Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE block

[Gary K.]

Starting to feel guilty about all the great advice I am getting but this is really the only place to find out: machine shop says my block is in great shape, surfaces straight, all good except out of taper .0035 whereas factory allowance they say is .0015 max. I have TRW #3888304 pistons which I think are forged and I also think that means higher taper up to .0055 is allowable? That's what the Tom Wilson big block rebuild book says but I am really not sure.

I don't want to bore 30-over a perfectly good block for a couple thousandths... any suggestions? My inclination is to rebuild the motor as-is and get on with my life. Thanks in advance for the help.

[Joe R.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Rebuild it once right or rebuild it twice! Hopefully correct the second time!!! Out of taper means different things to different machinists. I consider any block out of taper needing boring! Any block that is worn 0.004 should be bored.
In building a BB the following is essential:

1. boring
2. valve guides & seals
3. springs & retainers
4. cam bearings
5. cam
6. lifters
7. pistons
8. rings
9. main and rod bearings
10. rebuild the rods & new rod bolts
11. new oil pump & screen
12. timing chain & gears
13. dynamic & static balancing
14. surface flywheel

After that exercise the engine will be new!

JR

[Duke W.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

I don't understand what you mean by "out of taper". The bores have either "taper" or "out of round", or both. Which is it, and what are the taper and out of round measurements for all eight bores?

Why was the engine disassembled?

I also don't understand your statement about the pistons. Perhaps you mean piston skirt clearance can be up to .0055", but that's a totally different issue than cylinder taper and out of round.

Duke

[Larry B.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

I agree with Duke what does out of taper mean. A fresh bore with .0015 taper would be a horrible job. Your .0035 isn't all that bad but not great. Most ring manufactures allow .007 to.008 as in spec for used motors. Your .0055 should be piston clearance on a forged piston.

[Domenic T.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

When I worked at the dealership it was comon not to bore for a taper that small. What it will do is wear out the top ring land with any taper if it isn't already out of limits because of the ring end gap movement every time it enters & leaves the taper. Usually only the top ring land because of the higher pressure behind the top ring.
There was a special ring set that had 3 oil rings (of the 3 piece oil ring) in the 2nd groove that was good for a few more miles on a wore out block. The set was used for up to .020 taper and mainly sealed compression from the 2nd groove. That was one of the starts of the gapless ring because all end gaps never lined up in the 2nd groove. We had gapless rings in the 50's and never knew it when we would take a wore out ring land and turn it .030 larger and slip a .030 oil ring under the larger ring to space it in the groove. Now that is the gapless ring. The compression was as high as you wouls ever see.
Those were desperate rebuilds in those days but in a strairht bore 2 rings in the 2nd groove works great for compression.

DOM

DOM

[Gary K.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Thanks all - got plenty of Q's to bring to machine shop when I get the block. I have a '68 Chevy shop manual that gives some 'max taper' figures so I have to fully understand those specs too.

My #3888304 pistons I believe are forged. In reference piston-to-bore clearance the Wilson book 'how to rebuild big block chevy' says .0025 max wear clearance for cast pistons, but .0055 - .008 for forged "depending on horsepower....the higher hp the more P-t-B wear clearance allowable" etc etc, and that would make me happy. I dont put as much stock in Wilson as I do Chevy but it is still a solid reference. Would be interested in your opinions on forged vs cast piston clearances. Thank You!!

GK

[Duke W.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Piston skirt clearance is measured below where any taper wear typically occurs. Taper wear is usually limited to the top 1/2" of piston ring travel. There's really no relationship between piston clearance and bore taper.

You should follow the specs/wear limits in the '68 CSM and COM. Even if you have only one cylinder that has .0035" taper, I would bore to the next oversize if I wanted an engine to last for the long haul with final power honing to achieve the recommended clearance with proper stones for moly-faced rings. If the plan is to sell the car, then you can just through it together and let the next guy solve the problems.

Since what you have is basically a L-71, I would rebuild it with the Federal Mogul OE replacement pistons, which are forged, in the next oversize.

You have yet to tell use why the engine was disassembled. Some problem? If you put it together without addressing the problem, then nothing is solved and the whole exercise is pointless waste of money.

The soundness of the advice you get here is function of what information you provide.

