Why do we keep blowing motors?

Our 1979 Ford Fairmont 24 hours of Lemons road race car

So we blew up our first rebuilt motor (Direct mount 2bbl, .060" milled head, 264 cam, new bearings, sealed power hyper-eretic pistons) when we over revved it. We we leading our class at the end of Saturday's racing, then Sunday morning our driver left the C4 in 2nd when the green flag dropped. The factory tach was reading something like 7500rpm when it went boom, the piston disintegrated and took a good portion of the cylinder wall with it.

We swapped in a "rebuilt" motor from a 1979 Fairmont, bolted up an SROD (later a T5) and drove in 3 full race weekends, then put the cam thru the side of the block in practice for the next one.

The next motor we spared no expense with all the good parts from Classic inlines (ARP bolts and studs, Silvolite Tempo HSCC pistons, 274 hydraulic cam), plus the direct mount 2bbl head we had been using on all the other blown up motors, after checking it out and cleaning it up. This one ran great in the spring Sears Point race and we finished 44th out of 175 cars, and 2nd in class. The next race it blew up in 3 hours.

It really looks like the Tempo piston just came apart at the oil ring. The 2nd blown motor may have come apart just due to the number of race miles we had on it without checking the tightness of the rod bolts.

I already asked a similar question to the Lemons boards, but they did not come to any consensus. http://forums.24hoursoflemons.com/viewt ... p?id=27214

So you guys have more collective knowledge than anyone when it come to these motors. Were these just 3 different unrelated issues? Or is there some basic issue were aren't dealing with in our prep?

you answered your question on piston failure.... RPM or more precise, too high of piston speed for the pistons you are using

Isn't 60 thou a little much for those walls?

Seven reasons.

First, the skirt clearance for a race engine needs to be up on the stock recomendations.
(Without being critical of some of the most intelligent production engineers that Ford has, there has been no improvement in US Ford small six in-line six cylinder piston technology since 1963. We missed out on the massive amount of work done on the later 4.9 EECV engines, and the Tempo/Topaz/Taurus technology is not applicable to the small six. The 1983-1992 Tempo pistons were used in a block that had the cam spaced in the same position as the 250, but with a shorter stroke, so its under a lot less physical stress even if they have more hp per piston. [ At 90, 100 and 110 hp on the HSC and HSO's, that's 22.5 to 27.5 hp per cylinder, and our sixes are hard pressed to do 135 to 165 hp total]. Its the side loads that are 10 % higher, and unless the clearances are changed, the stock recomdnations don't suit the pistons.)

Second, the oil breathing slots need to be drilled holes like on the better Windsor pistons, not cut slots. The Tempo pistons commonly break skirts or freeze the wrist pins, and its this that takes out a cam, a rod,then a block. Back in the sixtees, its was common to tig weld the skirt until Mahale and Recpo (Duralite) supplied High silicon content drilled oil relief pistons for Falcons and Holdens with bore sizes from 3.625 to 3.680"


Third, I touched on the silcon content of the piston. Our Aussie pistons changed composition way back in the mid sixtees when the last round body Falcons were endurance raced, and when the 170 Hp M code 2v 250's and 190 Hp 202 XU1 Holden engines came out in the early 1970's, the pistons were high silicon, and ran bigger clearances than stock when used in Super Sedans and Improved production race engines,

Fourth, when a pistons cracks a skirt, it has no clearance space in a 200 or 250 Ford....the cams on the 7.803 200 and 9.469" deck 250 engine are right up against the rod bolts and the skirts can cause horriable maladies.

Fifth, our engines runs anglular, servere 1.5:1 rod ratios in 200 and 250 form, so there is heaps of side thrust.


200, 4.715" rod for 3.126" stroke = 1.508:1
250 , 5.88" rod for 3.91" stroke=1.504:1

The tall block, 250 spacing cam in the Tempo 2.3 and Taurus 2.5 allowed Ford to run pistons with large 90 thou wrist pin offsets, 53.5 thou more than the 200 and 250, and it has soft rod ratios of just 5.45" for 3.30" stoke (1.65:1) and 6.00" for 3.585" stroke (1.673:1). Putting a Tempo piston in a block with a stiff rod ratio means its overloaded compared to the orginal 4 cylinder requirments.


