I'm rebuilding my 67/327, .030 engine. Looking for opinions on piston choices as for material, Forged/Cast, Hypereutectic etc. Also source for purchase . THX! Al W. P.S. Car is a garage queen!

 

It would help if you would state what you are starting with and your final objective.

There were two 327s in '67. Do you still have an original engine or a replacement?

Are you looking for a stock rebuild? Blueprint rebuild with a little more omph or something entirely different?

Duke

 

Thanks Duke. This is a 350 HP. original numbers block using the stock 350 HP Cam. I'm more interested in drivability and idle than punch. (At 60 I've done enough burnouts!). More appropriately, should I spend the extra bucks for forged pistons? I intend to lower the Comp. into the 9's. My cylinder Bore is 4.030 and we will hone an extra .0005 out of the lower portion of the cylinder walls which seems to be current practice. Thanks for your input. (And to others who may join in.).

 

If you're an idler rather than a burnout guy, forget the forged pistons. Keep the L-79 stock cam and the domed pistons high compression. You'll never know the difference if you do get weak and do a burnout. Why would you want to hone your cylinders out bigger?

 

Years ago, during a previous engine rebuild, I had .030 over TRW forged pistons in my 327. Upon taking it apart for a refresh last summer (rings, bearings, etc.) I discovered the piston-to-wall clearance was .0065 -.007 .! .. It should have been .0035. .. Seems the previous machinist honed the cylinders way too much and didn't 'say anything' .! . .. The engine ran well though it did smoke a bit during throttle off coasting. I had wondered about that . . ? ..
This time I changed out those .030 over forged pistons for .040 over KB flat top hyperutectics. The cylinders didn't need much finish honing to bring them up to .0015 piston-to-wall clearance which is what the manufacturer recommendeds for a street driven engine using this type of piston. This time I checked it .! . . .. IMO, unless you plan on using nitrous or really hammering on the engine hard hyperutectic pistons are the way to go for a street driven engine. It's said they run quieter and seal better with less ring rock compared to the forged type.
In the KB line hyperutectic pop-up style pistons are available. I decided to go with flat top hyperutectic pistons mainly for 3 reasons. 1) The 91 PON premium here in CA. ........ 2) I plan to drive my car a lot (touring, etc.) and don't want to bump up against the detonation envelope, should it come to that ....... 3) My hearing ain't the greatest these days . . . . .. Would I even hear it .! .
Outside of reworking the heads (new valves, springs, etc.) and doing some 'backyard' porting on them this engine has the LT-1 cam. SCAT connecting rods replaced the stock units and the rotating assembly was balanced. With a frame-on resto project in the works my engine is still, well, on the engine stand!.

John

 

Maybe you want to detune it to 300 HP spec. Otherwise rebuild it to OE L-79 spec. Since that cam has a fairly late closing inlet valve you need at least 10:1 compression. You can achieve this either the OE forged pistons and a typical thick composition gasket or a flattop piston with a thin gasket.

There has been much discussion about rebuilding engines, especially in regards to establishing a target CR for the application and installed cam and then achieving it. First you plan, then you measure, beginning with engine teardown.

Time to search the archives.

Duke

 

What does Hyperuetectic mean? Were the OE pistons cast or forged? This is where I'm at. Seeing as this car is a "Garage Queen", and becoming more and more so, Octane related issues are of no concern as I can easily deal with that. Therefore, 11 X 1 it is. Lots of pistons makers out there. I've always, though years ago, had faith in TRW. Any opinion/opinions on this? I much appreciate all your comments. Al W.

 

Hypereutectic refers to a particular molecular structure that these cast pistons assume upon hardening, which is different than traditional cast pistons. They are used on nearly all modern cars. They are more durable than traditional cast pistons - nearly as durable as forged in a street engine, but can be fit tight like traditional cast types, and they also have better skirt and ring land wear characteristics.

They are a satisfactory choice for replacement (especially for conventional cast OE pistons), however, if you are sticking with L-79 specs I would use the OE replacement forged pistons and target a CR of 10.5:1 as I previously stated.

It would help if you would provide more information about the condition of the engine. Is it disassembled? Did you measure deck clearance on all 8 prior to disassembly? What is the measured bore size, and how much taper and out of round is measured?

Being as how it's already been bored .030", I don't understand why it needs to be rebuilt again.

Duke

 

Excellant advice- check all machined items. I'm preparing a suit now for the exact same thing. Guy did a lousy job, didn't think I'd check it and stole my pink rods on top of it! The bores were supposed to be .030 and I found them to be .036" and the ring gaps were .034" He's spun so many lies I can't even remember them all.
I went with the hyper speed pro w.125 dome.

Gary

 

Al,

Most 'aluminum' castings are Al-Si alloys. The maximum amount of Si miscible in Al is about 12.5%. That composition is the LOWEST melting point of any Al-Si alloy (~1070F) & is referred to as the 'eutectic' composition. An aluminum alloy with <12.5% Si is called HYPOeutectic; one with >12.5% is HYPEReutectic. Since the HYPEReutectic casting cannot retain all the Si in solution upon solidification, special techniques & equipment are required to ensure that the Si that is precipated is finely & uniformly dispersed in the piston.

 

Al,

To expand somewhat on your question; most 'aluminum' castings are Al-Si alloys. The maximum amount of Si miscible in Al is ~12.5%. That composition is also the LOWEST melting point of any Al-Si alloy (~1070F) & is referred to as the 'EUTECTIC' composition. It follows that an aluminum alloy with <12.5% Si is called HYPOeutectic -- your intake manifold & 'traditional' cast pistons fall in this category; one with >12.5% is HYPEReutectic. Since the HYPEReutectic alloy casting cannot retain all the Si in solution upon solidification, special techniques & equipment are required to ensure that the Si that is precipated is finely & uniformly dispersed in the piston to achieve high strength and decent ductility.

