Con Rods

[Joe C.]

What are the advantages/disadvantages over cap screw vs. thru bolt design rods? I see an advantage in the cap screw design, as they are lighter by 10 grams. What are the disadvantages, if any? Crower 5.7" rods, with small journals, weigh in at 585 gm for the cap screw vs. 595 gm for the thru bolt. This info is found in Crower's latest online catalog for SP91200B-8 vs SP93200B-8.

Also, is there a necessity to install a torque plate on the decks before honing the bores. I believe that most engine bores are honed in an unstressed state. How much better sealing is obtained by pre stressing the bores before honing? Are we talking deformations around .005" here?

Joe

[Clem Z.]

Re: Con Rods

the cap screw design uses hollow dowels around the cap screw to align and prevent the cap from walking around under load where the thru bolt design is not as positive a alignment device. if you put a torque plate on a cylinder that was not honed with a torque plate and run the hone thru the cylinder you will see where the head bolts pull in the cylinder wall and the hone does not touch these spots.

[Duke W.]

Re: Con Rods

My '95 vintage paper Crower catalog lists "Stroker" Sportmans with cap screws the design of which appears to offer more clearance, which is required in some stroker applications. The standard Sportsmans have through bolts.

I'll have to check the online catalog to see if this has changed.

As far as through bolt versus cap screw is concerned, this could probably be debated endlessly. IMO the through bolt design may be an advantage because the rod doesn't have to deal with thread stress - only clamping force, but many high-strength rods, in fact, probably most, are cap screw type.

I believe when a deck plate is used for boring it remains installed for final honing. The deck plate stresses the block as if the head was installed, but the deformations are probably measured in "tenths" (ten thousanths of an inch), and the bores should be slightly rounder when the heads are installed, which will improve ring sealing.

Installing a deck plate is good practice and doesn't cost much more than no deck plate.

Duke

[Michael H.]

Re: Con Rods

Duke is correct on the deformation of the block deck/upper cylinder caused by the head bolt when torqued. It caves in the tops of the cylinder at each bolt location and it becomes a pentagonal shape instead of round. This dimension can be as much as .004-.005" in some cases and it takes a LOT of run time to seat rings as the ring is trying to break in to two different shapes. If you look at a cyl in an engine with 50,000 miles on it, you can see a ridge in the cyl near each bolt location but no ridge in the span between bolts. The thickness of the ridge tells you how much the cyl has caved in at each location. In my opinion, deck plate boring/honing is a critical part of ANY engine build. And don't waste your time trying to hand hone cylinders. An automatic feed precision feed hone machine is the only correct way to finish a cylinder. People spend thousands of dollars on engine rebuilds for things that just don't matter but cut corners on the cyl wall finish. Use the money for cyl prep instead of things like roller rocker arms.

[Duke W.]

Re: Con Rods

I see that Crower has added a third variation to the Sportsman line - a cap screw type without the machining for extra clearance on stroker applications.

If the price is similar to the through bolt type, I would go with the cap-screw type just because they are lighter.

Don't forget to read the footnote regarding a suffix change for the pressed pin type. A pressed pin is the way to go unless it's a race engine that will be disassembled periodically, and the OE type pistons will require machining for a pin retention device with floating pins, which will add a couple of hundred bucks to the project price.

Duke

[Scott Marzahl]

Re: Con Rods

Duke,
This is the very cap screw rod that was ordered last time around, it was the SP93200PF (pressed fit).

I also ordered a set of the stroker clearanced rods which come with upgraded 7/16" cap screws and they weighed the same amount. There was a difference of 5 grams in the large end between the two versions but total weight was the same.

[Duke W.]

Re: Con Rods

What was price on all three types? I assume there is at least a small difference.

Duke

[Scott Marzahl]

Re: Con Rods

Duke,
The difference between the through bolt and cap screw rod is less than $1.00, however the stroker rods costs about $70 more.

[Duke W.]

Re: Con Rods

Thanks. Assuming the cap screw type (non-stroker) IS actually lighter, that's the way to go.

Duke

[Joe C.]

Re: Con Rods

Duke:

Both available from Summit Racing, $503.99. They will special order small journal rods (not in their catalog) for same price as large journal. Tax applies in CA, OH, NV. $9.75 S&H in continental US.

Joe

[John G.]

Re: Con Rods

Tax applies in CA when buying from Summit Racing?.. .. I wasn't charged tax on my last Summit order - about 3 months ago .. Did the law regarding out-of-state purchases to CA change? ..

[Joe C.]

Correction

State sales tax applies in OH, NV, and GA, not CA.

Joe