Sixth, Ford rod bolts and certainly the stock non forged rods are not that strong either.

Seven, I discussd this before, and you won't belive it, but crank windage is just huge in a hydraulic lifter pushrod ohv in line six at 4500 to 5500 rpm, its nothing for them to have 80% of the oil capacity in suspension as 60 mircorn balls of oil in a race situation. All the Aussie Ford Cortina's with 200 and 250 sixes, and the racing Torana GTR 202 XU1's had old grey haired guys desiging weired crack pot sumps with shutters, windage trays, crank scappers with as littel as 60 thou clearance, along with Brigs and Stratton fuel tanks as ears on the sump to increase oil supply height at the pickup point when the car was raced with slicks in sedan car racing. The I-6 Cortina and L-6 Torana sunps allowed those engines to cope with 7500 rpm in endurance situations.

The acid test for oil supply was when Mick Webb from Ford Special Vehicles resorted to cutting a hole in the floor pan, and then putting a persepx window in the sump, and viewing the oil horizon to the oil pickup in a hot lpa around Mount Panaorama at Bathurst. They did this in the early 70's when slicks started to be used in production car Falcons with the 351C Cleveland engine... it would fail in edurance racing, and overnight, they gained considerable durablity after a brace of better sump designs were homologated. The amount of time the oil had no supply to the pickup was huge, despite never showing an iota of oil pressure drop at 6000 rpm.

read X's post over very carefully, there is a lot of good information in there.

turning over 7000 rpms with stock type cast pistons is generally a recipe for failure, in part because the air/fuel mixtures in each cylinder is at best inconsistent. i am betting that part of your problem stems from inaudible detonation. generally at normal street rpms, cylinders 1 and 6 tend to run lean, 3 and 4 tend to run rich, and 2 and 5 can go both ways.

you can start to eliminate some of the issues you have by running forged pistons, two one barrel carbs, and arp rod bolts. i would also recommend using earlier forged rods as well. you also need to run a crank scraper, and a windage tray.

rbohm":2mfaovkl said:

read X's post over very carefully, there is a lot of good information in there.

turning over 7000 rpms with stock type cast pistons is generally a recipe for failure, in part because the air/fuel mixtures in each cylinder is at best inconsistent. i am betting that part of your problem stems from inaudible detonation. generally at normal street rpms, cylinders 1 and 6 tend to run lean, 3 and 4 tend to run rich, and 2 and 5 can go both ways.

you can start to eliminate some of the issues you have by running forged pistons, two one barrel carbs, and arp rod bolts. i would also recommend using earlier forged rods as well. you also need to run a crank scraper, and a windage tray.

Click to expand...

With respect to it running lean, to what extent would jetting it "fat" help.

Soldmy66":5k51bhje said:

With respect to it running lean, to what extent would jetting it "fat" help.

Click to expand...

that can help, but the problem is the mixture distribution more than anything else. fatten up the mixture, and cylinders 3 and 4 run even richer, as do 2 and 5. 1 and 6 then still have mixture issues, and you end up losing power over all. and that is why i suggested running two one barrel carbs. that way they can be spaced to where the fuel mixture distribution can be evened out so that each cylinder will get about the same fuel mixture at all rpms.

you can then tune the engine using the exhaust temps as a guide.

What engine is it exactly your running? If its a 250, at 7000 RPM it is about 1800 RPM too high for cast pistons, and for prolonged use you should really back it down another 10% or more below that. The recommended max safe piston speed for cast/hyper pistons is only around 3500 FPM, and at 7000 RPM you are imposing 4561 FPM on them. That coupled with the poor rod length to stroke ratio is not a good choice for your combo. Forged is the only way to go for what you are trying to accomplish, but is still subject to the problems associated with the bad rod length to stroke ratio if that isn't resolved also.

I'm an advocate of high silcon cast alloy pistons. Its only when you supercharge, turbo or run very high pistons speeds that I'd change. I'd never use anything but a proper competition proven high silicon cast alloy piston. In Australia, six cylinder engines were sedan raced since about 1963, so local suppliers like Repco, ACL, Duralite and now Mahale don't get away with making low spec cast alloy pistons or the customers revolt. So aftermarket Aussie pistons are a lot tougher than the US spec cast alloy ones because the engine builders rat out substandard pistons that don't hack 4000 feet per minute pistons speeds.


