Which head gasket for a 4.280 bore?
 

Looking at head gaskets for my 489 build - it has a .030 overbore on the 4.250 bore, for 4.280 total. Cometic makes two gaskets for this application in the .027 thickness - c5337-027 has an advertised bore of 4.337", and c5817-027 has 4.320. Which one is correct for my application?

Thanks!

either will work,but id go with the 4.320

Thanks Mike - just wanted to be sure there wouldn't be any chance of having any part of the gasket hanging into the cylinder with the smaller one. I guess the smaller displaced volume would help compression a tiny bit.

That gasket should get you around a .25 bump in static over a 4.380 .041 thick felpro mls 1071. How far in the hole are your pistons?

IIRC, they are about .008 in the hole. Flat top Mahle forged slugs with a 3cc valve relief, 088 heads with 118 cc chambers, 4.280 bore and 4.25 stroke. I think the .027 gasket will get me up to around 8.7 CR. That is the main reason I want to run the .027 cometic. That would also give me about .035 quench.

Sounds like a good combo, to ESP with flat tops, you can use any little extra compression you can.

Me too. Just make sure someone did not go ape**** with the chamfer at the top of the bore. I've seen some pretty large chamfers cut in to ease piston install.

Also I'd probably go with .030 thick cometic, .035 quench is getting a little snug. CR would go to 8.70 from 8.74 with the .027 vs .030 using the 4.32 bore gasket. Any reason why you are going so low on CR? Maybe a blower app?

That is just the way the CR ended up with the flat top pistons. They came with the stroker kit - other choice in pistons would have taken the CR up too high. I didn't want to mill a ton of material off the block or heads because then you get into issues with milling intakes, etc. Would have loved to end up around 9:1, but just not happening.

Is the tight quench a concern for piston / head contact, or something else? Engine will be limited to 5200 or so RPM. I thought the ideal quench was around .035 or so.

I think the chamfer is OK, but I did notch the bores to unshroud the intake valves. Guess I will need to double check that.

i've run them at .035 with good luck but usually look for .038 on a bbc. You just need to be aware of piston rock and other clearances so not to get into trouble. Plus if you don't have a rev limiter that 5200 may turn into a 8k unplanned.

Also what is the IVC angle on the cam you plan to run?

I hope it never goes to 8K - piston to head contact would probably be the least of my problems then! :eek:

Not sure what you mean by IVC. It is a custom Marine Kinetics cam - 226*/230* @ .050, 114* LSA, .612 / .596.

Its 'intake valve close' spec on the cam card. It helps determine how your combo will work. You have lower compression so you wouldnt want a cam IVC angle that bleeds off more compression. Plus it help you decide how exactly to index the cam in to get the best power at the rpm you want it at.

It closes 42.9* ABDC.

Do you have the seat to seat angle? Or the .004/.006? If not, the calculator below works with .050, its just for boosted apps usually. Just enter 0 for boost. You want you DCR around 8-8.1 which from my quick try of it with you numbers and cam suggested to advance the cam 3* to get a proper DCR. It would be best to have more compression to get the correct DCR number but you work with what you have. You might be fine advancing the cam 3* but Bob M should be the final answer on that.

http://www.rbracing-rsr.com/comprAdvHD.htm

It opens at 3.2* BTDC if that will allow you to derive the seat-to-seat angle.

Edit to my last post that should help you dial it in yourself with the .050

BTW, I like that cam selection. Should be a good combo if done right.