A forged piston, on the other hand, is made from a solid slug of an appropriate aluminum alloy (probably HYPO) that is heated & forced into die to achieve its rough net size/shape. Thus, there's more of a flowing 'grain' structure in the forged piston when compared to the 'crystalline' structure of a cast piston. They can take more punishment than a typical cast piston due to their structure, but may be heavier & perhaps more noisy than a high quality HYPEReutectic.

I am not sure, but would expect that the OEM flat-tops were cast (HYPOeutectic) & the domed hi-perf engine pistons were forged.

 

OK! It's established, I need to be more forthcoming with specifics. Here I go. My block is in the local machine shop, (well qualified), and it is here that the boring to .030 from 4.0 was done. As per my measuring the deck clearance prior to disassembly, no, not done. But I would like to know why I should do this in the future to add to my learning curve. The bore size and taper dimensions will be the responsibility of my shop. I do not have the tools to check these dimensions. Suggested dimensions for a .030 bore are requested with Thanks. I am going with the 10.5:1 CR with OE forged pistons. I am very appreciative to DUKE and ALL who have responded to my quiry. AL W. P.S. Duke, your local library historical director gave me the name of that ship carcass on the Redondo Beach. PPS. Suggestions for places to buy pistons?
BTW, .030 = what C.I.?

 

If the work has not been started, I suggest you issue a STOP WORK ORDER!

Proper results require a plan! Does bore wear dictate overboring?

The reason for measuring deck clearance prior to disassembly is to determine the deck height relative to OE nominal and have a dimension you need to compute CR. It will also tell you if your decks are parallel to the crankshaft axis. Usually they are and usually one is a few thou higher than the other. If the LH side is higher it can be cut down to the RH height. Of course, you DO NOT want to touch the RH deck or you'll destroy the numbers. Machine shops often do this contrary to specific instructions. Caveat Emptor - make sure your instructions are WRITTEN!

Shops should have tools to measure actual deck to c/s axis distance. The nominal blueprint dimension is 9.025", but most production blocks are high - typically by about .010 -.020", which is about half-a-point of compression.

Deck clearance/deck height combined with piston compression height, rod center to center length, piston dome or dish volume, head chamber volume, and gasket thickness allow you to compute CR to achieve your target.

Pistons must be evaluated and selected PRIOR to any machine work, then the engine bored and honed to proper fit for each piston using the proper final stone to achieve the specified finish for the type of ring face selected, which is usually moly face for street engines (OE was chrome.) ALWAYS follow the piston manufacturer's recommended clearance to the LETTER for the specific piston used.

Like I said, there is much discussion about the topic of engine overhaul planning in the archives - and many posts on the subject under my screen name, including a link to a good online CR calculator.

Duke

 

I went through the same questions when I did mine last year. I finally went with KB 10:1 Hypers, moly rings, stock cam (Crane), high pressure pump, new valve train (completly). Results were very good for both power and driveability.
If you have any other questions, E-mail me
Charlief
66 Coupe

 

I wouldn't go with forged, as the Hypereutetic pistons are plenty stout for your application and much cheaper. In fact, the new 2006 C6 Z06 with 427 Cu In and 11 to 1 compression and 505 HP and revs to 7,000 RPM, dosen't have forged pistons and GM has to worry about 3/36000 warranty repairs. I don't think GM is worried about piston failure with the cast pistons. In fact, I wouldn't be concerned with using the hypo cast piston and advance the cam 4 degrees because of the lower compression with flat top pistons. The cast pistions will not have the piston slap as the piston to cyl wall clearances are much tighter. As far as compression ratio, I would go with a flat top piston to end up with 9.5 to 10 to 1 compression and can use available pump gas.

 

Thanks to all who replied to this post thread. More opinions and past experience comments are still appreciated. I won't be ordering pistons etc. for another two weeks at least. This car has side pipes so I don't think the Forged Piston skirt slap will enter into my purchase decision. Anyway, my ears ain't so good anymore either! LOL Good Luck to All! AL W.

 

Al, your machine shop won't be done with the bores until they have the pistons in their hands; they have to finish each bore based on individual piston measurements to fit them properly.

 

Thanks John, I'll get my Butt in gear. Al W.

 

OK! After all is said and done heres how I went. TRW, Speed Pro Forged pistons, Flat Top 9.62 X 1 CR. (Approx.). I'll measure piston height etc. to compute final CR during assembly. Stock rods, stock heads with no porting or polish, high volume oil pump, and the OE L79 cam. Thanks to ALL who responded to my quiry!

 

If it's not too late, lose the high volume oil pump. The standard OE pump or equivalent with the OE 40-45 psi pressure relief is just fine. Much discussion in the archives.

Also, much discussion in the archives about the weak, early 327 rods. Your '67 vintage rods are better, but I hope you at least have them Magnaflux inspected to be sure none are cracked.

With the OE domed, forged pistons, typical OE machined deck clearance, and a typical thick composition head gasket, true CR will be not more than 10.5:1, and the engine will operate detonation free on unleaded premium with no more than a little initial timing diddling.

If true CR is about 9.6 the engine should operate on midgrade. The late LT-1s had a specfied CR of 9.0:1 (actual was probably closer to 8.5) and they were DESIGNED to operate on regular unleaded - 91 RON, which is equivalent to today's 87 PON.

Duke