwood
Old 02-16-2006, 03:29 PM
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Hi Sherwood,

You are right, the stroke is exactly the same, it was more about the head/piston clearance.

I will determine if I have to shim the cylinders when I measure the clearance between the heads and pistons using the 'ol solder technique...stay posted. The case is going together tomorrow (if I am lucky). I have a old friend coming over to help since it will go must faster and smoother with two sets of hands.

Cheers,

Mike
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Old 02-16-2006, 05:07 PM
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Mike, a little off topic, what did you decide to use for a case sealant?

Cheers
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Old 02-16-2006, 06:04 PM
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Welllllllll, I am still waffling since I have ALL the case sealants mentioned on the list in my shop. I am currently thinking the multi-approach as suggested by Henry, the Threebond stuff for the perimeter and Threebond silicon for the #8 and the old Locktite for the bearing webs. I might use the Curil-T for the #8 instead....

Its hard since its my first engine and I don't want to screw up. What are you planning to do for the next one? BTW I have a brand new tube of the Dow Corning 730 which I might not use.....I was going to go that way but the Threebond stuff has been used in the motorcycle world for a long time and it seems to work great...I got it from a Honda Dealer!

I am terrible with these decisions....

Cheers,

Mike
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Old 02-16-2006, 06:57 PM
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I am in the same boat, used loctite in the past and never had problems. But, after reading stuff here, I wonder if I am flirting with disaster! I am thinking about trying Henry's method as well. But I am tearing down my motor today and it has 240000KMS on it and the loctite never leaked......

I don't think I will use the 730 for case sealing.......

I did how ever have a very very very small leak in one spot on the cam tower on one side that was done with loctite when I did the top end 35000kms ago.......

Hmmmmmmmmmm,

Did High Performance do the rods for you? I have another set to do and just wanted to make sure you were happy with there work.

Cheers
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Old 02-17-2006, 12:32 PM
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Does shorting the rods = flycutting the heads?

Flycutting the heads increases compression ratio.
Shortening the rods decreases compression ratio.
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Old 02-18-2006, 08:42 AM
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Quote:
Originally posted by catca
Did High Performance do the rods for you? I have another set to do and just wanted to make sure you were happy with there work.

Cheers
Yup, Dave did the rods. He sized the big ends and the crank together and got the clearances (from what I can measure and a check with Plastiguage) perfect. The small ends were rebushed and he honed them out to fit the wrist pins and again the clearances are perfect. The only thing he did not do is adjust the small end offset to allow for the machining at the big end..it likely because I forgot to ask. This is why I am asking about the reduced rod length...but it only measures out to a few thousands anyways so I am OK. They are a great shop and I would definitely use them again. Prices are pretty good too!

Cheers,

Mike
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Old 02-18-2006, 08:51 AM
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Quote:
Originally posted by BK911
Does shorting the rods = flycutting the heads?

Flycutting the heads increases compression ratio.
Shortening the rods decreases compression ratio.
Yup, that is what I am thinking, so if you take 3 thou off the rods but take 3 thou off the heads then it should all equal out again...

Cheers,

Mike
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Old 02-18-2006, 08:52 AM
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I think what Mike was asking is if you end up with shortened rods, should you cut the heads to maintain compression?

Cheers
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Old 02-18-2006, 08:55 AM
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Quote:
Originally posted by catca
I think what Mike was asking is if you end up with shortened rods, should you cut the heads to maintain compression?

Cheers
Close....what I was asking is, given my rods are a bit shortened because of the machining on the big end, and my heads were fly cut to flatten them out, the next effect is zero on compression and clearances assuming they were cut the same amount...

Cheers,

Mike
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Old 02-18-2006, 01:57 PM
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