Trouble with Flat Tappets
Published in National Dragster
Written by David Reher
Faithful readers of this column know that my mantra on engine building is to spend your money wisely. Sometimes that means writing a bigger check initially in order to save money in the long run. A case in point is the decision whether to use flat tappets or roller lifters. In my view, a flat tappet is a ticking time bomb in a racing engine. Sooner or later, that bomb is going to explode with expensive results.
I’ll climb up on my soapbox and tell the drag racing world that a flat tappet camshaft is a bad investment for any serious competition engine. A typical flat tappet valvetrain may have a lower up-front cost than a roller camshaft, but it’s going to cost much more over the engine’s lifetime. The odds are that a flat tappet camshaft is either going to fail outright or cause a dropped valve. In either case, you’re looking at a thoroughly trashed engine and a major repair bill.
I know that some racers have run flat tappet cams successfully for years, and that NASCAR Nextel Cup teams are required by the rules to use flat tappets. The former are very fortunate, while the latter spend mountains of money on special cam cores made from exotic alloys, hardened and inlaid lobe faces, oversize zero-radius lifters, camshaft oilers and major block modifications to eke out 500 miles of racing from a flat tappet cam before it self-destructs. Flat tappets don’t save money for NASCAR teams – in fact, the total cost far exceeds the price of a roller cam and lifters. Moreover, the highly specialized components and machining procedures used in NASCAR Nextel Cup engines have virtually nothing in common with the flat tappet cams listed in retail catalogs.
Flat tappets survive in street engines because the valve spring pressure is relatively low. Consider that the seat pressure for a stock small-block Chevy V-8 with a flat tappet cam is 85 pounds. The generally accepted maximum spring pressure for a flat tappet cam used in competition is 130 pounds. That’s still much less than the 300+ pounds of seat pressure typically used with roller lifters in racing engines. Moreover, a low-tension flat tappet valve spring and a heavy steel valve is the worst possible combination for high-rpm reliability. The valve’s inertia can easily overcome the weak valve spring tension, producing violent, uncontrolled motion that can lead to a dropped valve and catastrophic failure.
Installing a flat tappet cam correctly demands considerable effort by the engine builder. Doing the job right requires making sure that every cam lobe is precisely positioned in relation to its respective lifter. The lobes on flat tappet camshafts are intentionally ground with a slight taper that causes the lifters to spin; if a lifter doesn’t rotate because of improper offset, the resulting friction will quickly destroy the lobe. The metal worn off the cam lobe is then embedded in the piston skirts and cylinder walls, causing scuffing and serious cylinder wall damage.
In contrast, a roller lifter can tolerate slight misalignment in the position and angle of the lifter bores. A roller bearing also requires much less lubrication than the sliding base of a flat tappet, so the amount of oil circulating in the engine can be reduced to minimize windage losses.
A flat tappet severely limits the camshaft profile and thereby restricts the engine’s performance potential. The diameter of a flat tappet dictates how quickly the valves can be accelerated. Years ago engine builders replaced standard .842-inch diameter GM lifters with larger .940-inch tappets and special mushroom lifters because the increase in lifter diameter allowed more area under the lift curve. But even the fattest flat tappets can’t produce the same valve acceleration as a standard diameter roller lifter. Consequently flat tappet cam profiles have long seat timing that bleeds off cylinder pressure without delivering the breathing benefits of increased duration.
In my experience, the people who tend to favor flat tappets are either newcomers to racing who are trying to save money or veterans who had a bad experience with roller lifters in the distant past. I think that both groups are making a grave mistake by not using a roller cam unless they compete in a class that specifically requires flat tappets. Yes, a roller cam and kit does cost more than a flat tappet cam and a set of lifters, but the cost of fixing an engine after flattening a cam or dropping a valve is much more expensive. It’s true that a roller lifter may fail occasionally – usually as the result of a broken valve spring or incorrect valve lash adjustment – but the likelihood of trouble is much less than with flat tappets.
I’m a frugal person, as anyone who has seen our Pro Stock racing operation can attest. My philosophy on engine building is that it’s much cheaper to do a job right the first time than to do it wrong and then fix it. It always costs more to repair something, and generally the effort and expense are a lot more than you ever imagined. For my money, flat tappets belong in the museum, not on the race track.

 

Makes sense to me.

 

"on a race track"

 

i would agree on a competition engine, in that case a solid roller would be my choice . on a street engine i would have no big issue with a flat tappet. i have many times in years gone by used the chrysler lifter in a small block chevy and used a Cam Grinder that could utilize the larger diameter lifter.

