What head gasket/piston combo for 10.-1 - Harley Forum

outback2970 · #1 02-08-2009, 10:40 AM

Got my andrews 37g's, SE billit support plate/ high vol pump and have decided to do the 95" BB. My question is what head gasket thickness and piston combo will keep me at 10-1 comp. I need part numbers for a good piston if you got em. I'll have to leave the heads for another year though. I'd like a little more comp than stock 9.2, to let the 37s work, but keep a nice easy starting, cool running reliable motor.

milindh · #2 02-08-2009, 01:13 PM

Hey Outback

I was going to tell you to call the makers of the pistons you're interested in and ask them directly but then I thought you might be an Aussie so that wouldn't be feasible. I'll bet you could get an email answer however. Either the piston maker or the head modifer guy would definitely be the best source for your answer. Keep in mind that if you do the head job later the configuration of the combustion chamber may (or might not) be modified and that will change your compression ratio again. I know as part of their basic hop up kit Branch Flowmetrics welds up part of the combustion chamber and grinds in a differrent shape as well as flowing the ports. In your case just going to a BB kit your CR will automatically go up and you're right in assuming that the appropriate head gasket will be needed to achieve your 10:1 CR. I would just make sure that the pistons you choose will be compatible with your planned head mods. The best way to assure that is to talk directly with whoever is going to do the head mods.

Good Luck Dude

outback2970 · #3 02-08-2009, 07:35 PM

Thanks for the advice...I'm just here in the Great White North, Koo Loo Koo Koo, Koo Loo Koo Koooooooo! Ah the call of Canada. I guess I was looking for some suggestions from those who have a similar build. What you used, how you like it etc. I hear ya though. Every builder seems to have a different opinion.

Wrightturn · #4 02-14-2009, 10:25 PM

Hey Out,

With stock heads don`t go thinner than .040 or even the .055. Give up a couple hp and run cooler longer. Have done a couple with the stock heads and 37g that run strong and cooler with the .040. The 37g will cover the 95 ci into stock heads.

northeastconfederate · #5 03-31-2009, 04:28 PM

I am building a similar engine. In my research I found Kieth Black has a forged piston, part number KB905C, with stock combustion chamber volume of 85cc they say makes 10:1. There is also a compression ration calculator on their page where you can input gasket thickness, bore, cylinder bore, deck height, etc., and get a calculated compression ratio. Personally I am running an S&S 585G cam and will have 10.25:1 compression. This I am going to achieve by running Kieth Black KB411 pistons, which are hypereutectic cast. The dome will need to be machined slightly, as they yield a 10.5:1 ratio out of the box. I am running Cometic MLS gaskets which are 0.030" before torquing heads. Stock gaskets are closer to 0.050". My local shop owner (who has been building hi-perf HD's for 35 years) says I should NOT use the thicker gaskets. Better to knock down the dome a little bit, as the thicker gaskets give more opportunity to fail with higher compression. Check out the KB website,
United Engine and Machine Co.
You have to select "motorcycle pistons" from the drop-down list under KB products to get to the HD pistons, but they are there, even though the site looks to be geared mainly to race cars.

I found the KB411 pistons to be inexpensive, and I also found a head porting guy who guarantees your stock heads will outflow anyone elses, including out of the box aftermarket heads. Obviously, if you start with S&S or STD heads and have them ported you'll be able to get higher flow numbers, but for the street who needs that?? This guy will tailor your porting job to your engine combination and your specific needs, as far as riding habits, etc.. Shoot me an email, I'll hook you up with him if you want. If you're careful, you might be able to get the whole thing done, heads and all for a lot less than you think. My pistons, boring (my cylinders were already 95", I only needed a 0.005" overbore), and head porting (including shipping) are costing me right around $1000. It'll cost you that to have Branch do just your heads.

northeastconfederate · #6 04-02-2009, 10:07 AM

Outback,
I am building a 95" TC also, and found Kieth Black pistons makes several affordable, quality pistons that will get you where you want to be. They have a forged, coated skirt piston, part #KB905C that yields 10.0:1 with stock combustion chambers. They also have a 10.5:1 version of the same piston, KB906C. This is the one I'm using and found it to be very affordable, only about $10 over the cost of their hypereutectic cast 10.5:1 piston, KB411. This is the one I was planning to use until my local bike shop owner discovered the price of the forged piston was so close. Also, I found a guy who hast vast experience with cylinder heads who does a very resonably priced job tailored to your specific engine combination and riding habits. I am going to be able to bring in the boring / fitting of pistons & rings and head porting both in for right around $1000, doing all the disassembly / assebly work myself. That's about what you would pay for Branch / O'Keefe to rework your heads. Email me and I'll hook you up with this guy if you're interested. You owe it to yourself to at least call him. As far as gaskets go, my local engine builder (who has 35+ years experience with hi perf and racing HD engines) says I should NOT run stock-type head gaskets, which are about .050" before being torqued, and so does the head porting guy. I'm running Cometic MLS gaskets, .030" before torque. Cometic has several different thicknesses available, and I think they'll make whatever you want. Check it out. Also, the Kieth Black website has a compression ratio calculator where you plug in head volume, gasket bore, thickness, piston dome volume (which their site gives for each of their pistons), cylinder bore, rod length, etc, and calculates compression ratio for that combination. Also if you plug in your cam timing it calculates dynamic compression ratio, which is really more important than static, which is what we all talk about.
United Engine and Machine Co.
There's a link to the KB website. Check it out and good luck!

VRYQUIK · #7 04-17-2009, 11:47 PM

Quote:

Originally Posted by outback2970

Got my andrews 37g's, SE billit support plate/ high vol pump and have decided to do the 95" BB. My question is what head gasket thickness and piston combo will keep me at 10-1 comp. I need part numbers for a good piston if you got em. I'll have to leave the heads for another year though. I'd like a little more comp than stock 9.2, to let the 37s work, but keep a nice easy starting, cool running reliable motor.

I ran across your thread and I thought I would put in my two cents.

You need to decide what "Dynamic Compression" that you want to run before you can correctly adjust "Static Compression" to make it work.

Most people want to make the DCR at around 9:1 which will give you a cranking pressure of around 185 lbs.

With the TW37G having an intake valve closing of 42 degrees, 10:1 compression ratio should be just about right to accomplish a 9:1 DCR.

I just built a 95" using a Woods TW-8 cams and a 5 degree advanced gear. That makes the intake valve closing the same as the TW37G at 42 degrees.
I used one of my .032" copper head gaskets which it had a cranking pressure of 185 lbs. using a stock combustion chamber and SE forged high compression pistons. We will be entering it in a dyno shootout 4-18-09 at Thunder Tower HD SC.

I've experimented with a lot of head gaskets but, I have found the copper head gaskets with copper sealer to be just about indestructible and no oil seepage. That's why I now stock and sell them.