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Change Pistons -- Must rebalance?

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RB401 View Drop Down
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    Posted: Apr/14/2012 at 5:22pm
I am still planning out my 401 power upgrade, but have largely decided to take AMX3098's advice of running '993 heads with 27 or 28 cc pistons and probably the Summit 8601 cam as well.  These parts would be installed on a rebuilt engine in good condition, (i.e. has good compression in all cylinders and no known problems).  Swapping out the present 502 heads for the 993s is no problem, but what about the pistons.  Will I have to rebalance the motor?  I guess if I could weight match the new and old pistons I could probably skip balancing but that might be hard to do.  If the new pistons were lighter than the old,
could I also skip rebalancing?
 
Another question is do I have to pull the motor to replace the pistons?  I pulled the motor twice when I did the T5 install (which is now a rather shocking 6 years ago), but I still have the hoist and engine stand in storage. So I can pull it again if necessary. I am sure its not reccomended practice, but the truth is I would like to make the piston swap in the car to reduce workload and downtime, if possible.
 
I am still a little leary of running 9.5:1 compression with iron heads, but stock piston choices are either 41cc or 27/28cc.  So, I guess I am going to go with 28 cc pistions unless someone knows of an off-the-shelf brand with a dish somewhere between the two.  After talking to a well respected local machinist, I have come to realize that using off-the-shelf parts keeps costs low whereas machine shop charges for custom work really boosts costs.
 
Thanks,
Dave
 
'73 Javelin AMX, 401, T5

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Post Options Post Options   Thanks (0) Thanks(0)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/14/2012 at 7:26pm
You do not "have" to rebalance the rotating assy. but it is the smart thing to do if you want the motor to last.
Can you do pistons in the car? yes you can but it will take you longer to do so. You would have everyting undone but the bell housing because you have to raise the motor, drop the crossmember just to get the oil pan off. You would also have to lean over the fenders to get the heavy heads on & off with the motor in the car too. Also would need to guide the rods and piston in the holes by your self from the top. In my book easier to pull motor and mount to the stand to do all the work and if you balance it has to come out any way!
Dave ----
TSM = Technical Service Manual

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RB401 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/14/2012 at 8:38pm
Ok, checked the TSM, your right.  To remove the oil pan, one has to raise the engine, pull the cross member, pull the strut rods, sway bar, etc.  It would be much easier just to pull the motor.  Not sure if I am up for that much work and car downtime this summer.  I may just put the '993 heads in storage or if I do install them, it will be just to see if the pistons are down in the hole and if so correct the quench by using a thinner head gasket.  
'73 Javelin AMX, 401, T5

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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 3:18pm

AMC uses pressed pins so you have to have the pistons replaced at machine shop.  And the only way you would not have to have a rebalance is if you can match the weight of the new pistons to the old. 

It will run fine with 9.5:1 with the newer style flat top pistons.  I run that compression ratio and even higher on all my cars running on Phoenix area's 91 octane premium grade panther piss oxygenated "city blend" gas and never have an issue with any of the car's pinging at all.


Edited by amx39068 - Apr/15/2012 at 3:21pm
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 4:07pm
You don't have to worry so much about the quench if compression is down. 9:1 or more you probably need to get the quench at 0.040" or less -- zero deck height and stock head gaskets(most stock type head gaskets compress to the 0.040"-0.042" range). You're right on the verge of having to have quench correct at 9.5:1, but you CAN get by with running up to 0.050" or so down from the deck, just have to retard timing a little. That means you're giving up a little power, but shouldn't be that noticeable. With the quench right on you can run a bit more compression without retarding timing. The important thing is to not have detonation, whether you do it by having a good quench dimension or have the piston further down in the hole and reduce compression to under 9:1 is really beside the point.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RB401 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 6:06pm
Thanks for the responses.  Since, I don't want to pull the engine or dismantle the short block at this time, I think I am going to just install the '993 heads, roller rockers, and a bigger cam than the small Edelbrock cam that's in there now.  Maybe the Summit k8600 cam, but I am going to call Lunati and see what cam they would reccomend for an 8.5:1 compression motor.  Comp has an online program that recomends as I recall the XE256 as the best fit.  I have read however, that the XE series of cams closes the valves quickly, i.e. hard, and thus makes a lot of noise, whereas the Lunatic Voodoo series of cams which are quite similar closes the valves more slowly and thus eliminates the noise.  Both the Comp XE and the Lunati Voodoo cams were designed by the same guy.  This guy explained in a post on another forum that he designed the Voodoo cams specifically to eliminate the problems in his earlier Comp XE designs. The Summit 8600 cam on the other hand appears more old school and while it may make less power has the advantage of probably being more likely to survive on modern low zinc oils without wiping out lobes.
'73 Javelin AMX, 401, T5

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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 8:36pm
I would go for the 8600 before I would do the XE 256 which is not a heck of a lot more than a stocker or the cam you already have.
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 8:44pm
What wipes out the lobes is valve spring pressure, not the cam itself. Stock valve springs shouldn't wipe lobes after cam break-in (additive needed for break-in, and I use it for the next oil change just to make sure, but not after that). You can go a little over stock pressure, but not much, without having to use an additive. A high rpm engine that needs high pressure springs must use an off-road/racing oil or an additive.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 9:00pm
Don't get sucked in by the "old or new school" BS. Bad lifters on a good cam is just as bad as good lifters on a bad cam.  The 8600 cam runs great on a mild build and has a nice little lope to it without being too choppy.  If you go with it, upgrade to better lifters than the summit lifters which are your basic $3.50 cheapie replacement lifter for grandma's car rather than for a performance application.  Get yourself a good set of antipump lifters and enjoy the cam for what it is, the best bang for the buck out there.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2012 at 9:43pm
Originally posted by farna farna wrote:

You don't have to worry so much about the quench if compression is down. 9:1 or more you probably need to get the quench at 0.040" or less -- zero deck height and stock head gaskets(most stock type head gaskets compress to the 0.040"-0.042" range). You're right on the verge of having to have quench correct at 9.5:1, but you CAN get by with running up to 0.050" or so down from the deck, just have to retard timing a little. That means you're giving up a little power, but shouldn't be that noticeable. With the quench right on you can run a bit more compression without retarding timing. The important thing is to not have detonation, whether you do it by having a good quench dimension or have the piston further down in the hole and reduce compression to under 9:1 is really beside the point.
 
I disagree, and have the documents that support my thoughts.
If you have a 9 or 9.5. to 1 with proper quench distance, then drop to say 8.7 but don't have good quench, you can ping worse than the 9.5 to 1.
 
"Retarding the timing a little" won't help with a piston down in the hole .050 - that's HUGE, by the time you add .035" of the head gasket. It's asking for trouble.  Even with compression as low as 8.7 or so - she can ping badly - and "retarding the timing" will do little but make it a really poor performer.
If you take the same engine and simply drop the piston down to reduce compression, it may well have detonation issues. You can actually make things worse by dropping the piston to drop compression.
Been through all this before - looks as if I need to post those documents again.
Basically, you offset the drop in compression with the loss of quench and on these old slower burn chambers will little swirl, you are asking for trouble.
 
If you change piston types, you'll be changing weight, get it balanged. I went with the step dish pistons on mine and the amoung of metal removed from the crank was amazing.
If you change bore, you change piston weight, if you change piston design, material, etc. - you change the weight. Get it balanced. It's CHEAP compared to pulling it down again, and/or finding broken parts, or dealing with weird vibrations.
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