Waystro
No Class
Ok. So how would it be if I Did .020 with .10 reduced height like Soro71 did.
Are would I be better off to do 030 bore with 010 reduced height.
Are would I be better off to do 030 bore with 010 reduced height.
Pulled heads little bit of carnage
- Thread starter Waystro
- Start date
Do you have a machine shop you are going to deal with? Most of your questions will be answered when you take the parts in for evaluation. The oversize will be determined by the amount of material needed to clean up the bore. Earlier you were planning to use stock pistons. If your bores are still good you wont need oversize pistons. The same applies to your heads. They need to be checked for cracks before you decide on the next step. If the block needs to be decked to clean up the head gasket seal you may need to mill your pistons. What started out as a budget build can get pretty expensive. A late 6.9 block is a good base for a rebuild. I was expecting the expense but was surprised at how long it took to get everything back from the machine shop.
Waystro
No Class
Not yet still trying to find a decent shop. That's what I'm planning to do have the shop look everything over and tell me what they think on the cylinder bores and heads. What is the benefits of going say .030?
In my case the machine shop needed .030 to clean up the bores. I didn't order pistons until I knew what size was needed, then the machine shop measured the pistons and did the final bore within the tolerance spec.
Sorro71
Full Access Member
Similar to TahoeToms mine needed 020 oversize to clean up the bores. The reason I purchased 010 reduced height pistons was so the machine shop could take a 004 swipe to clean up the deck of the block for the head gasket.
KnockKnock
Registered User
Try Ted's in Conroe, it's a reputible shop and they come highly recommended.
As far as cylinder boring and relieving the pistons for milling the heads or decking the block, you're going to have to see how much meat the cylinders are going to need cleared to become round & smooth again. In the gasser-world a +.030 bore will net you a few more horses, and still leave you with another .030 of meat to grind again later after another 100K miles. Now I'm no expert here (as I'm sure you've deduced already), but from what I gather, you'll only want to bore from the 6.9 block what is absolutely nessecary - ie: you don't want to go +.030 if +.010 is what gets everything back the way it's supposed to be, and I say this for two main reasons: 1) If a 6.9 is similar to the 7.3 IDI, then boring the cylinders could lead to the dreaded 7.3 cavitation problem, and 2) You'll want to keep as much meat on the block as possible for 20+ years from now when you want to rebuild that engine again.
In a perfect world, you'll only need to hone the cylinders, but when the machinist looks over the block, he'll tell you if the head mating surfaces are flat, and if not, then how much he's going to have to deck the block to get it right again. He'll also do the same thing with your heads, and check the mating surface to the block, and if they need to be milled flat again, he'll be able to tell you by how much.
Once you have all that information, I'm sure the more knowledgable folks here can chime in a little better with what is & isn't within tollerence, and whether or not you need shorter pistons, bigger piston valve reliefs, thicker head gaskets, recessed valves, fairy dust or a flux-capacitor. HTH, good luck & keep posting!
P.S. Sorry for the poor spelling, my spell checker is on strike
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fsmyth
Full Access Member
Ted's (Engine Machine Service) has been around a long time.
I used him exclusively when I was in the business (late 70's/early 80's).
Dunno when he started, but he was in business when I was in high school
(late 60's). Ted is not there anymore, but his son is.
I used him exclusively when I was in the business (late 70's/early 80's).
Dunno when he started, but he was in business when I was in high school
(late 60's). Ted is not there anymore, but his son is.
Waystro
No Class
Thanks for the shop recommendation I'll give him a call later.Try Ted's in Conroe, it's a reputible shop and they come highly recommended.
As far as cylinder boring and relieving the pistons for milling the heads or decking the block, you're going to have to see how much meat the cylinders are going to need cleared to become round & smooth again. In the gasser-world a +.030 bore will net you a few more horses, and still leave you with another .030 of meat to grind again later after another 100K miles. Now I'm no expert here (as I'm sure you've deduced already), but from what I gather, you'll only want to bore from the 6.9 block what is absolutely nessecary - ie: you don't want to go +.030 if +.010 is what gets everything back the way it's supposed to be, and I say this for two main reasons: 1) If a 6.9 is similar to the 7.3 IDI, then boring the cylinders could lead to the dreaded 7.3 cavitation problem, and 2) You'll want to keep as much meat on the block as possible for 20+ years from now when you want to rebuild that engine again.
In a perfect world, you'll only need to hone the cylinders, but when the machinist looks over the block, he'll tell you if the head mating surfaces are flat, and if not, then how much he's going to have to deck the block to get it right again. He'll also do the same thing with your heads, and check the mating surface to the block, and if they need to be milled flat again, he'll be able to tell you by how much.
