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Old 03-15-2009, 08:07 PM
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sbc 355 low piston height

I'm building a sbc 355 from a Targetmaster crate engine. I had it bored to 4.030 in and installed Federal Mogul / Speed Pro 345NP pistons. They have a modified compression distance of 1.540 in. and with a stock deck height, they should be .045 in. in the hole. Mine measure between .049 and .055 in. in the hole and I thought that the deck had been machined at some point in the past. Guess I was wrong.

Is there a way to measure the block deck height without pistons and crank installed?
Can I measure the piston compression height to double check the manufacturers figure? If so, how?
How does a machine shop know how much to take off the deck to get a desired piston height?
Assuming the piston compression height figure is correct, any ideas on why the block deck height would be .010 in. higher than stock?

I don't know if this amount is significant in the manufacturing process, but it seems to be a significant amount for my compression ratio calculations.

Thanks. Any help would be greatly appreciated.
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Old 03-15-2009, 09:29 PM
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Quote:
Originally Posted by callmejavier
I'm building a sbc 355 from a Targetmaster crate engine. I had it bored to 4.030 in and installed Federal Mogul / Speed Pro 345NP pistons. They have a modified compression distance of 1.540 in. and with a stock deck height, they should be .045 in. in the hole. Mine measure between .049 and .055 in. in the hole and I thought that the deck had been machined at some point in the past. Guess I was wrong.

Is there a way to measure the block deck height without pistons and crank installed?
Can I measure the piston compression height to double check the manufacturers figure? If so, how?
How does a machine shop know how much to take off the deck to get a desired piston height?
Assuming the piston compression height figure is correct, any ideas on why the block deck height would be .010 in. higher than stock?

I don't know if this amount is significant in the manufacturing process, but it seems to be a significant amount for my compression ratio calculations.

Thanks. Any help would be greatly appreciated.
I'll answer this for others as well as you......
In the first place, you should have measured the piston deck height with the old pistons still in the block. This could have been accomplished with nothing more technical than a steel rule and a set of feeler gauges. Span the bore about a 1/4 inch from the edge of the bore with the steel rule placed on edge. Do this at 3 oclock or 9 oclock as you stand at the side of the block. Checking at noon or 6 oclock can allow the piston to rock on the wrist pin and give you an erroneous measurement. Bring the piston up to TDC (you may have to turn the crank a few degrees forward or backward to get to exact TDC) and insert feeler gauge blades between the crown of the piston and the underside of the edge of the steel rule to find piston deck height.

Measuring a bare block will require an 8" caliper, either vernier, dial or electronic. With no bearing shell in the main bearing bore, measure from the bearing bore to the deck of the block. Add 1.320"/1.321" (half the bare bearing bore measurement).

To measure the piston compression height, use your caliper to measure from the top of the wrist pin bore to the piston crown. Add half the wrist pin bore (0.464").

The nominal block deck height on a small block Chevy is 9.025", however, I believe these Targetmaster blocks are from Mexico, so anything is possible.
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Old 03-16-2009, 06:59 AM
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The way I do so is install the number 1, 2, 7 and 8 pistons, then I check each to see how far down in the hole they are. if you get a different measurement such as the 1 and 2 are .040 in the hole and the 7 and 8 are .055 in the hole this tells you the decks are tipped .015 from front to rear. Factory block decks are often tipped and are often taller by a few thousanths so this comes as no surprise. Besides decking the block or going with a 1.560 compression height piston will be the only ways out. I would go with a 1.560 piston, check the deck tip, and have it machined to set them flush at around .015. This way you can use a .028 thickness head gasket and have a .043 quench distance.
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Old 03-16-2009, 12:29 PM
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Quote:
Originally Posted by callmejavier
I'm building a sbc 355 from a Targetmaster crate engine. I had it bored to 4.030 in and installed Federal Mogul / Speed Pro 345NP pistons. They have a modified compression distance of 1.540 in. and with a stock deck height, they should be .045 in. in the hole. Mine measure between .049 and .055 in. in the hole and I thought that the deck had been machined at some point in the past. Guess I was wrong.

Is there a way to measure the block deck height without pistons and crank installed?
Can I measure the piston compression height to double check the manufacturers figure? If so, how?
How does a machine shop know how much to take off the deck to get a desired piston height?
Assuming the piston compression height figure is correct, any ideas on why the block deck height would be .010 in. higher than stock?

I don't know if this amount is significant in the manufacturing process, but it seems to be a significant amount for my compression ratio calculations.

Thanks. Any help would be greatly appreciated.
A Gomer Pyle moment; "surprise, surprise, surprise"!

To start with the dimensional variation you're seeing isn't that uncommon. A lot of machine tolerance in the block, crank, rods, and pistons all come home to roost here usually to net you less compression than you think you have.

The aftermarket sells a lot of 1.54 SBC pistons which is .020 short of the OEM spec. The assumption they make is that the mass rebuilders will remachine all decks by some number and .020 is pretty good average. For the hobbyist or custom builder, this means you have to shop carefully.

So there you are, you can disassemble the engine and have the decks milled, or you can buy new pistons that are 1.56 from the middle of the pin bore to the crown.

