volvoclearinghouse SuperDork
6/28/15 9:17 a.m.

We run a 460 BBF in our '75 LTD Landau 24 Hours of LeMons car, and before the next race we decided to unbolt the intake and heads, clean everything up and put it back together with fresh gaskets.

Obligatory picture of the old girl:

If you've got 23 spare minutes, here's a video of the full teardown:

If you don't want to sit through the whole video, skip to about the 21 minute mark or so. Upon yanking the heads, we discovered the pistons were stamped "0.060", which means they're 0.060" oversized. Now, the stock bore on the 460 Ferd is 4.360", so 60 thousandths over would be 4.420". However, Summit Racing sells no gaskets in this size; in fact, I can't really find any head gaskets for this bore size. I'm either pulling up gaskets for 4.400 bore, or bore sizes over 4.420 (like 4.500").

Two questions:

1) Assuming I should use a gasket with a bore size slightly larger than the actual bore of the engine, how much larger would be OK?

2) What thickness gasket should we use? Obviously, I'd like to use the thinnest gasket possible, to increase compression (this engine still runs stock heads and pistons with valve cutouts, plus overboring generally slightly reduces the static compression ratio). But I wonder how thin is "too thin" for this setup.

Any help would be greatly appreciated!

Streetwiseguy PowerDork
6/28/15 10:24 a.m.

I've never considered the problem, as a stock gasket is generally large enough to accept the overbore size. Its entirely possible I'm wrong, but I've never encountered a problem.

As thin as possible, without it leaking would be my second answer.

dean1484 GRM+ Memberand MegaDork
6/28/15 12:34 p.m.

If nothing else is changed over boring a motor increases compression

volvoclearinghouse SuperDork
6/28/15 2:29 p.m.
Streetwiseguy wrote: I've never considered the problem, as a stock gasket is generally large enough to accept the overbore size. Its entirely possible I'm wrong, but I've never encountered a problem. As thin as possible, without it leaking would be my second answer.

I guess I'll have to measure the bore on the old head gasket and see what it is. I'm also unsure if head gaskets are sold in "actual size", or "meant for this size bore".

I guess I'm wondering if anyone has any experience with figuring out how thin is too thin, i.e. will the valves hit, etc.

Thinking about going a slightly un-LeMonny route and taking the heads to a shop to have them cleaned and checked for flatness. A hot tank would do them wonders.

44Dwarf UltraDork
6/28/15 8:54 p.m.

Bike motors you normal order gaskets by bore size but its rare in car engines unless you go copper. Measure your old ones or check carbon pattern on the heads if you can't find the old gaskets. Also check the short block at TDC is the piston still down in the hole a ways? if so as long as the fire ring (folded over metal ring) is siting on the block and not the fiber part hanging in to the cyl you'll be fine.

volvoclearinghouse SuperDork
6/29/15 9:02 a.m.

I'm thinking I'm going to make some more accurate measurements, cc the heads, etc. Thanks for the input!

volvoclearinghouse SuperDork
6/29/15 12:56 p.m.

Not sure how cc-ing the heads costs money. I can do it for free myself. Its more just to know what we have, rather than toss a gasket on and call it good. It's LeMons, so we'll do it cheap, but no half-assing it, either.

curtis73 GRM+ Memberand UberDork
6/29/15 8:37 p.m.

Going with a larger gasket bore is fine. Going really big will add noticeable combustion volume, but not really much of an issue.

Going too much thinner is a potential issue, however most 460s have plenty of piston-valve clearance since they have pretty open chambers. What I like to do is use the old play-doh method. As long as your pistons have negative deck height (don't pop up above the deck), I like to check them without a head gasket to be safe. Just bolt them on snug (maybe 20 lb-ft) and rotate it with the play-doh on the piston. If you need additional distance, just order the thinnest gasket that gives you enough space.

Another thing to consider; you do reach a point of reduced returns. If your pistons/chambers have a quench area, going too close can increase chances of detonation and is theorized to reduce power because of trying to squish air out of such a tiny spot. Think of it like dropping a sheet of plywood on the floor. It freefalls until the last little bit as the air has trouble escaping from under the plywood. Less than .025" starts getting squirrely. .035" is kind of a sweet spot. Anything more than .080" starts rapidly diminishing the benefits of quench.

volvoclearinghouse SuperDork
6/30/15 7:01 a.m.

The general interwebs consensus is the heads on our engine (from an '86 Ford truck, last of the carbed heads) are closed chamber, about 95-97 cc according to teh internets. Piston dish is 22 to 24 cc. Deck to piston is cited as anywhere from 22 to 32 thousandths- ours look pretty close to flush (slightly "in the hole", as they say), but it's easy enough to measure once I find my calipers. So, bored 60 over, and assuming a 40 thou' head gasket, we'll be running right around 8:1 compression. Since our pistons sit in the hole, quench is kinda terrible, and 8:1 should be pretty safe to avoid detonation. We'd have to either deck the block or get taller pistons to really hop this engine up, and we're not in for that kind of work before the next race. Also, the exhaust ports on all 460 heads kinda suck, and need much cleanup to make real power.

MrChaos GRM+ Memberand Reader
6/30/15 8:41 a.m.

add boost to make up for low compression.

volvoclearinghouse SuperDork
7/1/15 8:03 a.m.

In reply to GTXVette:

No worries, I'm still learning the BBF. What I'd really like to do is buy a second engine and tinker with it in the spare time and build up a nice spare in case this one craps out. Find a set of D0VE heads, play around with porting, get some zero deck height pistons, run a lumpy cam and 10:1 compression, etc. I think a 400 HP/ 500 ft-lb monster would be pretty easy to build, even on a LeMons budget.

volvoclearinghouse SuperDork
7/31/15 8:11 a.m.

Thanks for all the input and help, the 460 now lives!

It's like 2 months before the race, and our TO DO list is surprisingly short. knock on wood

Oh, and I did pick up a spare 460, out of a mid-80's motorhome, with like 31,000 miles on it. And a spare C6 transmission. We're actually prepared.

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