700cc headgaskets

jay760

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According to Gary at Hoos racing a regular factory head gasket will work for a 700cc kit that he sells.
 
According to Gary at Hoos racing a regular factory head gasket will work for a 700cc kit that he sells.

The guy who makes the head gaskets says they should be slightly larger than the bore, I asked hoos what gaskets he was using and didn't like the thought of the gasket overlapping into the combustion chamber, there is also a thread on here with a 700 with a noisy top end, could be where it's hitting the gasket
 
The guy who makes the head gaskets says they should be slightly larger than the bore, I asked hoos what gaskets he was using and didn't like the thought of the gasket overlapping into the combustion chamber, there is also a thread on here with a 700 with a noisy top end, could be where it's hitting the gasket

I can't say I disagree with you. I'll be ordering the proper 700cc copper gasket for my XS1 big bore motor, but I just wanted to pass along the info from Gary.
 
The guys at Hoos know what they're doing, and I can tell you from a few years of experience with 700 cc. motors that gaskets with stock ID work fine with 77.5 mm. pistons. I've posted this before, and I'll post it again. Stock 75 mm. pistons are very forgiving when it comes to dropping the deck, but most 77.5's and 80's are not. If you drop the deck even .010" with 700 cc. pistons, you'll need to check clearance between the edges of the piston crowns and the squish bands (beveled edge of the combustion chamber) with solder crushes. Unless you're using performance rods you need at least .050" clearance there, and most big bore pistons won't give it to you when the deck is lowered by any significant amount. If you don't have adequate clearance you'll need either to use OEM thickness gaskets or have the squish bands recut. That isn't an easy modification; the machine shop will need the whole engine brought in so that combustion chambers can be centered to the pistons, and achieving accuracy will involve several rounds of cutting, assembly, and measuring. There are few shops that can do it at all, and fewer still that can do it well.
 
If you have squish issues from decking,head milling,boring why not run a thicker base gasket? Maybe the place mentioned above can make them?
 
Right, if squish clearance has been closed up due to mods, you can regain clearance with a base plate or thicker gaskets. It doesn't matter if you lift the deck from the bottom or the top.
 
Right, if squish clearance has been closed up due to mods, you can regain clearance with a base plate or thicker gaskets. It doesn't matter if you lift the deck from the bottom or the top.

But, just thinking, wouldn't it be safer based on compression and combustion to do it from the base?
 
Doesn't make much difference re. compression whether you move the cylinder base or the head. Either way, you're moving the head relative to piston dome position at TDC, and thus changing combustion chamber volume and static compression ratio. Moving the head would have a very slightly larger effect than moving the base the same distance, since the clearance between piston dome edge and fire ring of the head gasket figures into the volume calculations, but the difference is pretty minor.
 
Nope. Athena replacement head gaskets run a full .060" thick (versus OEM thickness of .047"). The late John Underwood used to use two head gaskets puckied together with Indian Head shellac to drop his CR to 8:1. As far as heat's concerned, that's why the fire ring is there.
 
Nope. Athena replacement head gaskets run a full .060" thick (versus OEM thickness of .047"). The late John Underwood used to use two head gaskets puckied together with Indian Head shellac to drop his CR to 8:1. As far as heat's concerned, that's why the fire ring is there.

OK,

Boy there's a blast from the past!
 
You can still get Indian Head at NAPA, if the guy behind the counter is old enough to know what it is.

Guys, I had to learn the hard way that boosting the CR by dropping the deck on big-bore pistons isn't a good way to go. You're better off to start with a high compression race piston and mill the top down to get to a streetable CR. I used that procedure on my first 700 cc. engine, knocking .030" off the crowns of a pair of 11:1 Venolias. The CR was still pretty stout, and after a few nervous episodes with octane boosters in places where high octane gas wasn't to be had, I decided to used a pair of 8.6:1 JE's from Shell and drop the deck by .030" to get to ~9.5:1. I'd done up to .040" on stock pistons with room to spare, it ought to work fine, right?

Wrong--there was hard interference at the squish the first time the crank was turned. Then there was the trip with the motor to FTZ Racing in Cape Girardeau, MO, where they cut the squish bands for a very low price--and left me with clearances that would have grenaded the motor. Then there was the trip to Charlie Southgate's shop in Nashville, TN, where the right motor cover was stolen, my wrapping was removed so that the motor got covered with grinder debris, and I took the motor back after 10 months with no work done. After stripping the motor to the cases and flushing Mr. Southgate's grit out of every part, I decided I'd jumped through too many hoops over a half point or so of compression, and reassembled with OEM deck height.

But it's all good. The 8.6:1 CR is enough to work well with Jack's port work and the Taiwanese camshaft, and performance is lively enough to be satisfying. When I get ambitious I'll have another .010" milled off the crowns of the Venolias and drop those on with fresh cylinders and the trusty old Shell #1 cam, but for now I'm satisfied.
 
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One more clarification. OEM replacement head gaskets are made to accomodate overbores, and there's plenty of clearance left at 77.5 mm. They get nowhere close to overlapping the bore.
 
One more clarification. OEM replacement head gaskets are made to accomodate overbores, and there's plenty of clearance left at 77.5 mm. They get nowhere close to overlapping the bore.

Wish somebody told me that when I asked the question in another thread.
 
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