Block line boreing. Does it work w/ new/used caps?

[Ray G.]

Hello;

When Line boreing/honeing engine blocks is it possible to get a true bore?

What are the numbers such as "15 , 23 , 24 , 27" stamped into the caps on the bottom surface between the main cap bolt heads?

Does removeing some material from the mateing surface of the cap give enough material to get a true bore?

Thank you in advance for your help.

Ray

[Jack H.]

Re: Block line boreing. Does it work w/ new/used c

"When Line boreing/honeing engine blocks is it possible to get a true bore?"

Yes, and a good machinest can actually get the align bore BETTER than factory original as they had allowable tolerances that would accept slightly 'off-true' alignment...

[Duke W.]

Re: Block line boreing. Does it work w/ new/used c

I don't recommend align boring the mains unless it's absolutely necessary. Inspection of the main bearings and crankshaft journals will give you a pretty good idea of alignment, and a further check with a machinist's bar and .0015" feeler gage will confirm if the main bearing bores are in good alignment or not.

Most OE machined blocks are okay, but some machine shops want to do this job on all blocks as SOP, which I don't agree with. Only align bore if inspection/measurements dictate that it's required. Align boring will throw off the deck heights, which can be a problem when you assemble the engine and are looking to achieve correct deck clearance equal on both sides.

Duke

[Ray G.]

Re: Thank you Jack & Duke. Anyone else ?? *NM*

[Todd A.]

Re: Block line boreing. Does it work w/ new/used c

Ray,
I support Duke's opinion to avoid line boring unless necessary - if you are missing a cap or two from the block or get new caps, there's a chance you'll have to bore them to bring all in line.
As for the block, in my experience with medium speed marine diesels I haven't had much luck if the main bores are misaligned outside of factory spec. It has usually meant the block was tweaked/twisted. In the two specific instances I have seen, the bores were found out of spec after overheats. In each case metallurgists said the blocks were materially fine, so the blocks were align bored with new main caps throughout. Both engines ran approx 3000 hours before locking up. The engines were align bored and rebuilt again but only ran 5700 hours and a little over 6000 hours before locking up. (Normal overhaul interval is 20,000 hours). Might be apples to oranges, as Im not sure if anyone has seen congruent problems with car engines.

[Verle R.]

Re: Block line boreing. Does it work w/ new/used c

As Jack said, GM had tolerances for the crank bore.
I suspect they compensated for those that may not have been true by installing GM factory only inserts with "unusual" sizes.

I have disassembled untouched engines with GM insert sizes of .001 .002 007 and possibly other sizes that I don't now remember, not all the same but varying top and bottom across the mains.

Duke is correct in stating it will alter the deck height. For an "average driver" that is probably not significant. For a high performance engine it is.

In answer to your question: "Does removing some material from the mating surface of the cap give enough material to get a true bore?" Yes, unless it is seriously out of true, in which case the block is almost certainly toast.

Verle

[Clem Z.]

Re: Block line boreing. Does it work w/ new/used c

"line honing" if done correctly will remove very little from the block side. if the block has spun a bearing "line boring" may be necessary and this will remove more material from the block side.they sell timing chain kits with gears that have a larger "pitch diameter" gears to make up for the difference the crank is closer to the cam. if a crank installed in the block with the caps torqued to specs can be spun freely by hand the block needs no line honing

[Joe L.]

Re: Block line boreing. Does it work w/ new/used c

clem-----

Your last sentence describes the "ultimate test" which is often overlooked.

[Pat #25912]

If the crank has been moved closer ...

to the cam, will the front of the crank be slightly off centre in the timing chain cover?

[Clem Z.]

Re: If the crank has been moved closer ...

you are only talking about .005"/.010" which will not cause a problem with the seal.