Bogie
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Old 03-16-2009, 03:33 PM
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Thanks for the detailed responses.

I have never read or heard to measure piston to deck distance at 3 and 9 o'clock. Makes perfect sense. Sometimes one just needs to be reminded of the obvious. I don't think that was my problem, but I won't forget it.

If I tell the machinist my connecting rod length and piston length, he can mill the block down to my desired piston to deck clearance without guessing and checking by installing the pistons. Is this correct?

Also, I was told by the machinist that he needs to know whether I plan to use a steel shim or compostion head gasket in order to determine how to finish the deck surface. Is this true?

I don't recall ever reading that on this board or in any book. And I planned on using whichever thickness gave me the proper compression and quench regardless of material.

Thanks again for the help.
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Old 03-16-2009, 03:47 PM
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IMHO, the engine should be mocked up to see exactly what the stack height is. Telling him the supposed dimensions would be guessing.

Most everyone uses a composite gasket, not steel shim. Average thickness in the neighborhood of 0.040"- but this varies w/app. and manufacturer, the thicknesses are published.

And yes, the type of gasket effects what he needs to do, both how much and type/smoothness of the deck finish.
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Old 03-16-2009, 06:43 PM
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Quote:
Originally Posted by callmejavier
Thanks for the detailed responses.

I have never read or heard to measure piston to deck distance at 3 and 9 o'clock. Makes perfect sense. Sometimes one just needs to be reminded of the obvious. I don't think that was my problem, but I won't forget it.

If I tell the machinist my connecting rod length and piston length, he can mill the block down to my desired piston to deck clearance without guessing and checking by installing the pistons. Is this correct?

Also, I was told by the machinist that he needs to know whether I plan to use a steel shim or compostion head gasket in order to determine how to finish the deck surface. Is this true?

I don't recall ever reading that on this board or in any book. And I planned on using whichever thickness gave me the proper compression and quench regardless of material.

Thanks again for the help.
3 and 9 O'Clock is above the pin so the distance isn't being affected by the thrust clearance rocking of the piston on the pin which would or could change that dimension as it rises or droops around the pin as an axis.

Yes the gasket thickness is needed if he's just taking off some material to dial in the volume for compression. Steel shim gaskets are usually less than .020 inch, composition gaskets are available in varying thicknesses from about .030 to almost .060 inch. Steels can be stacked for greater thickness, which of course isn't your problem.

Compression can be hand computed; (bore diameter/2) times pi (3.1416 is close enough) equals area. That times thickness gives volume; it doesn't matter if the thickness is the stroke of the engine or the thickness of a crown clearance to a deck, or a gasket.

cubic inches times 16.4 gets ccs

linear inches times 25.4 get milli-meters

Bogie
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Old 03-16-2009, 07:14 PM
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Quote:
Originally Posted by oldbogie
A Gomer Pyle moment; "surprise, surprise, surprise"!

To start with the dimensional variation you're seeing isn't that uncommon. A lot of machine tolerance in the block, crank, rods, and pistons all come home to roost here usually to net you less compression than you think you have.

The aftermarket sells a lot of 1.54 SBC pistons which is .020 short of the OEM spec. The assumption they make is that the mass rebuilders will remachine all decks by some number and .020 is pretty good average. For the hobbyist or custom builder, this means you have to shop carefully.

So there you are, you can disassemble the engine and have the decks milled, or you can buy new pistons that are 1.56 from the middle of the pin bore to the crown.
e
so true...sometimes the truth hurts...i learned the hard way!
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Old 03-17-2009, 09:17 AM
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Thanks bogie.

I know that I need to consider the gasket thickness when deciding how far down to machine the deck but I was wondering if the decks could/should be finished differently based on which type of head gasket is going on there.

j
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Old 03-17-2009, 09:44 AM
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Your machinist should be told what gaskets you intend to use, the gasket manufacturer can supply the requirements for the "Ra" finish that's needed for whatever gasket is being used.

Hopefully your machinist is up to speed in regard to this- and doesn't just belt-sand it and let it go at that.

Preferably, he has the ability to do a variety of finishes and a profilometer to measure the results with.
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Old 03-17-2009, 10:59 AM
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Quote:
Originally Posted by callmejavier
Thanks bogie.

I know that I need to consider the gasket thickness when deciding how far down to machine the deck but I was wondering if the decks could/should be finished differently based on which type of head gasket is going on there.

j

Where are you getting your machine work done?

You need to talk with the machinist, this can be tough to do if you go to B&B for example (for the rest of the world that's B&B in Seattle/King County Washington state, I'm not refer to any other business potentially named B&B) you talk to a counter guy, drop off your work, hope for the best, they send it to a back shop somewhere and you can only hope your concerns get communicated. If you go somewhere like Engine Machine, you can talk directly to the man.

So you need to talk with these people as the material and to some extent the year of manufacture of your parts has an effect on gasket selection which has an effect on the machining. Keep in mind this is a communication, a lot of these guys in the machine shops have a lot of experience and are willing to make suggestions to you.

