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post #1 of 38 (permalink) Old 06-17-04, 01:03 AM Thread Starter
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302 rods in 351

Whats the deal with using 302 rods in a 351, whats the advantages of this combo?

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post #2 of 38 (permalink) Old 06-17-04, 02:39 AM
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Re: 302 rods in 351

Originally Posted by XRMAN
Whats the deal with using 302 rods in a 351, whats the advantages of this combo?
This has been discussed at length by regular forum users but for the newbies heres the deal once again. 351 rods measure 5.778 in centre to centre. 302 rods come in at 6.025 ,so you are looking at an increase of some 250 thou or quarter inch.Are they better than 351 rods?Well ,this has been the subject of all the conjecture.Some people believe the longer rod has the added benefit of producing more power .The reality is that if there is a power gain it would be so slight you would not notice anyway.On the other hand you have those who believe in using a rod length 2 inches longer than stroke length(kaase)and if this were right then the standard 351 length is just about spot on.The only ral benefit I can see in using the 302 rods is if you are upgrading from a 302 to 351 and you need to buy crank only for this swap.The downside is you need special piston with a lower pin height to accomadate the extra lentgh and these cost around 400 bucks as compared to 200 for standard pistons.Imho no real advantage using these rods at all.Some will beg to differ.
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post #3 of 38 (permalink) Old 06-17-04, 03:08 AM
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Re: 302 rods in 351

I am with russ
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post #4 of 38 (permalink) Old 06-17-04, 04:30 AM
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Re: 302 rods in 351

Longer rods give better rod angle, less wear on bores and pistons.
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post #5 of 38 (permalink) Old 06-17-04, 04:45 AM
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Re: 302 rods in 351

A bit more info from the ACL site.


- Development of New Ford Cleveland Duratech® Performance Piston

A new Duratech® performance piston for 351 Cleveland engines for use with 6" 302 rods has been designed and developed for addition to the existing ACL performance piston range. Although this is a derivative of the original piston (9351) a separate development and testing program has been undertaken to verify the suitability of the design. In addition dynamometer validation has been conducted at ACL Technical Centre.
          • Figure 1: Short compression height performance piston for Cleveland 351
            with 6" 302 rods. Part number 9356. The revised groove details
            and lands still incorporate a pressure balancing groove. The
            external oil drains are visible as is a horizontal groove for pin lubrication.
Design Objective:
The intention was to obtain a short compression height version of the current Cleveland piston 9351 to allow use of Ford 302 connecting rods. This conversion is popular because these rods are more abundant and the shorter compression height results in a lighter piston. Most of a piston's weight is concentrated in the ring belt region so any shortening here can make a big difference to mass, thereby improving power and reducing loading on other components.

Design Obstacles:
The reduction to compression height was only going to be obtainable if the ring belt region could be appreciably shortened. This necessitated a change to the ring widths, and a reduction in height of all lands. In all a reduction of 6 mm was needed. However the proximity of the valve recesses to the top ring imposed a limit on the reduction possible on this land because of the danger that breakage would occur at this point so this meant that the second land also had to be shortened to less than the ideal.

To obtain a robust design capable of withstanding combustion pressures that would occur at an elevated compression ratio (say 10.0:1) meant that the radial depth of the top ring had to be reduced to less than standard for the diameter. This allows the groove depth to be reduced with a consequence that the bending moment on this land is smaller. Larger than normal groove root radii were specified to improve fatigue resistance at this point. The reduction of groove width coupled with minimised back clearance lead to an improvement in strength of the second land of some 25% which compensates for the reduction in land width.

          • Figure 2: Comparison of land and groove details of existing Ford Cleveland
            piston with new short compression height version. The reduction
            of 6 mm was enabled by reducing top and oil ring widths, and the
            1st, 2nd and 3rd lands.
The top ring specified had a width of 1.2 mm and a radial depth of 3.3 mm. The normal radial depth of a compression ring of this diameter and width is 4.3 mm. It was conceivable that side wear could be increased and some instability could occur with these "non standard" dimensions so it was considered necessary to subject sample assemblies to engine testing to validate the design.

(An alternative option was to dispense with the second ring. This would have allowed much more room for the rings and lands. However this option was tested in ACL Technical Centre and while blowby was, as expected, much improved, the oil controlling function which is the primary purpose of the second ring was lost and oil consumption was considered to be excessive. It could have been restored with further development of the remaining rings but this would have delayed introduction of the piston).

Engine Test:
A 351 Cleveland engine was obtained and built for the planned test. This incorporated modified 302 closed chamber heads with 57 cc combustion chambers, larger valves, a high lift camshaft, and four into one extractor exhausts. A 750 Holley carburettor was fitted to the standard inlet manifold. The compression ratio was measured at 11.3:1, much higher than would be recommended for eventual customer use of these pistons. Avgas was used throughout the test to enable the engine to operate without detonation occurring.

The engine developed 250 kilowatts at 5,000 RPM and 502 Newton metres of torque at 3,500 RPM.

After run in and power checking the engine was subjected to 94 hours of running at the maximum torque condition. This imposed greatest combustion loading on the narrow lands and the thin section between the valve recesses and the upper side of the top ring groove.

