I am in the process of thinking through the machine work for my rebuild project. I have seen several references to having a shop fit the pistons to the cylinders. I am not familiar with this process, and I know it was never done when I helped my Dad do garage rebuilds (years ago). What's the scoop? Necessary for an engine in a driver, or unneeded hype?
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Re: C2 piston fitting?
You should procure the pistons and give them to the shop, so they can measure the pistons and then hone the bores to an exact fit.
The clearance is a function of the type of piston you use. What engine option do you have? Forced pistons used on SHP engines must be looser clerance than cast pistons used in medium performance engines. These specifications will be in your applicable shop or overhaul manual.
In production, pistons were sized into several groups - four I think. Each cylinder was then measured and matched up to the proper size piston to keep tolerance within a few tenth thousanths of nominal spec.
Proper piston clearance is important for both noise and oil control and to prevent scuffing.
Duke -
Re: C2 piston fitting?
My engine is a '64 L75. A prior owner had the engine bored 30 over. I really don't want to go any more over than that. Does this mean I need to go 40 over so 40 over pistons can be fit to the block? (do they even make 40 over pistons?)Comment
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Re: C2 piston fitting?
Depends. If the pistons skirts and walls are in good shape and there is no more than say .002" taper at the top you could use the old pistons with a new set of moly rings.
If you want everything "as new" you will have to bore to the next available size that will expose fresh metal on the walls, which will depend on how worn they are.
L75 is a medium performance engine with conventional cast aluminum pistons fit at about .001" clearacne. These are perfectly good pistons and readily available, but modern hypereutectics alloys are better.
You should always follow the pistons manufacturer's clearance recommendations.
When doing a rebuild you don't know for sure what new parts you need or required machining until you take it apart, and you should do lots of diagnostics while disassembling. As a minimum you should measure piston crown to deck clearance to get an idea of whether the decks are level, and this measurement will aid in computing compression ratio for new pistons regardless of type.
Thorough inspection and measurements of all bearings, journals, and cam lobes is a good idea. My basic rule is that if there is something I can measure that has a spec I measure it. If there is no spec, I measure it anyway and compare with other similar machined features. If they are all about the same, there is likely no wear.
DukeComment
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Re: C2 piston fitting?
OK. Sounds like I either reuse the old pistons or I use new ones + bore the block. I will take measurements as I disassemble. Can a shadetree (at best) mechanic measure the piston crown to deck clearance or do I need the shop to do that?
Thanks for all your advice.
MarkComment
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Re: C2 piston fitting?
A depth mike is the best tool for measuring piston crown to deck clearance, however, a machinist's bar and feeler gages work about as well, but take a little more time. The piston will rock a bit in the bore, especially a loose clearance forged piston, so you rock it back and forth until the measurement is the same on both sides.
You can measure off and mark the balancer at 90 degree intervals to determine all the TDC points and tweak it at each position to insure that the distance is at the minimum, which means you are at true TDC.
Be sure to record all your data for future reference.
I think on of the biggest mistakes amateur engine restorers make is not inspecting/measuring various dimensions on teardown. The piston to deck height is particularly important in order to determine deck parallelism to the crankshaft centerline and to compute the compression ratio with whatever pistons are selected.
Since it's important to NOT deck the block in order to preserve the pad numbers, if you determine that the deck is parallel, the block can be bored with equipement that indexes off the deck, which is what most shops use. Most shops like to "take a cut" to insure parallelism, but if your measurements show that it is parallel, no cut is needed, and knowing the deck clearance along with the head gasket thickness will allow you to accurately compute the compression ratio when combined with the compression height and dome/valve notch volume of the chosen piston.
Duke
Duke
DukeComment
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Re: C2 piston fitting?
Mark:
To measure crown to deck clearance, clean the decks thoroughly, lay a steel rule across each bore, and with each piston @ TDC, get a feeler gauge between the ruler and the piston crown. Measure each bore seperately, as there will likely be small variations.
To calculate compression ratio, use the following formula (again, each bore must be independently calculated, as there will be differences):
CR= (PD+CV)/CV
where PD=piston displacement (bore x stroke)
CV=compressed volume
CV=Combustion Chamber Volume-Dome Volume+Deck Clearance Volume+Head Gasket Volume(compressed)+Ring Land Volume
Combustion chamber Vol should be measured (cc'd) to obtain accurate readings.
Dome Volume, in your case will be negative, since you will be using flattop pistons, and you must subtract the volume of the valve clearance "eyebrows".
Deck Clearance Vol=3.1417 x (bore/2)(bore/2)x crown to deck clearance
Ring Land Vol=3.1417 x distance from crown to top of upper compression ring x [(bore/2)(bore/2) - (piston dia/2)(piston dia/2)]
Recommend that you have a reputable machine shop prepare your block, size the pistons (+.040" are common, but I wouldn't go any bigger), balance the rotating assembly, machine the journals, and match the bearings. If you decide to assemble the engine yourself, remember MAKE SURE EVERYTHING IS AS CLEAN AS A BABY'S BUTT BEFORE REASSEMBLY.
Good luck,
Joe
PS: If you have an original motor, DO NOT let anybody mill the decks, unless they can preserve the stamp pad.Comment
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Interesting Trivia....
You might want to shop/interview machinests as to their approach and see the myraid of differences that abound! I just rebuilt my '71 SB 350 and consigned the short block + head machine work to a local machinest/shop with recognized competition race reputation. I did final assy with help from local chapter mates. Here's a few trivia tidbits I learned:
(1) The block bores were at 40-over and I presumed I was in for an additional bore to 60-over. The machinest chuckled and told me not to go so fast with 'presumptions'.... He ran a bore scope down all 8-holes and concluded I could get by with a 42.5-over system and save 'meat' in the block.
(2) Said most folks presume pistons only come in discrete pre-packaged sizes, but that's NOT true! You can special order, hand selected pistons for a modest sorting'handling fee from many piston houses.
(3) Once the 42.5-over decision was made, he did NOT proceed to bore but waited for the pistons to arrive.
(4) Then, he did balance work BOTH internally and externally with a 1/10th gram target.
(5) When balance work was done specific part location assignment was defined. He proceeded to bore the block TAILORING each bore to its mating piston (yes, even with custom/hand-selected pistons there IS a unit-to-unit variance).
(6) The above, along with a handful of other 'racer' tricks, produced the smoothest running engine I've ever experienced. I can virtually balance a coin, on end, on the hood when she starts and idles! Plus, steady state engine temp has dropped a solid 15 degrees....Comment
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Re: Interesting Trivia....
If you are willing to share, what did that amount of machine shop work go for?Comment
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Re: Interesting Trivia....
Jack - my machinist working on my 66 427/425 engine is using the same hand fitting of each piston to cylinder bore by bore. However, I don't know how much variance there will be as I went with a 30 over matched set of TRW L2268F pistons....which was my choice. I suspect I could have used 0,010 over given the condition of my block, but I couldn't find any pistons in that size, maybe I should have gone and special ordered a set from one of the custom manufactureres...but since it is still 0.030 on a BB with heavy meat cylinders I didn't worry. I was impressed with the precision of his operation, he builds mostly racing engines but does do precision work for fussy customers. The biggest problem is he is so backed up, it takes him 2 months to get to many operations...so I have been working on my block since last fall but I should be done next week...CraigComment
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