Scenario: block was decked an unknown amount, forged (Diamond) pistons are now protruding approx .012 to .015" above deck. How much clearance is needed betwixt the piston top and head surface under high heat/boost conditions? Want the good quench, but not too close... thinking Cometic HG to keep 'em separated?
Decked Block
- Thread starter Jay J
- Start date
Cometics will go up into the 140's. You're not that far out of the hole. Some plain 60+ish thick gaskets will keep you safe. Just try not to go closer than 40 unless you piston-to-cyl clearances are nice and tight.
Make sure to clay the pistons if you're running a big cam or one that's not ground for a turbo engine.
If you're that far out of the hole, you've either got longer rods, larger compression height or a BUNCH of meat taken off the deck.
Make sure to clay the pistons if you're running a big cam or one that's not ground for a turbo engine.
If you're that far out of the hole, you've either got longer rods, larger compression height or a BUNCH of meat taken off the deck.
Are you sure about that compression height? At 1.738, it's be .062" shorter than a normal piston. (your rod length is stock)
If that number is accurate and you have a stock stroke 109, it would take .114" to .117" off the deck to get you @ 12 to 15 out of the hole.
If that number is accurate and you have a stock stroke 109, it would take .114" to .117" off the deck to get you @ 12 to 15 out of the hole.
So--do you think the block was cut approx .025" or thereabouts?
All your parts mathematically stack up to 9.5205" tall. The published deck height of a 109 block is 9.510. Undecked you would be .0105" out of the hole. Using you numbers it looks like they took off 1-1/2 to 4-1/2 thou. Any chance your deck finish is a punch of parallel lines? Or is it a bunch of concentric half circles?
Keep in mind that's assUming the block was actually 9.510 on all four corners to start with (and it wasn't)
Keep in mind that's assUming the block was actually 9.510 on all four corners to start with (and it wasn't)
OK. Dug the T-gauge out to get an accurate measurement. Once I played with it a bit, and rocked them back and forth, the high side was .021", low side was .004" I did measure all of them, but that was the furthest one out of the hole by a couple of thou. That was a good call on the amount of rock on the Diamond pistons.
turbodave231
Moderator
- Joined
- May 24, 2001
Earl,
All the documents I've seen (A Buick Bulletin and AERA database) say the stock deck height on a 109 is 9.522. I've also measured quite a few on the CNC at work and I've finished (square decked) several in the 9.510 range.
All the documents I've seen (A Buick Bulletin and AERA database) say the stock deck height on a 109 is 9.522. I've also measured quite a few on the CNC at work and I've finished (square decked) several in the 9.510 range.
Odd.. the numbers I've found are 9.510" on the 109s and 9.535 on the 'tall decks'. I don't have my build notes for my 4.1 handy at the moment to see if they agree.
Then again, we all that published deck heights and real life deck heights are two completely different animals.
Then again, we all that published deck heights and real life deck heights are two completely different animals.