The engine ran well for 67 hours but the crankshaft broke at this point. Fortunately only one piston was damaged so the remaining 7 and a new one were installed in a replacement block assembly and continued the test. A valve broke after 94 hours and terminated the test. The piston in the affected cylinder suffered some crown damage but did not break. Throughout the interrupted test the blowby was monitored and was fairly constant at 50 litres/minute. This is a good figure considering amount of bore distortion that is usual for this engine type. The lower than usual radial depth of the top rings no doubt contributed to this good result as they would have been more conformable than deeper radial rings. Oil consumption averaged between 80 and 100 cc/hour, also a reasonable result for a performance engine.

On inspection of the piston assemblies at the end of the test there were no signs of excessive wear on the skirts, the pin bores or ring grooves and all rings were in excellent condition. The pistons were crack tested with dye penetrant and found to have no cracks.

Shortened compression height pistons have been developed for use in Ford Cleveland 351 engines using longer 302 connecting rods. The shorter lands and special narrow top rings with lower than standard radial depth have proven satisfactory in an engine running at much greater compression ratio (11.3:1) than would be recommended for the pistons in service (i.e.10.0:1). The weight saving (after adjusting for the 12 gram increase in weight of the 302 rod) is 60 grams.

The compression ratio will be 8.8:1 if the combined volume of the combustion chamber, deck volume and gasket volume is 85 cc. (The valve recess volume is 2 cc as per 9351).

The test was the equivalent of a round Australia trip towing a heavy van at full load and proved that the piston is very durable despite the compromises that had to be made to land and ring dimensions. However obviously detonation would have to be avoided as this would most certainly lead to piston failure as it would with any piston.

As a result of this project the piston has been released for production and will be added to the existing range of Duratech® performance pistons produced by ACL. Duratech® is the name assigned to second generation Duralite® ACL pistons.

Oversizes will be limited, at least initially, to 020 and 030. The part numbers are as follows:
  • <LI type=disc>Piston: 9356 <LI type=disc>Piston and Ring Kit: 8MKRY9356. Oversizes 020, 030 (Contains pre-gapped rings) <LI type=disc>Ring Set: M9122, 020, 030 (Note: A file back set is not available).
  • Pin: R3340T-STDM
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post #6 of 38 (permalink) Old 06-17-04, 01:33 PM
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Re: 302 rods in 351

I have just installed these pistons in my 302 rodded 351! I managed to chip the edges of a couple of middle rings Get a good ring compressor and push into the block do not knock them in. I had to buy a new set of rings +.020 I have some spares!!
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post #7 of 38 (permalink) Old 06-17-04, 03:54 PM
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Re: 302 rods in 351

Originally Posted by northiam
I have just installed these pistons in my 302 rodded 351! I managed to chip the edges of a couple of middle rings Get a good ring compressor and push into the block do not knock them in. I had to buy a new set of rings +.020 I have some spares!!
You must not have had the ring compressor tight enough. As they'd only chip if they'd have hit the block deck on way down. I've installed pistons and rings in a mazda 1200 engine only using two people and gently depressing the rings with 4 screw drivers and pushing the pistons down with my hand.

Moral of your story is: The piston should push down easily. You shouldn't need to knock it down excessively hard. Otherwise it breaks.

Make sure the piston/bore and ring compressor are oiled. That way everything slides nicer.


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post #8 of 38 (permalink) Old 06-17-04, 04:49 PM
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Re: 302 rods in 351

the thing is there is a lot of theory about long rods but no one has ever done anything to prove or disprove the theory ( to my knowledge )
Someone with the $$ needs to build a motor with stock rods, engine dyno it ... the do nothing but change the length of the rods in it and dyno it again.
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post #9 of 38 (permalink) Old 06-17-04, 07:04 PM
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Re: 302 rods in 351

Originally Posted by XRMAN
Whats the deal with using 302 rods in a 351, whats the advantages of this combo?
As everyone else above has said, there are many arguments for whether this combo is better or not.

The 302C rod is longer in length than a 351C, and so as above a shorter height piston has to be used.

Some believe the better rod angle provides better bore/piston wearing, and the longer tdc time allows for better burning, as well as many other pros and cons, including a better/stronger feel through the mid-range rpm.

I personally have a stance of neither for or against.

I have built many 351 rodded 351 Clevo's and one 302 rodded 351 Clevo.

I must say (although it could be a couple of things) that in the 302 rodded engine it does feel much smoother and stronger during the midrange rpm and slightly better in top end.

I am comparing this against an engine that had same cam, same exhaust, similar exhaust setup, same g'box, diff ratio, similar carby, same ignition.

As I say, it may be put down to differences else where in the engine, but it does definatly feel stronger.

Personal Choice??

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post #10 of 38 (permalink) Old 06-18-04, 03:55 AM
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Re: 302 rods in 351

If you have the choice, the 6" rod is definitely a better option.
Less side loading, less prone to splitting bores and cracking pistons, broader power range, and more free revving.

Plenty tests have been done on this and always point to an advantage using the 6" rods.


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