The calculation is on http://www.wallaceracing.com/piston-speed-velocity.php


General Motors Holden in Australia upgraded our Our Aussie cast ACL/Repco/Mahale/Duralite pistons in 1972...prior to that, any Holden, Chrysler Slant was a perpetual piston top loser in competition environs. That's when the engine builders experiemented with TIG welded piston to oil relief skirts, which helped a lot.

Since that time, GMH genuine ACL XU1 GTR Duralite pistons can handle 7500 rpm with a flash to 8000 rpm okay with a 3.25" stroke on a 202 Holden. Proviso is that the ring and piston clearance is more than the stock recommendations for GMH and Ford I6 engines.

Piston sizes are basically short deck Rover/Buick at 1.75" compression height, and they have been race proven for 600 mile 6 hour 100 mph per lap races subejeted to 230 plus horsepower. Rod ratio is 5.25" for a 3.25" stroke, or 1.615:1.


Running the calcs, Piston Speed: 4,062.50 feet per minute or 67.71 feet per second.

In the VB/VC/VH/VK Commodore Cup engines, under very hard conditions, the 253 cubic inch V8's with ACL/Mahale/Duralite cast pistons do 7000 rpm and 155 mph with 330 hp on cast pistons with a 3.0625" stroke, 1.75 compression height, and 5.622" rods. They can do well over 7 grand in certain situations.

Running the calcs, Piston Speed: 3,572.92 feet per minute.
Piston Speed: 59.55 feet per SECOND


When put on a 250 with long stroke, the same kind of Duralite pistons with 1.531" compression height and 3.68 to 3.74 are only good enough for about 6250 with a fash to 6500 rpm possible, but plenty of racers are using cast hi silicon alloy pistons in 350 to 400 hp 250 naturally aspirated sixes at 6500 rpm all the time.

Running the calcs, Piston Speed: 4,072.92 feet per minuteor 67.88 feet per second

If revs are kept down below 5500, turbo engines in the 500 hp area can cope with cast alloy pistons.


Forged pistons tend to be more of a risk if the engine is not heated up, they suffer freeze problems, and you'll see more damage from cold piston slap with forged pistons, but a cast alloy piston will explode, a forged piston won't.

X, i too am a fan of hyper eutectic pistons, they are strong, and a bit less expensive compared to forged pistons. and they do allow for quieter engines due to tighter P/W clearances. however the reason i suggested a forged piston to the OP is that his engine is a race engine, i doubt he drives the car on the street. as such he doesnt need quiet operation or extra long life.

either way however the HE and forged pistons would do well in his application.

With the exception of the piston choice and the oiling system your bottom end should be very strong!

1. I also think a better piston will be a big help Hyper Eutectic or Forged I do like a forged piston for a race only combo.
2. Max on over bore .030 the less the better unless you want to sleeve it back to Std.
3. A longer Connecting Rod would be very helpful reducing the cylinder wall loading.
4. A much better oil pan design (read custom), with at least a deep sump with baffles to keep the oil around the pump pickup or The better T style with trap doors look at what the race GT350 Mustangs used. Also a windage tray, and a crank scraper too.
5. Double Roller timing chain
6. A Rev Limiter, it looks to me like you are way over revving the engine far beyond where the cam is still working (torque range) or pulling.

What type of bearing and piston clearances are you using? What oil pump? Good luck looks like it could be a lot of fun to race.

I think our friend from the southern hemisphere brings a whole nother perspective.
This is good info for sure.

here in the states as a whole the ford little inline six was used for racing by VERY few people and that is why the weak points (at hier RPMs ) were completely over looked by FoMoCo.
Here the RPMs the sixes were run were much lower in my opinion compared to those "down under" and in NZ.

Having said that I definitely think you have found the limits of the stock pistons & rods.
I'm surprised those cast pistons lasted that long at at that RPM.


Side note.
When I was at Crane we had a few 5.0 test engines for Dyno work. one (A non H.O. 5.0 (cast pistons))was sacrificed on the dyno to see the upper RPM limits of the Crane EFI computer.
The Rods REALLY let go in a big way about 7000RPM. It went higher than some non ford fans predicted. They gave me the engine to scrap.
As far as time at RPM dyno engines have no time at all compared to road racing.

Wouldn't an MSD box be an easy way to limit revs?

