[Harley]

Lord knows I've built my share of SB chevys
but this one has me stumpted.
No 7 exaust lobe eaten before end of break in. Safe to asume that I did all the basics
i.e. Cam lube, pre oil etc. 45-50 lbs. oil
pressure at gauge. But pressure weak on top
just little trickle at rockers.
I thought I had left the gallery plug out on
the front of block but they are there.
plenty of flow at oil filter ( all over the floor, but there ). Cam bearings installed as
usual.
What do Ya'all think?
Harley

 

[tony leep]

harley,
I have only seen this happen on high milage motors.maybe it was a faulty cam from factory.If you bought it yourself it should have came with a warranty! good luck tleep

 

[BillK]

Harley,
I have seen it happen. Usually it is caused by the lifter being too tight in the lifter bore and not being able to rotate properly. You can usually tell this by the "straight" wear pattern on the lifter.
I have also had cams that were just plain defective, sent them back to the maker and they tested them and said that the heat treat was not correct. If it is a name brand cam, send it back. Most of the manufacturers are very anxious to keep you happy and will replace the cam and lifters.
Hope this helps,

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

 

[Harley]

I figured the cam problem was metallurgic.
Lifter bores are within .0002 of each other.
Doesn't explain low oil pressure at the top
end. I pulled the last two lifters (8 ex-7ex
) plenty of flow there, just not getting
through to rockers.
Oh well have to start over with new bearings
anyway, maybe I'll find the problem during
that process.
Harley

 

[DUANE OCHS]

Harley: Been there and done that. Last year in Sept. I did a cam and intake change in BBC 454. The motor ate 3, yes 3 cams during break in. Did all of the basics also. Tried 3 different cams from 3 different manufacturers. No body could tell me why or what was wrong. I was about ready to say screw it and put old one back in, at least it was working. Found a guy that had one of the same cams that I wiped out. Bought it from him already broken in, put it in in Jan. and am still running it and am very happy with it today. I have gotten many opinions on why the cams might have gone bad, but still do not know for sure. Good luck on your next try. Hopefully the next one works and you don't have the frustration that I went thru.

 

[Gene Chas]

You guys ever heard of "core shift"? I'm really not that familiar with it but here's the deal. My 427 ( 1973 454 block )during it's last rebuild was found with a wiped cam. Strange since the engine probably had 10,000 miles on it ( and lots of trips to 7000 ). Anyway, block was measured and found to have core shift. New cam/lifters installed ( same Crane grind Commander solid 310 )with special .002 ground lifters From Crane). This engine now has another 15-20K on it. It's going into my '67 without a rebuild. Cam is barely shiny. Looks real fresh as did #1 rod bearing. Core shift. Yup. I've never heard of this before. Apparently one of the benies of rollers is that core shift is a non-issue. If you need more detail, I'll ask my buddy who rebuilt the 427. Back in the mid to late 70'2 when I worked in a machine shop, we had never expereicned such a thing.

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[This message has been edited by Gene Chas (edited 06-10-99).]

 

[Harley]

Bill, it was spinning as the wear is cupped
at the lifter and cam was rounded at what was left of the apex of the lobe.
Harley

 

[Gene Chas]

Harley, check my post, that was the same wear pattern as the 427.

------------------

 

[DUANE OCHS]

Someone mentioned to me about core shift but did not know much about what happens. Maybe BillK can explain this phenomon to us. He seems to know a lot about machining and building motors. How about it Bill. Ever heard of it, and explain what it is.

 

[Harley]

Core shift is the the incorrect position of
one part of the mold in relation to other componits of the mold. It's total effect on
the rough casting depends on which one and
how much. However it's effect on the machined
block is only an issue if there is no material there to cut or not enough.
Try to imagine that the machines are going to
try to put a bore in or deck a surface, whether there is iron there or not.
The problem lies with the factory machining.
the tolerances allowed by GM on some locations is pretty big i.e. cylinder locations, Lifter locations. Etc. etc.
( too involved to try to explain factory tolerences here). Suffice it to say that
"Core shift" is predominatly a rough casting
problem and has little effect on the finished
block ( unless the shift is so bad that it
deflects the cutter.) Another fine point is
"Stress relif". ( before and after machining).
Harley

 

[jholp]

Core shift occurrs at the foundry where the engine block was cast when the cast iron is still molten - during the cooling cycle.

You can do some preliminary visual inspection by looking at the cam bore on the front of the engine. If the cam hole appears to be in the center of the boss, meaning the band of material around the flat of the cam bore is equal to the eye then there isn't any obvious core shift.

But to be sure have the block sonic tested, it will give you a lot more information.

You never stated what the cam specs were. If it is a killer grind then coming with that will be killer spring pressures and things can get very tricky then you do the break in. They make special break in springs for instances like this. You do the break in around 2000 to 2500 rpm so valve float is not a problem with the whimpy springs. When the cam and lifters are use to each other (broken in)then in go the killer springs.

Some of the high dollar shops will "blue print" the lifter bores by drilling them over size, insert sleeves and last drill/hone the sleevs so the centers are spot on.

John

 

[DUANE OCHS]

This is all good info. but if the core shift occured at the foundary, you would think that the original cam when the motor was new would probably have been wiped out also. The cam that I pulled out was the comp 292h, a fairly radical cam and went with a smaller grind and less lift and wider lobe separation angle. I went from .550 to .540 lift, 224 int. and 238 exhaust @ 114 deg. lobe sep. I primarily wanted less overlap from intake to exhaust for emissions. The previous owner said that he had no problem breaking in the 292h cam. All of the cam manufacturers assured me that the springs for the 292h would not need to be replaced with the cam that I selected. Also, why would I not be able to break in a new cam yet be able to use a broke in cam with less than 200 miles on it?

 

[jholp]

Harley & Duane Ochs,

Sorry my thoughts don't seem to apply. You have very unique instances here and frankly I am at a loss after reading all of your postings.

Sometime back there were postings on how to do a break in and many of us went to lenghts of how to do it, with others being rather cavalier and indicating that we were pole valuting over rat s#@t with all of our antics.

It appears that you did everything I would have and still came up with cam failures. It kind of shakes the confidence factor.

John

 

[BillK]

Hi guys,
Core shift does not affect the position of any of the machined holes in the block. It only affects the amount of metal around them. This can be a concern when trying to bore a block way oversize, such as in a .125" oversize 454. Core shift cannot contribute to a cam wiping out prematurely.
What core shift means is that the casting molds have moved around some during the casting of the block. The molds are made out of sand held together with a binder. The holes in the side of the block that everyone thinks are for "freeze" plugs, are really to allow the casting sand to be removed after the block in cast. Anyway, if the cores for the cylinder holes move during casting, the walls will be thicker on one side than on the other, affecting the amount you can bore the block without it getting too thin.
As John mentioned, one of the easiest ways to check for core shift is to look at the front of the block where the camshaft bore is. If it is centered pretty well in the casting, there was not much core shift.
Bottom line, core shift is not really a concern in most street and even race motors. It only comes into play when you start making serious HP.
Hope this helps,


------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

 

[Tom Mobley]

Harley, How much clearence do you have between the bottom of the valve spring retainer and the top of the valve guide at full lift? How much clearence is there between the individual coils of the spring at full lift?

If you know these numbers, are they the same for all cylinders? Have you checked it yourself?

One engine I looked at recently had one valve that was 0.100 shorter that the others, nobody noticed it during assembly. This engine ate a couple cams too.

Tom