ARP main cap studs kit# 203-5403

[AKimball92]

I am in the middle of getting all my parts aligned to reassemble my engine. I recently purchased the APR 203-5403 kit for the main cap studs and waiting for them to arrive in the mail. However, I just remembered and after verifying from my block and the OEM Manual, the engine came with main cap bolts and we all reference this stud kit.

Google is our friend. I found this forum with a quick search.
http://www.chevelles.com/forums/13-...arp-bolts-really-add-bottom-end-strength.html

If you look at how a main stud engages and tightens vs. a bolt the difference is readily apparent. As Carl said, bolts tighten via course threads into the block, you have torque on the bolt head as well as the threads while tightening. Studs thread into the block, then when tightening, threads in the block are completely engaged, the only friction is the small surface area under the head. You are pulling machined metal directly into machined metal via fine thread (more threads per inch). Just no way to compare the two.

What are your thoughts on this issue for the Fiat SOHC? I do agree that replacing the original bolts is important but uncertain on new studs vs new bolts.

 

[Huey]

Opinions will vary on this topic but I'll give you mine. Studs are certainly better and are desired on race cars or other high performance cars where the head may be removed more often. For average daily drivers and occasional-use cars, bolts are fine. I ordered a new bolt kit from Bayless and it will be good for many years.

So my opinion is unless you are building an engine where you expect frequent removal of the head, you can save some money and stick with bolts.

 

[AKimball92]

Opinions will vary on this topic but I'll give you mine. Studs are certainly better and are desired on race cars or other high performance cars where the head may be removed more often. For average daily drivers and occasional-use cars, bolts are fine. I ordered a new bolt kit from Bayless and it will be good for many years.

So my opinion is unless you are building an engine where you expect frequent removal of the head, you can save some money and stick with bolts.

Huey, just to clarify. this is concerning the main cap fasteners not the head fasteners. I don't believe any of us want to make opening up the bottom end a regular thing. I know I want to make the lower half a one and done build for sure.

 

[Huey]

Oh wow, I completely misread that . Sorry about that.

 

[AKimball92]

Oh wow, I completely misread that . Sorry about that.

No problem. Its good to know that studs on the head makes removal and assembly easier. On a side note, do you do studs on both the front and back (right and left) side of the head or studs and bolts as it came from factory? My current build plans do not include frequent head removal but if I ever plan in the future to do frequent track days that would change.

 

[EricH]

No problem. Its good to know that studs on the head makes removal and assembly easier.

We're off on a tangent here, but it's an interesting one.

Studs make things easier if the head is going to come on and off with any frequency, but they make things much worse if the head is going to stay on for years at a time. Given time, galvanic welding between aluminum head and steel studs can seriously lock the head on.... There's a reason why the head pulling tool is designed for motors with studs.

 

[Simon Oaten]

probably more important to chk the holes are round with the different clamp load .....................

rgds
simon

 

[AKimball92]

What torque do you use for the main cap studs? The OEM manual calls for 58 Ft-lbs or 80 Nm. I know the studs can and are designed to handle more. With this new torque would any of you feel comfortable with using plastigage to test roundness or is a more precise measurement a must? Who else has gone though the process of converting from main cap bolts to studs?

Someone might really object to me once again taking the block out for machining. This build is wracking up a small fortune in receipts and someone in the house hold is not too happy.

 

[fiatfactory]

I like using main studs...

But there is a correct way to install them, and if you want to get pedantic (which i most definitely am) there are a few checks to do.

Installation... read this section of 'ultimate SOHC' and scroll down to the section on stud installation... it's specifically for the head studs, but pretty much the same installation method is used for the main cap studs. I like to 'set' studs into the block semi-permanently, and this method ensures the are perpendicular. Following ARP's recommendations to reduce issues due to 'torque scatter' is time well spent.

Once the studs are installed and 'set' correctly, there are a couple more checks... platigauge doesn't really cut it for these as that will only measure overall clearance between the bearing shell and the crank journal... what we want to measure for is any distortion or out of round in the bearing bore. To do this properly you need precision measuring equipment... an internal bore gauge and a good micrometer (50 - 75mm) are needed and after the studs are installed and lubed and torqued to spec (without the crank or bearings installed) you need to carefully measure the bores at 12/6 o clock, 10/4 o clock and 2 / 8 o clock and determine if there is any out of round or distortion to the main bearing housing bores .... if there is, then yes, it's more machining to align hone the bearing journals back to size and round and alignment...

Just as a side note when you plasti-gauge the crank to bearing clearances, measure at 12 / 6 o clock... as the bearing clearance at the 'parting line' is usually slightly greater...just something to look out for.

SteveC

 

[Rupunzell]

Re-using the old threaded hardware during a engine rebuild is not wise as threaded hardware and small hardware in general is one of the lowest cost items for any performance engine. Those ARP main studs (Originally for Toyota 4AGE, suggested by Chris B at ARP years ago when we discussed ARP engine hardware for the Fiat SOHC) are the way to go as the OEM Fiat main cap screws (screws go into a threaded hole, bolts have nuts on the threaded end). Either follow Steve's instructions in the "ultimate SOHC" write up or follow the ARP instructions. As for torque, follow the recommended ARP chart using their lubricant as lubrications WILL affect clamp load produced by set torque values.

*Avoid clicker type torque wrenches for engine building as they can go out of calibration easy, and difficult to control. The ideal torque wrench to use for precision engine work is a precision flat beam.

The OEM Fiat main cap screws are special. They are M10x1.25 threaded, grade 10.9. Under the hex head is a small relief between the shank-grip length of the screw and hex head. This was done to effectively compress and act as a spring once loaded. The idea is similar to a stove head bolt-screw where there have been slots put across the hex head. This allows the hex head to flex under load effectively resulting in a over-sized conical spring. In the case of the Fiat OEM main cap screws, that relief under head and the pre-load they have been installed to are reasons why they should not be re-used.

Studs come into their own as the length goes up. Long cylinder studs can be found on air cooled VW and Porsche engines used to retain the cylinders to block. While long studs like this is one way to deliver clamp load and fastening in the case of VW & Porsche, they have been loads of problems for both due to long stud length (metal fatigue, dimensional changes with temperature even when alloys used in the studs have been temperature compensated. twisting as they are torqued and more).


Bernice