Solid Rocker Arms?

[Mike_Moore]

Hi all, can I just pick your collective brains over there and ask what people are doing for solid rockers these days.
Is there anyone re-manufacturing these at the moment and if so what are the specifications, price and availability?
Any advice and ideas really appreciated!



thanks, Mike

[chris_seven]

Mike,

They are being re-manufactured in the UK and have been for around 5 years now and last time I checked they were about £250 each but I suspect price has increased as this was 2 years ago.

I have just had a tooling quote to re-forge the original 1965 forged rockers to take an Elephant's foot and the tooling cost is surprisingly reasonable.

I am just putting the final stages in terms of machining and surface treatment before placing an order.

[ajwans]

Excuse my ignorance but why would you want non-adjustable rockers?

Also, why would you want forged rockers? I'd prefer that the rocker
broke rather than the valve bent in case of valve-piston collision.

Is this a weight thing?

andy

[Zeke]

Excuse my ignorance but why would you want non-adjustable rockers?

Also, why would you want forged rockers? I'd prefer that the rocker
broke rather than the valve bent in case of valve-piston collision.

Is this a weight thing?

andy

There are more informed than me, but the solid rocker is not non adjustable. Just a different way by adding shims to a cap on the valve stems. Not user friendly, but OK for race engines. And, yes, it's weight thing.

I'm surprised we aren't talking titanium.

[chris_seven]

Always an interesting debate.

The inertia of the valve train will affect the ability of the spring to control the valve and the higher the revs and lift the greater the problem and the stiffer the spring and the greater the cam wear. The Elephant's foot adjuster must be a much higher proportion of the inertia particularly at a rocker ratio of approximately 1.4.

I would imagine that the 911R, ST and RSR all benefited from using the 'solid' rocker and shims in terms of valve control.

I believe that the original forged rocker is safe for around 8000rpm but it may not work as well with some of the higher lift cams particularly if they have high ramp rise rate.

I would agree that Titanium rockers with a very light valve train would be great but the coatings used to prevent galling on the cam lobes would be quite fancy and the cost would be quite high. A typical F1 Ti rocker has a manufacturing cost of about $750 and if this were to sold in the retail market I could easily see them being $1000 each.

Some BMW race engines now use a forged aluminium rocker with rollers and these seem quite and elegant solution but probably a fairly short life.

Titanium may also be a problem as it has no effective Fatigue Endurance Limit and so would need a replacement life if it were to be designed to a serious weight limit.

The dabate about 'sacrificial' rockers I find even more interesting.

I have never believed that Porsche designed an Investment Cast Rocker to protect the valve train and the pistons in the event of a tensioner failure. Surely the correct design approach would be to correct the tensioner problem.

This material change took place relatively ealry on in the life of the 911 motor at a time when Porsche was suffering from tensioner design problems.

I believe that the 'new' design was introduced as a cost savings as many other manufacturers were moving to Investment Casting at precisely the same time and they only did it to save cash, the safety of the valve train was also a marketing benefit.

From a material science point of view a sacrificial design of this type is very, very difficult to implement, particulalry in the late Sixties and early Seventies when process control was relatively poor.

A rocker needs to operate at stresses which would be below the fatigue endurance limit of a material if they are to have an 'infinite' life and I believe that this is an essential requirement.

This stress would be around 50% of the tensile strength of a forged steel if assessed using very basic fatigue data such as a Goodman Diagram.

A sensible manufacturer would then apply a reasonable Safety Factor so it is likely that the breaking force would need to around 3 times the operating force.

If we now consider investment casting as opposed to forged steel it is likley that the fatigue endurance limit becomes around 40% of the tensile strength as cast materials are generally significantly more brittle that wrought metals.

This means that ratio of operating force to a mean breaking force may now be 4 times.

I would also expect to see a scatter of materials properties of 10-15% for an investment casting.

I find it difficult to accept that the breaking force of a cast material can be controlled to the relatively low level needed to avoid valve damage and still provide an aceptable fatigue life.

I think that it may be common that when an engine is buzzed the extra force applied may cause a rocker that is already supporting a short fatigue crack to fail and we all then feel grateful for this excellent feat of engineering.

The rate at which the rocker is loaded will also have a large effect on the failure mechanisms operating. If the rocker is defect free when it is subjected to an overload it may be relatively difficult to induce a brittle fracture but when a short and sharp fatigue crack is present impact strength will be quite badly affected.

After many years working with metal fatigue and fatigue testing machines this is the only example I have seen of this type of protection, hence my basic disbelief.

I will now go an hide in a bunker for a short while.

[edmayo]

Seems like I have read/heard that another benefit of the solid rockers was that the clearance remained stable much longer. The combination of thread fit, and elephant foot wear would cause an eventual clearance change, the solid rocker and shim was more stable. I have a set in one of my engines, but so far I can't attest to the above theory since there isn't much time on the engine.

[JKH]

IMHO There is a big difference between Fnishing the race and Repair before its too late.(runs really bad with a broken rocker) Along with the ease of service.
The forged rockers with lash caps are way lighter for sure.(runs with a slightly bent valve from miss shift, that a pro should not make.)