Why do I feel something pulling my leg? Why engine break? Because Racecar.......

Cast rods + cast pistons + 7500 rpm = BOOM!

Like he had to ask. I guess the little light inside hasn't turned on yet.

MustangSix":1prty6z8 said:

Why do I feel something pulling my leg? Why engine break? Because Racecar.......

Cast rods + cast pistons + 7500 rpm = BOOM!

Click to expand...

Ditto :nono:

Three things that may help in addition to the above especially oil pan baffles are an oil cooler, on the old forged rods there is an oiling hole at the top of the rod for extra oiling to the piston area. Make sure your upper bearing inserts are drilled for this oiling orifice.
Last contact King bearings for main bearings which supply oiling 270 degrees instead of the stock 180 degree oiling to the rod bearings. this is a band aid fix for cast pistons vs forged pistons.

So perhaps I wasn't very clear.

The first motor which I built with new hypereutectic pistons in the stock 1979 bore size block after a race and a half (1500? miles) when one of our drivers ACCIDENTALLY left the C4 in 2nd when the race went from yellow to green flag. It sounded spectacular at the end of the front stretch when according to the stock Ford tach and our GoPro it his about 7500rpm. Ans then the piston came apart.

In normal racing we had imposed a 5000rpm shift point on ourselves, or were just leaving it in D instead of shifting it ourselves. This motor had all new bottom end bearings in a 100k mile block, plus a Clay Smith 264 degree hydraulic cam. On top was a 1979 big log head, milled .060" (but with the addition of a Fel-Pro gasket works out to about a .030-040" reduction in combustion chamber), with a 6 into 2 header, and a 2bbl Rochester 2GC carb mounted directly to the log.

Next we picked up a mystery "recently rebuilt motor" from craigslist for $300 from a guy swapping a V8 into a Fairmont. I pulled the head off, and swapped on our modified one (after getting the damage from the blow up fixed), and noticed it still had crosshatching on the cylinder walls. We also installed a rev limiter and a big aftermarket tach. We managed to mount an SROD behind this one (bad 3rd gear) and eventually a T5 and ran it in about 3 races. At one point the head gasket started to leak somewhere, but not into the oil, cylinders, or exhaust, so It got a head gasket swap at the track during a race from a modern composite to a Ford stamped metal service replacement. We never touched the bottom end and we're pretty sure either the rods or bolts stretched, or the nuts just loosened up, because this it the one the kicked the cam right out the side of the block.

The next motor we built was meant to be the good one. We started with a late 1960s block which had the good rods already. We took it apart to confirm this. We used all new bearings, ARP rod bolts and studs, and had the block bored to .030 over. I now realize we never should have used the Tempo HSCC pistons, but at the time it seemed the thing to do. Since we needed a new cam anyway we went up to a 274. Again we set the rev limit to about 5000rpm and ran it in one full race, where it was strong and sounded great. We finished second in our class that race and 44th out of about 180 cars at Sears Point.

And then the next race it came apart ruining another cam and a block we had spent a bunch of money on.

...But all the info you have posted so far has been very useful. The only thing that has kept me from building a baffled pan has been the fear that my design will keep oil from the pick up, instead of keeping it where it is supposed to be.

A lot more goes into building an engine for this type of activity than just replacing parts. Your bearing clearances are equally important along with good fasteners and some specialized machine work. For instance, you could have initially grabbed a bearing causing the rod to fail which then broke the cam and then shattered the piston blowing out the cylinder. Determining what happened first is often the key to ensuring the right parts get upgraded the next time. It's possible that if these initial problems hadn't cascaded that the cast pistons may have endured indefinately. But again, it could have been the first part to fail and then caused the other parts to follow suit. I know the purpose of the Lemons races is to be on somewhat of a budget, but sometimes there are corners that just can't be cut. It isn't very realistic to think that your going to take a junkyard engine and throw some new parts in it an race it at 5000 RPM for very long. You really haven't done anything to upgrade the engine to prepare it do this. You never did say which engine exactly you keep having these problems with. Is it a 200 or a 250?

cnc dude gave me another thing to check when you assemble the next engine, and that is the piston pin clearance. if you are not getting enough oil to them they will seize and shatter a piston as well. so you might want to open up that are a bit to allow more oil to flow into that area to prevent seizing.