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351w engine rectification - Club Cobra
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  • 1 Post By blykins
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  #1 (permalink)  
Old 07-15-2017, 06:05 AM
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Default 351w engine rectification

we took my 1972 351w block to the machining guy and after few measurements, he gave me a list of items to get for him.

piston: +40
piston ring: +40
Main bearing : +10
Con rod bearing: +10

Based on this doctor prescription, i bought the following items from summit racing which came as a set:

SLP-E-251K40 PREMIUM PISTON RING SET Speed-Pro Hypereutectic Pistons H336CP4

SLP-1204M CAMSHAFT BEARING SET

SLP-224-41143 OIL PUMP

SLP-260-1028 GASKET KIT

SLP-3380CPA10 CONNECTING ROD BEARING PAIR

SLP-381-8015 BRASS EXPANSION PLUG KIT

SLP-5107M10 MAIN BEARING SET

SLP-55403B PRELUBE

SLP-H336CP40 CAST PISTON SET OF 8

SLP-SPG1 PLASTIGAGE

Final result and readings:

no problem observed in the working of the engine (visual and sound)
"compression readings improved from 9 to 12"

Question: i am reading in this forum that we should actually not have pushed for pistons bigger than +30 after engine machining / boring...am also reading in this forum that 1972 is relatively stronger block to be able to sustain +40.....am i really risking all the great work we have done so far by pushing for +40 (which means i should actually have changed the block instead of boring it....) what are the first symptoms i should be watching out if we have over-bored ? is there any preemptive measure i can take to avoid problems? i am not planning to race the car, it will be used for daily commute, yet i will not refrain from pressing the pedal ?
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Old 07-15-2017, 06:18 AM
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It's perfectly fine. I do .040" 351W blocks all the time. No worries.
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Old 07-15-2017, 12:17 PM
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Thanks for the comforting feedback blykins. The doctor / magician said he needed to "little shave off" the top side of the pistons to fit them - does this make any sense to you at all ?

Any different feedback from you great small block builders or else, I am really on my way to relaxing on this itching matter.

Thanks in advance.
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Old 07-15-2017, 12:20 PM
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Not unless he was trying to raise the compression ratio.
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Old 07-15-2017, 12:50 PM
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Dear Blykins, thanks - no, he said it was purely due to size consideration just to fit it right...for me, it's close to speaking French cause from my side, I check what sort of pistons would fit into my 351w based on the listings I found in summit racing and finally decided to use Speed-Pro Hypereutectic Pistons H336CP4...ironically, I didn't know about summit racing's online help and chose these Pistons for my 1972 block just based on my shallow understanding of these matters ! Then obviously it was a surprise to me why this "size suitability problem" happens cause they were just supposed to be made for 351w....anyways, it's a mystery for me (1) why the size would not fit and pistons need to be shaved off and (2) if these my piston selection is right for my vehicle and what makes these pistons different than others (I read that it's a mix with silicon which increases durability but again, not sure they are right choice for my daily commute application)

Pls feel free to keep your answers short and longer versions are always most welcome !

Thanks again
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Old 07-15-2017, 12:54 PM
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You may be asking yourself why I'm not asking these qsts to the MAN himself - unfortunately in dubai, we do have communication problems with engine masters n they most of the time don't speak our language or communication is very basic !
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Old 07-16-2017, 07:30 AM
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Since he said shave off the top of the piston, I would assume he was trying to gain some piston to valve clearance, which is very common with larger cams. I didn't see the cam specification in your list.

If that is the case, it makes perfect since. Having worked with language communication misunderstandings, make it fit does not necessarily mean fit in the block. I fit may mean no collision of parts, when it rotates.

Back to your original 0.040 overbore. That is commonly done, but most people choose to bore no bigger than they have to, so they have some room to bore again on the next rebuild.

0.040 overbore is a general rule of thumb that almost any engine can be bored that far, with out any issue. Today it is easy to check cylinder thickness with ultra sound before you bore. Back in the day before ultra sound was invented, shops were confident that you would not get into trouble at 0.040, and in the rare case that you did, it was a bad block anyway.

Drag racers used to bore engines 0.060 back in the day. The problem with boring more is if the cylinder walls get too thin you can have problems with the engine overheating. They don't run long enough to overheat in a drag race only situation.

Last edited by olddog; 07-16-2017 at 07:32 AM..
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Old 07-16-2017, 09:16 AM
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Quote:
Originally Posted by olddog View Post
Since he said shave off the top of the piston, I would assume he was trying to gain some piston to valve clearance, which is very common with larger cams. I didn't see the cam specification in your list.

If that is the case, it makes perfect since. Having worked with language communication misunderstandings, make it fit does not necessarily mean fit in the block. I fit may mean no collision of parts, when it rotates.

Back to your original 0.040 overbore. That is commonly done, but most people choose to bore no bigger than they have to, so they have some room to bore again on the next rebuild.

0.040 overbore is a general rule of thumb that almost any engine can be bored that far, with out any issue. Today it is easy to check cylinder thickness with ultra sound before you bore. Back in the day before ultra sound was invented, shops were confident that you would not get into trouble at 0.040, and in the rare case that you did, it was a bad block anyway.

Drag racers used to bore engines 0.060 back in the day. The problem with boring more is if the cylinder walls get too thin you can have problems with the engine overheating. They don't run long enough to overheat in a drag race only situation.
Shaving material off the top of the pistons would only be to increase compression ratio, or trying to make a 9.500" piston fit into a 9.480" block. Increasing piston to valve clearance would be considered "fly cutting" the valve reliefs.