Duke

[Gary K.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Engine was torn down due to a loud Ka-PING and subsequent undrivable condition while cruising at about 70mph. I suspected a valve train issue but when heads were taken off a couple bent pushrods, spent lifters, flattened lobes, a burnt smell and two 'toasted' top of pistons cyl 2&4 were evident. Worried about cracked block, rods, who knows what I tore it down to find a pristine short block, crank, bearings, etc like factory new. If I had known that I would not have torn down but it looked bad - but only on top. Car will never be sold - but I really do not like to fix things that don't need fixin'.
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[Timothy B.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

I am no where near as good as others here but If it were mine I would seriously consider a deck plate and smooth hone and put it back together.

[Gary K.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

The 68 Chassis Service Manual by chevy states .005 cylinder bore taper limit for service, then it actually does state higher 'service' piston clearance the higher the HP as other references state. The 435hp 427ci max piston clearance does not state skirt or top just 'clearance' max .0085. Going to see the machinist today so will find out more but as usual appreciate any opionions.

[Duke W.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Piston diameter is measured at the skirt, perpendicular to the pin bore, and bore diameter is measured about the same depth in the bore with the piston at TDC. The difference is what is commonly called "piston clearance". Forged pistons require greater skirt clearance than cast pistons. Top land clearance is much greater with greater tolerance and usually doesn't need to be measured.

.005" taper service limit is a lot. If the car is a keeper I would bore the block to the next oversize piston, but you have a bigger problem.

The type of catastrophic failure you experienced calls for a forensic investigation. There was something wrong inside the engine that caused the failure, and you need to positively identify and correct that problem or it may happen again. My hunch is that there is something amiss in the oiling system.

Look at the nearby thread on big block cams and rear cam bearings. A '73 CE block should have the single hole rear cam bearing, and even though a cam with a rear groove should work I expect the cam has no rear groove.

What is the cam ID number cast into the blank and does the cam have a groove in the rear journal? ID 3904366 was cast without a groove, but a note on the drawing says it can be used to make a 3863143 by machining the groove.

ID 3863144 was cast with a rough groove, but required finish machining to produce the 3863143 cam/pin assembly.

Duke

[Larry B.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Piston clearance is determined by the piston not the bore. Example if you bored the block .030 over you would have a finished bore of 4.280 and the new piston would mike 4.275 for a f orged piston if that's the clearance the manufacture wanted. Remember when you just hone the block you will increase the clearance. Your cam to lifer wear is not caused by the rear cam bearing or a grooved rear journal. That rear journal would shut the oil off to the lifters and rockers but in no way feeds the area of the cam to lifter wear area. You did not say the rockers were worn or dry. A loose valve train can cause excess lifter wear and bend pushrods.Some of todays oils are missing the zinc and won't take wear pressure on the cam and lifter with the 1.7 ratio and the higher spring pressure. That cam to lifter wear is the highest scuffing action in the motor and it is basically only oiled by splash. You can get cams with .002 taper across the lobe which helps a bunch as the lifter will rotate better.

[Gary K.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Larry, before the "ka-PING" moment at 70mph I had covers off because I heard valve train noise. Should have adjusted lash at that time but oil was flowing beautifully throughout the valve train and I chalked it up to "mechanical lifters are noisy" but perhaps I have learned a tough lesson. The Sealed Power cam I'm ordering CS165R w/AT-992 lifters - does anyone here believe this stock setup needs any work like the small taper you describe. If any advice on pushrods, springs & rockers too, I'm getting geared up either for a valve job on my incorrect-but-excellent '67 -391 heads or correct -840 heads purchase. Normally I won't do anything to stock parts but that's why I'm here - to get the best ideas from experienced people. As usual - THANK YOU for all your time & advice.

[Larry B.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

All cams have the taper, that's what rotates the lifters. Most stock cams have .001 taper. The pushrods and rockers no big deal but don't get carried away with springs, and use a good break in oil. One thing I like to do is put a white paint dot on each pushrod then start it with the covers off. If the dot ain't spinning the cam will not last. A lot of times you can see the pushrods spin on assembly when adjusting the valves. You can always see it with the .002 taper. I'm just not a big fan of 50 year old cams ground by Sealed Power or others .I prefer CNC with a little more technology .

[Gary K.]

Re: Big block out of taper .0035 should I bore or leave it alone? '68 Vette, '73 CE b

Thanks everybody for the input. For the benefit of the next guy with his block at a machine shop: results were .003 to .0035 piston clearance all 8 cylinders at skirt area which is well within the chevy 'service' specs for 435hp 427, not out of round, no excessive taper in upper cylinder, no cracks no issues it's going to be a re-ring w/new bearings all around, new cam, lifters, pushrods, timing chain & valve job. Shop of course wanted to bore .030 over but Chevy overrides them in my book. Thanks again!