 

Given that popular flat tapper cams are of "XE" variety and oils have less zinc all the time even on a street engine, a roller cam is a better option IMO.
If a guy is good with the old school ramps on a cam with the associated performance penalty, and ensuring they use the right oil I suppose you could skate on a flat tapper cam for quite a while. Especially if you only drive 1000 miles a year, it could last for the life of the car.
I would prefer to t advantage of the current technology, and have the reliability of the roller cam.

 

FWIW to anyone else contemplating a similar dilemma...


As one who is building up a streetable trackday (AX/RR) BBC for my shark, despite that I'd honestly prefer to do another SFT this further reinforces the conclusion to which I keep returning that a solid roller is more likely in my best interest. Sure, an SFT will make all the power I need and rev higher than I intend to do, but popping the valve covers in the middle of a road course session to see how the valvetrain is getting on isn't quite so practical an option as is doing so between blasts down the strip. And, even if it's the only thing I might know about flogging SFTs, I've learned the value of frequent inspection (especially with highly-stressed mushroom setups).


Next question, rev kit or no?

 

You want to run a solid roller? Better ante up for some Isky EZ Roll or Crower bushed lifters......even Comp Cams is finally caving in on this system.....needle bearing solid roller lifters don't have the best reputation for reliability in an endurance application.

Do you ever wonder why there are so many solid roller lifter sets on Ebay? It's the NASCAR team takeouts out of the Busch/Truck motors ( whatever those series are named now a days ) those races only go 350 miles maximum and after that length of time those parts are pretty much used up.

While I deeply respect the author as a drag racer, I'm not sure of his experiences in building motors for series like USAC Silver Crown, NASCAR or other endurance series, in those applications solid roller lifters are always a reliability concern for engine builders.

If you want to play.....you've got to pay......hydraulic rollers last because they don't suffer the impact that a solid roller gets, this is especially severe at lower engine speeds below 2000RPMs so, if you plan on running a solid roller in a street/strip car....plan on spending some money to keep things reliable.

Anyone who doesn't believe this....I have a set of takeout solid rollers with under 5000 miles on them.....2 pairs are starting to fail due to brinelling of the needle bearings, these were the best lifters Comp Cams offered at the time of the motor build, spring pressures were 180 seated......440 over the nose on an older Ultradyne cam with pretty stable lobe acceleration, these lifters died because they were beat to death while idling

 

You would not need a rev kit sir.

 

The thing you have to keep in mind when you bring up NASCAR is they are not going to reuse a part that cost them $2800 a set over and over again. It's not worth it to loose a $30M a year sponsor over something like this. Every part in a NASCAR engine is cycle tested. They know how long they can run something. When the part reaches its life span it is cycled out of the engine and new goes in.

All lifters in NW, Truck, and now Cup are Needle bearing.

 

about 1976 or 1977 i asked the question of why they didn't make a hydraulic lifter cam . this was a group of people that included people as knowledgeable ( At the Time) as Reher and Morrison. one of them laughed outright , one looked like i grew three heads , one said there is no reason for it. well technology marches on and my question wasn't as ridiculous as it once seemed.
i have run every kind of cam in my life except a hydraulic roller and have gotten good service life ( for the application) made good power . i can't ask for more than that. will i ever try a Hr, maybe maybe not. with the extra weight and additional wear and tear as a result i think with a flat tappet with the Chrysler lifter i liked to use because they are cheaper than others , and the cam ground to take advantage of it i think im on my way to narrowing the perceived advantage . the availability of proper lubrication is my only issue, but then we search fore more octane in our fuel don't we? flame on

 

The aftermarket for decades enjoyed the QC that the OEM's demanded from suppliers. The aftermarket ordered lifters on the coat tails of the OEMs.

Today the OEM produce no flat tappets. Hence the QC is not what it was. Some lifter companies have seized making some lifters. Chrysler .904" are only produced by 1 company that I know of now.

Weight on the cam side of the fulcrum is not as critical for reduction as on the valve side.

 

I got 20,000 miles out of the old 818's from Comp Cams that didn't have the oil hole to the bearings in my old 406ci. I check them every year and around 20,000 miles I found a couple of them with needle bearing brinelling.

I now have quite a few years on their "new" design 818's that have the oil hole to the bearing. Both engines were/are high HP pieces with 1/4 mile runs on them and street miles which are supposed to kill these type of lifters.

I had to rev it up a little at the lights or waiting in traffic with the first set but I don't do that now.

I don't read any magazines or articles on the net, just bolt them together and they seem to run fine for a long time. These lifters are not expensive.

Run whatever you want to run

 

Yes, sir. Given there are now alternatives proving themselves superior I wouldn't plan on using needle rollers in this build.