Recently installed a rebuilt 400 to replace a worn out 351M in my 1978 F150. Had a lot of trouble with timing issues and received a lot of advice from the forum to getting it running without a miss under load.
Rebuilt 400 has an RV type cam from Summit....their 5200 to be exact. Stock valve springs were used on the reworked heads and this is where my dilemma has led me. I've had two broken springs springs on #1 cylinder in less than 200 miles. Thought the first one was a fluke, but number two shows I have a problem. Checked with Summit and got their recommendation for the cam springs. Ordered new springs and now just trying to get some feedback on best way to install all sixteen. One wasn't too bad using the rope process. Facing doing all sixteen in the truck makes me think the air in the cylinder may be best. I don't mind buying any tools I may need and I have a shop compressor that's big enough for the job.
Guess my question for some more experienced folks is will these new springs solve this issue from continuing to occur? Will the compressed air provide enough pressure on the valves to hold them up while replacing the springs? Does each cylinder need to be on TDC with the compressed air method?
Thanks for everyone's help and support!

 

If you have power brakes and air this could be a major undertaking. I used three different spring compressor, modified two of them and several clamps etc to do it in my truck. Took several days of blood sweat and beers. Id'd be tempted to pull the motor part way out next time. The back ones are a bear. Were the springs old? Have you checked your preload? Using air to hold the springs works well but be carefull not to rotate the crank at all. It can turn the crank and break a wrist.

 

No A/C on my truck and I don't mind pulling the master cylinder and booster if it makes it easier.
Little more information to add since my earlier post. Yesterday truck was running fine while I ran a few errands. When I got home bled the brakes because had a soft pedal and wanted to make sure no air in the system. When I finished up was going for a test drive. Started up and was running rough..like missing a cylinder. Pulled #1 plug wire and no change in how it was idling. This was what I experienced with the broken valve spring a few weeks ago so I knew what was happening..I thought. Today I pulled the valve cover and expected to see another broken spring...nope. Everything looked okay so I started her up and she's running fine again. I'm kinda at a loss to what's going on now. Still plan on ordering the correct springs and replacing them, but if I've got more serious problems I need to address I'd rather tackle it all at once.

 

Does your engine use a rotator on the exhaust side or are all the retainers the same? If it does you'll need different springs for each valve. .486 lift won't be too difficult to get to clear with reasonable loads but it might be difficult to get .512 to work if you've got the short exhaust spring. There's a Qualcast spring that would give you nice loads and it'll fit on the head easily if you only need one spring. I don't have the part number here but I'll look tomorrow when I get to the shop.

 

The Qualcast spring part number is 32-909-D. I had a customer bring in a set for his engine and I really liked them so I'm using a set on the 460 that I'm rebuilding for my truck. These test in a really tight group with an average rate of 303lbs/in. This gives an average seat load of 84lbs@ 1.830 and an open load of 231lbs @ 1.346 with .484 lift. That would give 241lbs open with .512 lift and that would be workable.

 

Thanks Dave for the info on the springs. I'll check those out.

 

The cylinder you are working on needs to be at or very near it's TDC so both valves are shut so air can keep those valves shut while springs and etc are off of them ... and also, if bumped at the stem while you are working ... a valve can only drop a little bit and you can easily grab tjhe stem end and just pull it back up and the air compressor pumping will close it again.

I put a Crane Fireball Cam in my 351M back in the '80s, have 50,xxx miles on it easy now, used stock springs on it. Valve lift with Factory Rocker Arm Ratio was advertised at 0.484 int./0.510 exh. Other specs look like the same cam as sold by many as a RV cam ... like Summit's 5200. I later had a miss in 1990, but it was a couple burnt valves after I tried to lean out my Holley 600 on a trip, I was experimenting with jets. I did swap heads off a wrecked Thunderbird after they were freshened up, but I used stock springs from the same car as I was building my home when I was swapping the heads, time was short. My truck's OEM springs are in a coffee can on a shelf. Maybe you just had some bad springs, or some other anomaly. I have never intentionally let my 351M exceed 4500 rpm ... or even get up to 4500 rpm, then only very rarely in a quick pass on a two lane road .... it has held about 3500 when pulling a loaded trailer on I-81 from Ironto to Christiansburg or on I-64 over Afton Mtn ... in 2nd. I know ... the cam is said to be good to 5500 rpm, but such is just not in my needs.

I once replaced valve springs, keepers, retainers with umbrella seals in a Chevrolet big block ... I used air ... but that car was a 4 speed manual and 4th gear with the parking brake set easily held the crank locked in a chosen position after getting both valves closed on the cylinder I was working on.

On my truck with 351M and auto, I've looked and looks like I could make a "lock" to engage flex plate ring gear teeth to lock it easy enough after removing the inspection plate for access.

I don't recall any rotators on mine ... but they might have 2 groove locks.

 

I'd just take both of the rocker arms off at the same time. If the piston goes all the way down in the cylinder from the air pressure that's ok.

 

Sure ...if one is real careful, you can just remove both rockers but with just air pressure holding those valves shut, even @ 100-150 psi ... if you accidentally knock a valve off the seat before getting the keepers back in place, it has a long ways to drop if that piston is at BDC. Why I would seek to somehow lock the crank at TDC on that cylinder.

 

It's not that scary and you've got a valve seal on there that's adding some drag to the valve stem if you do break the seal with 150psi in the cylinder even a 2 inch valve will have 470lbs holding it to the seat.

 

Held tight ... until bumped .... then more like the pressure relief valve on a compressor.

 

You're going to bump it with more than 400lbs of force?

 

Some people use a socket to tap the retainer to loosen the retainer from a wedged in place stuck lock so the magnet grabs the just previously stuck lock out when you compress the spring with your compessing apparatus next as the air holds the valve up. That 400+ pounds is only applied by the air trying to get past the valve until the moment the valve lifts off the seat. Driving a nail takes a bunch of 400+ pound whacks with that 16 oz. hammer. You just start a 16D into just plain white pine, then you stand on it and you'll see how much force that little 16 oz. hammer was delivering as the nail still stands proud. Use red oak and a 20D and invite a buddy to stand with you. Only takes one tap with the valve, then the 400+ pounds is gone out the port, maybe even pushing the valve further away from the seat in it's rush to get by and turn through the port past the guide. If it happens, suddenly the valve drops, if there is a loose stem seal, a not tight guide maybe, 3 or 4" vacant stroke in the bore to the BDC piston top ... time to pull a head. Not so scary, more like just an uneeded risk and a PITA. Maybe it's not worth the effort to prevent but I for one would be more than just a little happy if a TDC piston stopped a valve from dropping down out of sight if it does happen.

 

How many times do you think that I've changed valve springs on the dyno with air in the cylinder? You're worry is unfounded but you can do whatever you want. If the locks are wedged in the retainer the retainer is still in place. Just give it a few sideways taps to loosen the keepers and then use your spring compressor to take it apart, no big deal.

 

Oh, I have no real idea but 43 two valve V-8s do have 688 valves. I can say that heads are a lot easier to pull on a dyno in a building ... than under the hood out in the driveway. "On the dyno" is a better place to experiment as every thing is just so much more ... "accessible". I have no knowledge of your shop or shop habits and I don't question your experience with engines. I will bet dolars to doughnuts though that even valve covers are easier to pull off of a head while the engine is on a dyno. If I were doing it in my little shop space, even if just on a stand, I might well "wing it" on just air too. Out there in the driveway though, it's better for me to limit the reasons for me having to revisit some things when a simple effective precaution was available to me. I find I'm usually left with cleaner thoughts that way.
Take Care