Older engines, pre mid 1970s with old style gaskets will tolerate fairy rough finishes. The move to thin wall castings, mixing aluminum heads with cast iron blocks, using rubber coated steel shim or Multiple Layer Steel (MLS) gaskets is pushing toward smoother, flatter finishes.

- The old finishes ran about 60-100 RMS for iron and 50-60 for aluminum.

- The newer (thin wall) castings, especially when combined with rubber coated steel or MLS need smooth finishes on the order of 30-40 for iron and 10-20 RMS for aluminum.

In addition to surface finish, you need to worry flatness and waviness.

- For length or width dimensions of a head or cylinder deck I like to see not more .00024 inch per linear inch of measure for both flatness and waviness.

- For overall length point A is a one end and B at the opposite end, this includes twist that would be the inside of one end measured to the outside of the opposite end.

- For waviness point your measuring the low place between high places. A is the first high spot measured to the next high spot which is B, the next high spot would be C and so on.

Bogie
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Old 03-18-2009, 09:44 AM
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I had Napa on Lake City Way (Seattle) do the machine work the first time. The experience was terrible, starting similar to what you describe. And the work that came back was wrong or bad or not done.

Now the block is at an factory rebuilder. A friend who works there should be able to direct the machinist with my info.

How do you measure deck flatness laterally (perpendicular to the crank) without being thrown off by piston rock?

Thanks.

j
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Old 03-18-2009, 12:18 PM
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Quote:
Originally Posted by callmejavier
I had Napa on Lake City Way (Seattle) do the machine work the first time. The experience was terrible, starting similar to what you describe. And the work that came back was wrong or bad or not done.

Now the block is at an factory rebuilder. A friend who works there should be able to direct the machinist with my info.

How do you measure deck flatness laterally (perpendicular to the crank) without being thrown off by piston rock?

Thanks.

j
Parallelism to the crank is a whole other issue. When the factory machines the raw casting they start with the crank saddles and oil pan rail. The crank and pan rail are assumed by subsequent operations to form a plane in space that is accurate unto itself which will hold the casting in proper alignment for further operations. This assumption is used to set up the following machine operations of milling the timing case and bellhousing faces, the cam bore, lifter bores, head decks, and cylinder walls. So each operation assumes the first operation was completed properly and the casting there after is aligned for the next operation always off the crank bores and pan rail. Castings that fall outside acceptable tolerances are removed from the line to be repaired or scrapped.

Depending on the equipment, the corner machine shop can either in part or in whole duplicate the factory machine experience, or if not well equipped it starts with the assumption that the crank bores and head deck are essentially correct and machine cylinder walls, or even the deck based on the assumption that the decks are centered around the diameter of the crank bores and are parallel to it. In these shops, like NAPA on Lake City Way, cylinder boring is often indexed off the head deck, so if that's out of line to the crank, so are the resulting cylinder bores.

Now the data I gave you on flatness and waviness do not take into account the surfaces alignment to anything. All we're talking about is how flat the surface is compared to laying a straight edge on it and measuring deviations between the two objects with a feeler gauge.

Typically a large rebuilder shop like your friend has access to, has machines that can duplicate the factory experience. The down side from a race engine builder stand point is that they run hundreds, if not thousands, of blocks thru the shop and make some dimensional assumptions to which all blocks are subjected whether they need the fixes or not. But some shops will do custom work on the night shift or on weekends and with this equipment that can do some extraordinary work.

A lot of the work you're intrested in having corrected really falls into "blueprinting". This adds a lot of expense to an engine because each operation is measured back to a corrected crank position. This takes a lot of time to make measurements and adjust the machine setups when your not in the one size fits all mode.

Getting good work can be difficult, especailly at afforable prices. I'm not adverse to sending parts all over the west coast to get things done, but around home in Western Washington I hang with these shops a lot.

- Autosport in Seattle for general machine work

- Castle Automotive Machine in Longview (a bit of a drive from Seattle, or Portland, but worth it. On the surface old man Castle does big rigs, heavy equipment, and shipboard; but he's a chronic racer which you can take advantage of)

- Delta Camshafts in Tacoma (What can you say when you break a cam in an old MG during the Tacoma Gran Prix and the guy from Delta looks at it and says well there's nothing like that in the northwest, but it looks a lot like a Massey Ferguson tractor cam, I bet we can re-machine one and get you racing, and so he did)

- Heads Up in Auburn (these guys weld on cast iron or aluminum, have heat treat facilities, and offer full machine shop services, they've forgotten more about head construction than most of us ever knew)


Bogie

Last edited by oldbogie; 03-18-2009 at 12:25 PM.
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Old 03-18-2009, 12:28 PM
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Quote:
Originally Posted by oldbogie
- Delta Camshafts in Tacoma (What can you say when you break a cam in an old MG during the Tacoma Gran Prix and the guy from Delta looks at it and says well there's nothing like that in the northwest, but it looks a lot like a Massey Ferguson tractor cam, I bet we can re-machine one and get you racing, and so he did)Bogie
Now, I find that extraordinary, to say the least.

I wonder who'll carry the torch when these brain trusts "move on"?

You prob. get this a lot, but thanks for yet again, another informative post.
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