Hi Guy’s, I’m looking for advice. I have a 427 that I’m building, . Everything is original to the block except the pistons which are L2268 TRW 427+30. Upon rotating the crank throws appear to hit the bottom of the piston under where the wrists pin sits. It’s not buy much with a little help the crank will turn. When it warm however it will not rotate past those points in the rotation where it appears the crank throws are coming in contact. I took the crank to the machine shop who is baffled as to why this is happening. I’ve ask them to remove some metal from the front and back throws for clearance. The say it can be done but the assembly will need a rebalance. I believe the 427 is internally balanced. So will need the balancer and flywheel as well? . Will just taking 50 thousands off the front and back throw require a rebalance? This is a 4 bolt block so I don’t want to screw it up. Any ideas and input would be appreciated. Thanks

 

Yes it will need a balance...no you do not need a balancer or flexplate as 396/427 is neutral balance......
The piston number checks out correct...does it hit on all eight pistons?

Jebby

 

sounds like crank or rod is wrong

 

Nope just hits on the front and rear crank throws. Thanks

 

Depending on where it hit....you could get away with removing a little under the wrist pin boss.......

Jebby

 

that is better,
I would compare orig piston to new.
orig crank rods so,,

 

I would recommend first, as mentioned previously: "sounds like something is wrong"; figure this out before doing anything!

Next, I would not recommend just cutting down the counter-weight O.D.'s; as if balanced now (?), this weight sum will have to be returned; how (?), heavy/mallory metal addition (expensive!)? TRW pistons were what we had back-in-the-day, and everyone used 'em (and they didn't hit!), but today, c'mon! If your going to spend some money start with a set of better pistons (yep, you'll have to re-balance, but it wouldn't need metal added!) and they should fit the crank.

Scott.

 

Somebody offset grind the rod journals for a little bit of extra stroke at some point? (Thus bringing the pistons lower in the bore)

 

is it assembled correctly? are the rods n correctly?
are the pistons correct for 427 or are they 454?
is the crank the original...is it a 396/437 crank?
What are the crank numbers?

you shouldn't have to machine anything for clearance if the parts are correct

 

L2236 is the correct part number for L72/L71 427 pistons, if that helps.
It would be time well spent to verify the stroke and the rod length.

 

If the crank main bores were align honed could it raise the crank high enough in the block to cause this interference?

 

No....max movement up from line hone is just a couple of thousands......and even if it could be more, the rod length, crank pin centerline, and rod journal centerline doesn't change......if you move the crank up it move the piston up relative to it.....

Jebby

 

I'd overlooked that little bit of the geometry...
Thanks.

 

Not sure what's going on...but if everything cks out otherwise with rods etc....I'd probably clearance the pistons as needed and leave the crank alone. You're not going to remove much weight from aluminum and the very slightly lighter piston won't impact balance much overall as long as it was close to start with.

How close to the top of the deck are they at TDC?

JIM

 

I would double-check deck height, stroke length, crank journals, etc.
All eight should be the same.
The crank could have been re-ground badly. Which crank do you have? Pics?

Lots of possible issue areas. Lots of weird things could have happened in 50 years.
I would want to be sure which part is "off" before I started grinding on anything.
Let us know what you find.

 

Something is off here. How far in the hole are the pistons at TDC? I’m wondering if you have a piston or rod mismatch.

 

I wonder how many times have the Rods have been re ground. A set of modern CNC machined Pistons with there shorter skirts would quickly remedy or just chuck the Pistons in a Lathe and take off a little bit on all 8.

 

Plus one on checking this

 

Yes check each piston deck height. Actually twice. Piston all the way up, and all the way down. That will also give you the crank's stroke for each cylinder and tell you if the crank has been re-ground sloppily, or offset ground on purpose, etc. I have seen that happen more than once. A good 6" digital caliper would be accurate enough for this. You are looking for one or more cylinders having too long of a stroke by say .010", or even .020" , "ten or twenty thou". They should all match and be stock stroke length. Unfortunately many machine shops do not check this, and some do not get the rod journal well enough centered when rod journals are repaired. Only good race engine builders check this. That could definately get you in trouble with piston skirts.

Now if the rod journals are still uncut this is not likely. If they have been cut, you should check. A 427 should have exactly a 3.7600" stroke length. Many 4 bolt engines have been raced over the years and any number of racing "tweaks" could have been applied. Grinding the rod journals .010" or more offset was an old racing trick to give you a couple of more cubic inches, and a little more compression, in the days before inexpensive stroker cranks.

A 4 bolt block should have a steel crank in it also. With 1/2" wide cast lines that look to be ground off. And rings like a bell if you tap it.