It's also a wives' tale to say that overboring a cylinder causes the engine to overheat. That is pretty far from the truth. The only issue that extreme overboring causes is a thin cylinder, which decreases ring seal.

I've sonic tested two 351C blocks recently with less than .050" cylinder wall thickness, as they came from the factory, standard bore. The only detriment there is cylinder weakness.

Combustion heat takes place in the head. That's why you can put block filler in an engine block and the water temperature doesn't increase. The oil temperature will go up, but the engine will not overheat.
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Old 07-16-2017, 01:16 PM
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Originally Posted by blykins View Post
Shaving material off the top of the pistons would only be to increase compression ratio, or trying to make a 9.500" piston fit into a 9.480" block. Increasing piston to valve clearance would be considered "fly cutting" the valve reliefs.
Bad day, typo, I don't know, but taking material off of the top of the piston will decrease compression not increase it, just to keep us all honest and factual. That or pop up pistons are for low compression.

I do believe "fly cutting" the valve reliefs may well translate from one language to the next as shaving material off the top of the pistons, when a person cannot think of the correct words. French is sometimes lacking technical words in mechanical terms. I work with a sister plant in France. I could be flat wrong, but offered it as a possibility. I wish I could recall an example. We have had some big misunderstanding when everyone thought we were clear.

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Originally Posted by blykins View Post
It's also a wives' tale to say that overboring a cylinder causes the engine to overheat. That is pretty far from the truth. The only issue that extreme overboring causes is a thin cylinder, which decreases ring seal.
Well I have questioned the overheating claim in my own mind. I assumed the less cast iron, the less mass to make a thermal flywheel - so to speak. Under steady state conditions it make no difference, but in a short burst of maximum power, the temperature spike would be shorter and higher with less mass. It is hard to see how a few thousands of an inch would matter, so I accept that I have continued to pass this old wives tail about. Guilty as charged.

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Originally Posted by blykins View Post
I've sonic tested two 351C blocks recently with less than .050" cylinder wall thickness, as they came from the factory, standard bore. The only detriment there is cylinder weakness.

Combustion heat takes place in the head. That's why you can put block filler in an engine block and the water temperature doesn't increase. The oil temperature will go up, but the engine will not overheat.
I read a bit on that on your C website. I wondered if you could put an epoxy filled block on the street. Sounds like you can.

By the way thanks for the free education. I appreciate all you do for this forum.
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Old 07-16-2017, 01:36 PM
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No sir, think about it again.

What happens if you take a flat top piston with two valve reliefs and whack the top off? You get smaller valve reliefs. You may put the pistons a few thou further down in the hole theoretically, but decreasing the piston volume makes a nice difference.
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Old 07-16-2017, 03:42 PM
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Originally Posted by Artur View Post
You may be asking yourself why I'm not asking these qsts to the MAN himself - unfortunately in dubai, we do have communication problems with engine masters n they most of the time don't speak our language or communication is very basic !
The stack of (1/2 stroke + 5.956"rod + 1.772" new Piston Comp Height ) is 9.478".
Since your 1972 block nominal block height is 9.480" which gives you 0.002" piston to deck clearance, now if your block had the decks machined in its previous life your builder might be concerned about the piston to head clearance .... this should ideally be between 0.040" min to 0.055" max with the head gasket you have chosen.

If the above is the case then he might have to machine piston crown to get correct piston to head distance, Valve to Piston clearance with your camshaft should also be checked if this is reqd.
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Old 07-17-2017, 06:25 PM
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No sir, think about it again.

What happens if you take a flat top piston with two valve reliefs and whack the top off? You get smaller valve reliefs. You may put the pistons a few thou further down in the hole theoretically, but decreasing the piston volume makes a nice difference.
Well yes, but you kinda left off some important information or we were in different gears. So yes if you bought a piston that was for the taller deck and milled the top off, to the height of the shorter deck, leaving less of a dish, then yes you would have a higher compression ratio than had you bought the shorter deck piston. Yep I see that.

However if we are talking a given piston and you milled the top off of it, that piston will have a lower compression than had you not milled the top and put it in as it came. That was where my head was at. Guess we were in different gears.

My wife does that to me all the time. Maybe it is me and I missed something (because I usually have her half tuned out), but I suspect she thinks I should be able to read her mind.
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Old 07-18-2017, 03:11 AM
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It's actually a common way of "customizing" a setup like that. You cut the volume of the piston down, adjust the deck height, and you have higher compression. It's the poor-boy way of doing it. In a world of cheap custom pistons, it's not really viable, but you can make it work.

On a piston with a large bowl shaped dish, the gains in compression would be significant.

Even in a situation where the pistons were at zero deck or below deck, you can get to a spot where the amount of material you're taking off is greater than what you're skimming off the piston, *if* the shape of the piston "volume" is conducive to that.

Most piston volumes are not "straight-walled" cylinders of volume. If you look at them closely, they have taper to them, or a "bowl" setup, where if you start whacking off the top, you really start making gains.

You are correct, I'm speaking in generalities, simply because the same principle can't be applied to every single circumstance out there.
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Old 07-18-2017, 03:45 AM
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OK. I'm with you, when you adjust the deck height. Makes perfect sense.
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