Once you have all that information, I'm sure the more knowledgable folks here can chime in a little better with what is & isn't within tollerence, and whether or not you need shorter pistons, bigger piston valve reliefs, thicker head gaskets, recessed valves, fairy dust or a flux-capacitor. HTH, good luck & keep posting!
P.S. Sorry for the poor spelling, my spell checker is on strike
Now that's what was confusing me in the gasser world like you said going .030 or similar would net some extra HP
Like I said most of my engine tear down etc has been on a Dualport 1600cc VW so the idi is definitely a learning experience.
Oh by the way how is it coming on your end have you pulled it yet?
KnockKnock
Registered User
I'm waiting until I have another engine to drop in before I go tearing mine apart. If I can't find a good '87 6.9 runner around here at a good price (or fair trade), then I'd consider going with a '85½+ short-block w/ 7.3 heads. The 6.9 is supposed to be more resistant to cavitation, and the 7.3 is supposed to have the pre-chambers that are better designed to work with alternative fuels which I may or may not dabble in later. I'm more inclined to try and find a runner since I am a sucker for the term "while I'm here." That seems to get me into a lot of trouble wallet-wise. Besides, I'm waiting on you to get done with yours so I can know what hurdles to expect.
EDIT: Seriously, keep on posting, this thread is great
EDIT: Seriously, keep on posting, this thread is great
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30 over pistons may make a larger % difference in VWs 1600 CCs (97 cubic inches)... for a 351w/c its nets about 6 cubic inches... not really worth "power" Nobody knocks their bores out to .060 or .080 (Pontiac) JUST to get a couple more cubes from a motor...
The lowest overbore (least material removed) you can go is the best idea. If roughing out .018 cleans up the bores, go with .020. Whatever shop you pick had better hog out the worst bore first to see what the oversize will be! If they muck around with tanking it and other crap before a rough bore, they're trying to hook you for work on a block that may not be usable.
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Knuckledragger
blowing chunks and grabbing porcelain
The need for lower pistons is usually for turbo boost insurance, not valve clearance. That should all be done with the valve protrusion. A good head and proper valve job will net you plenty of clearance, it is when you run WMO that the gunk builds up on the pistons and creates interference. The carbon can be blown out from time to time but needs to be done carefully. Even if you are reusing the current pistons (and if you saw crosshatching, that is a very good sign), check for wrist pin bushing runout. They are not expensive, and can save you a lot of headaches. I think it makes the engine quieter, too. Consider having the rotating mass balanced. It should cost about $250 and is worth every penny. Check for roundness of the rod journals, have them rebuilt if they are out. Also a worthwhile expense.
The 6.9 engine rarely has any cavitation problems because it has plenty of wall thickness around the pistons and does not create a harmonic pulse like the 7.3. The only pitting you should encounter is from a leaking head gasket.
Machining these engines takes a long time relative to gassers because the iron block has much more nickel (also part of the reason they do not wear much in the bores) in it and it is fairly tough to machine in comparison. My machinist whined about having to change bits while boring the engine and ended up doing considerably more honing than he expected.
The 6.9 engine rarely has any cavitation problems because it has plenty of wall thickness around the pistons and does not create a harmonic pulse like the 7.3. The only pitting you should encounter is from a leaking head gasket.
Machining these engines takes a long time relative to gassers because the iron block has much more nickel (also part of the reason they do not wear much in the bores) in it and it is fairly tough to machine in comparison. My machinist whined about having to change bits while boring the engine and ended up doing considerably more honing than he expected.
turbo elk
Full Access Member
Well ford did bore the 6.9 120 thou. to get the 7.3..... right?
Or is it do to a longer stroke?
icanfixall
Official GMM hand model
Nope... They did not bore out the 6.9 to 7.3. The 7.3 block is a different block. So are the heads. Now I realize most every part of each engine fits the other. but the blocks are different. Can you bore a 6.9 out to 7.3.. Sure you can but why??? Just use a 7.3 block because it has the larger head bolts. Now can you sleeve a 7.3 down to 6.9.. Sure can and its going to end up a built proof engine for sure. All idi blocks are a high nickle material and they machine very slow. If you speed up the cut the tool chatters and gives a horrible finish requiring lots of honing to finish.
KnockKnock
Registered User
Oh really... My 7.3 died of cavitation, and I wonder if this might be the fix. I've always been leery of sleeves though; heard too many horror stories about them dropping & breaking things. Are the rod journals the same from a 6.9 to a 7.3, I wonder. I think I should look a little deeper into this. Sorry for the thread-jack Waystro - back to you buddy
