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Max RPM on Bridged heads? |
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tmancusi
AMC Nut Joined: Apr/25/2009 Location: WI Status: Offline Points: 432 |
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Posted: Oct/18/2009 at 9:53am |
After reading through a lot of replies to my past posts and others, it looks as though you can't go much higher than 490 lift to keep it safe. How about RPM range? I read a post by AMX39068 that part of the problem with the later model heads is:
"not the rocker rather it is the pathetically small 5/16ths bolt size which can snap off or pull out of the threads with too much strain from a larger lift cam combined with the associated heavier valve springs. To correctly equip for a bigger "performance" cam you should also upgrade the valve springs which in combination is where they can exceed the strength rating on either the bolts or the dinky little bolt hole threads in the heads.
For a near stock cam and close to stock valve spring tension they will probably be fine. Go much higher than stock for either and you are asking for trouble." So how what kind of RPM range can one go? I have a Comp 280 cam I would like to run, up to 6 grand RPM, not sure if the 5/16th bolt size will hold up? Thanks. |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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I think the 280 cam is pushing it. I seem to recall an earlier post from someone on the forum that said the 270 cam is about the max. The problem is that most original bridge rockers were made of very fragile white metal and as you mentioned the bolts are very small. I have purchased a couple of later model 401s over the past year and they all had issues with the bridged rockers when I pulled them apart. There were a couple of broken bridges on one engine and one or more snapped bridge bolts on the other. Both had jumped time from an old sloppy timing chain and one had a very mild 264 cam in it. Each engine had at least one broken bridge and the engine with the cam had a snapped bolt that we had to remove with an easy out on one bridge. I realize that the problem with the bridges in both cases were caused by jumping time but I have seen many old engines with a sloppy timing gear jump time with non bridged heads and they had no issues with the rocker arms or rocker studs and nothing on the top end broke other than a bent pushrod and bent valves here and there but no broken parts.Edited by amx39068 - Oct/18/2009 at 11:26am |
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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FuzzFace2
AMC Addicted Joined: Jul/05/2007 Location: Angier, N.C. Status: Offline Points: 10343 |
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I run a 280 cam & kit in my drag 360 motor only because I did not have time or money for machine work to go bigger. Now I do run roller rockers, 291c head, but think it runs the smaller bolt to hold them on. I run the rollers because I had them on another motor. It also cross the line at 6000+ 4.56 gear. I wounder what the class racers use as they have to use stock parts. gman from CT runs a 70 390 (291c heads IIRC) so that is not the bridge type. Did they make steel ones?
Dave ----
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TSM = Technical Service Manual
75 Gremlin X v8 for sale 70 Javelin 360/auto drag car 70 Javelin 360/T5 Street car |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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The 291C head has rocker studs with a 7/16ths base and 3/8th shaft and it has individual rockers and so it can handle even more than your 280 cam with no problem if kept below 7K RMP
The bridged rockers are just a U shaped piece of stamped or white cast metal with a pair of rockers in them and those seemingly way too small 5/16ths shaft size bolts. They are very very small and break off pretty easily...ask me how I know!
The metal on the bridges are either a fairly thin stamped metal or a relatively brittle white cast metal although I have found the stamped metal bridges seem to be less brittle thus less inclined to have problems than the awful cast white metal rockers. A fairly easy upgrade on the bridged rocker heads may be to drill and tap the rocker hold down bolt holes on the heads up to at least a 3/8ths bolt and to use a set of Hardland Sharpe bridged roller rockers that also have the bolt holes drilled out to the same size as the enlarged hold down bolt. You would oviously need to pull the heads and preferrably have them broken down while enlarging the holes in order to ensure that none of the filings from drilling out the bolt holes ends up floating around inside the engine.
With HS roller bridges and larger bolts, you can probably eliminate a good portion of the fragile nature of the bridged heads in combination with a bigger cam. Edited by amx39068 - Oct/18/2009 at 2:02pm |
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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tmancusi
AMC Nut Joined: Apr/25/2009 Location: WI Status: Offline Points: 432 |
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Why did AMC change over to the bridged style?
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Fluffy73
AMC Addicted Joined: Nov/21/2007 Location: Castlegar, BC Status: Offline Points: 3059 |
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Upgrading to steel bridges on my 304 solved all the valvetrain noise/clearance issues I had with that engine. Now it runs perfectly quiet with no ticking whatsoever.
This same motor had some broken bridges when I first got the car. It always had some valve train noise. |
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I am genetically incapable of being Politically Correct.
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tmancusi
AMC Nut Joined: Apr/25/2009 Location: WI Status: Offline Points: 432 |
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What do steel bridges go for?
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hoosieramc
AMC Addicted Joined: Jul/03/2007 Location: Indiana Status: Offline Points: 1233 |
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What about using a 7/16" stud instead of a bolt on the bridged rockers? In theory a stud would have more clamping power and be less likely to break.
Edited by hoosieramc - Oct/18/2009 at 3:36pm |
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idrambler
Supporter of TheAMCForum Joined: Jul/02/2007 Location: Nampa,Idaho Status: Offline Points: 2942 |
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4$ per set from APD or 32$ a set for a V8......
http://www.americanpartsdepot.com/valverock.htm you can get the steel sets from your local parts place......may have to wait a day or so for them to arrive..... they were used in later Jeep/AMC V8's..... |
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Jim....AMCRC
Treasure Valley AMC Club, Pres 69 AMX 401/727 74 GremlinX 401/727race only 73 Matador 2dr HT 360/727 |
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tmancusi
AMC Nut Joined: Apr/25/2009 Location: WI Status: Offline Points: 432 |
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Thanks!
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