Rocker arm to poly lock clearance

[Ohio Rob]

Hello all.
I’ve been doing some preliminary mock up on a pair of SBF trick flow, twisted wedge R, heads for a drag race application.
I’ve got a Bullet cam with gross valve lift of .740 on the intake and .725 on the exhaust. Bullet recommend a PAC 1225 spring. (I’ve got an appointment to see Ken at Motorsports unlimited to source my springs and components. I’m leaning toward a PSI DR454R that has similar specs)

I’ve got some older comp cams ultra pro rocker arms, and some trick flow stud girdles with poly lock fasteners.

In mocking up the rockers, I pushed the intake valve to 1/2 lift and used a small square to get a visual estimation of 90 degrees, valve stem to fulcrum / roller center centerline. I set my poly lock there. It looks like I’ll be just about centered over the stem with the correct pushrod.

Then, I pushed the valve to full lift (minus .010 for lash. Bullet recommends .018 hot) when I actuate they rocker arm, I’m about .065 from the valve tip with the roller tip.

I suppose I can take up the .065 with a .080 lash cap. I stacked up .080 of feeler gauges to simulate a lash cap and see what clearance it would give me between the poly lock and rocker body.

My concern is that I only have .025 (I clayed it) clearance between the poly lock and the inside of the pushrod end of the rocker arm. That concerns me, but I’m not sure it’s a valid concern.

I have new Ferrea valves and the port work and valve job is done, so I’d prefer not to buy a longer valve.

The exhaust will have a bit more room as the lift is .015 less, but not much more.

Anyhow, I’m looking for some experienced input into what I’m up against.

Thanks in advance.
Rob

[BillK]

Where is it at the other end of the travel, valve closed ?

[Ohio Rob]

With the valve closed, the poly lock to valve side of the rocker arm has plenty of room.

[ProPower engines]

Are you running 7/16" studs??
If so you have the option to clearance the back or push rod sire of the rocker enough to clear. I would not go less then .020 for any possible flex while it appears you are running a stud girdle I have been here before and .010 was not enough in my case .
The steel rockers seem in most cases to have less flex in the center body part of the rockers but depending on spring load which may be greater then 500 lbs open based on you comments about cam&spring choices I had an issue with Crower steel rockers years back at 600 lbs open ans that's when I found .010 was not enough when they are that close.
I would test fit them at max lift with your spring choice and confirm. But that's static. Things change when the engine is at 7500 RPM and the valve train is really working hard

[Ohio Rob]

Yes, 7/16” studs.
I have .025 clearance
The open pressure will be around 700. The PSI part number is DR424R not 454. The 454 was a typo and I can’t edit it now.
I do plan on spinning the engine to 7500.

Thanks for the advice.

[ProPower engines]

Yes, 7/16” studs.
I have .025 clearance
The open pressure will be around 700.
I do plan on spinning the engine to 7500.

Thanks for the advice.

After you dyno it be sure to check for shiny spots on the poly locks. That was how I learned. plus it bent a couple push rods as well

[Ohio Rob]

Yes, 7/16” studs.
I have .025 clearance
The open pressure will be around 700.
I do plan on spinning the engine to 7500.

Thanks for the advice.

After you dyno it be sure to check for shiny spots on the poly locks. That was how I learned. plus it bent a couple push rods as well

Yikes!

[Ohio Rob]

Thinking on this, I realized that adding lash caps will effect the poly lock height in order to keep my geometry correct.

I may be at the design limit with a stud mount rocker arm and this cam / application.

It may be that some shaft mount rocker arms are the way to go.

Any other thoughts or advice is welcome and much appreciated.

Thanks
Rob

[Paul Kane]

I’ve been doing some preliminary mock up on a pair of SBF trick flow, twisted wedge R, heads for a drag race application...
...I’ve got some older comp cams ultra pro rocker arms...I pushed the valve to full lift (minus .010 for lash)...My concern is that I only have .025 (I clayed it) clearance between the poly lock and the inside of the pushrod end of the rocker arm....

...you have the option to clearance the back or push rod side of the rocker enough to clear.

Ohio Rob, from a mechanical clearance stand point this is the simplest and most immediate fix if you are dead set on running your current valve train setup. Yours is a custom engine build and there is no harm in customizing the component in question (rocker arms) for the intended application.

Having said that, the other option is that you could delve deeper into the culprit(s) of your situation; see next post below...

[Paul Kane]

The unfortunate situation(s) you are observing with your valve train setup is compounded by several factors, including not just the rocker arm but also the cylinder head design itself.

You may recall that the very first generation SBF Twisted Wedge heads were so geometrically out of whack in the area of valve train design that they were destroying valve guides in just a few short hours of run time. That First Gen head was very quickly revised to address much of its oversights. While there are certainly exceptions in high performance racing applications where compromises in valve train geometry might be intentionally applied in exchange for competitive performance gains, I suspect the design of that first head did not only forego proper valve train geometry in the interest of port design & flow gains but also was developed by someone who had insufficient understanding of the deepest intricacies of geometry in the first place. Because if they had greater insight, they would have foreseen the issues of the First Gen head and would not have released it at all.

Anyway, assuming you have the later revised head then the biggest issue in your setup is the rocker arm choice itself. That rocker arm was originally offered as an SBF rocker to be utilized with 20-degree valve heads. I don't know precisely which TW heads you have but I think they are the ones with the 11* intake & 13* exhaust valve angles. By its own design the currently utilized rocker arm already has geometric shortcomings for a 20-degree SBF head, and now when you install it onto a 11*/13* head it becomes further yet removed from ideal.

That's what your seeing; that's why the rocker slot-to-poly lock is not correctly positioned relative to the valve lift the rocker arm is accommodating; that's why there are rocker slot-to-poly lock clearance issues, roller wheel-to-stem tip reach issues, etc. And...should you continue beyond a simple mock-up and actually set the rocker arm up on a completed long block engine you will then observe a massive amount of over-arcing of the pushrod cup side of the rocker arm. That over-arcing will hurt the valve opening velocity more and more as the valve opens further and further, will reduce area under the curve, will increase rocker stud deflection, etc, so much, in fact, that it's conceivable a far more applicable 1.6 ratio rocker arm could match the area under the curve over your less applicable 1.70 ratio rockers while also maintaining better opening velocity and valve train stability throughout the lift curve.

Personally I would try other brand rocker arms until I found the design that best accommodates the TW head.

[Ohio Rob]

Thanks Paul

[Ohio Rob]

I’m updating this post, so as to not leave it open ended.

I’ve decided to sell all my stud mount stuff and buy Harland Sharp shaft mount rockers. They make a shaft setup specific to the architecture of the Trick Flow twisted wedge R heads.

I’m local to HS, so I stopped by yesterday. Randy, from HS, gave me a pair of arms and mounts to take home and mock up. No deposit, nothing but a handshake.

When I had them mocked up, I Could achieve a nice centered narrow scrub and nice pushrod side geometry.
I will get very close to a “mid lift” lift setup with some shims. They set up with a low fulcrum geometry with no shims.
The quality is very nice and all the parts are made in the USA!

I’ll post some pics when I get them, as they are made to order.

Thanks for all the help with this, as I’m a hobbyist, and I appreciate the experience and the willingness to help that this site offers.

Regards,
Rob