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Cracked Forgies???

6K views 25 replies 11 participants last post by  davis 
#1 ·
hey guys just want to know if anyone has ever heard of cracking forgies i just got my new motor it is a 351 with 4mab crank arp rod bolts standard shot peened rods some sought of forgies its .30 over and has been decked to some height with 2v closed chambers that have been polished single valve springs with collets and retainers can you tell me whether you reckon it is possible the pistons are cracked as previous owner has said they are and can you also tell me what sought of block it is JG41ML26734K thanks guys any answers would be appreciated
 
#4 ·
do you need forgies in your engine? if you need to pull it down i'd recomend not to put them back in i am using kb signature series hypers in my new engine but many other good brands out there acl race series are quite good from what i here. how noisy is the engine on start up as?
AZZA
 
#6 ·
azza said:
do you need forgies in your engine? if you need to pull it down i'd recomend not to put them back in i am using kb signature series hypers in my new engine but many other good brands out there acl race series are quite good from what i here. how noisy is the engine on start up as?
AZZA
If forgies were noisey on startup? So would have hypers. It's called piston slap. More common with LS1's. It's not unheard of with a clevo but it's not a good practise to rev a clevo with piston slap. Piston slap is cause by bad clearancing (too much clearance).

I run forged pistons without any issues. I run them due to my 11.2:1 compression. Hypers supposedly aren't made to take more than 10.5:1 punishment? I honestly can't see why but I am not an mechanical engineer.

Brenden
 
#7 ·
Brenden,

Agreed. But forged do require slightly more clearnace due to their tiny bit more expansion. Depending on what type. We're not talking much here, but hey.

I had always been told that forged require more warm up time/ or gentler warm up.

Hopefully hypers can take more than 10.5:1 punishment as I'm around 10.4:1 with gas? :idk:

Time will tell I suppose. But at this stage I'm on your side saying why not??

Back to the original post, why did the previous owner think the pistons were cracked?
 
#9 ·
hey guys the previous owner bought the engine ioff another fella who said they were cracked neither me or the previous owner have heard the engine started only been told so i am speculating to pull it down but yeah i dont no much about the state of the pistons at the moment and have got a set of hyper's lined up if needed they are a set of sterling high performance pistons model number h555p if anyone knows anything about these pistons could they please share it and also can someone tell me what sought of block i have
 
#11 ·
JG41ML26734K

JG - Ford, Broadmeadows VIC
41 - Falcon 500 Ute
ML - Sept 1972

Seems like I enjoy doing this stuff... ;)

So that makes it an XA Ute engine block. If you suspect the pistons are cracked and the engine hasn't been run for a long time (sounds like it), pull it down. I have no idea about forged pistons cracking, but one answer could be freeze/thaw...

Basically, when something freezes or is subjected to very low temps, it will compress - doesn't matter if it's aluminium, steel, titanium or ice; it's all the same. Then if it heats up fast enough, then it will expand. If this happens regularly enough, and with regular frequency, it will eventually crack. This might not be prevelent to pistons, but it happens the world over with other things, so it's one theory.

Regardless, if the engine hasn't been running for a while you're best answer as to whether you have a problem is to take it to bits and find out if anything is actually damaged. And an engine that hasn't run for a while deserves a bit of TLC. Take it apart - see what you find.
 
#12 ·
hey guys just got the heads pulled off today and there are no wear marks on any of the bores they all have fresh honing marks the pistons dont have enough carbon on them to look like they have ran for more than 1000k's you can take the pistons back to bare metal just using your fingertips and there are no wear marks on the rockers or pushrods so at the moment it lookslike he hasnt even ran the motor in so any ideas would be good i would like to get this motor back together as soon as possible as i get my licence in three weeks and would like to have this motor in the car by then
 
#13 ·
Sounds like too much clearance....OR pistons put in wrong way around and pin off centre is wrong, causing side thrust-piston slap....
 
#15 ·
UCNTRY said:
the pistons are L2379 030 the block has a .030 overbore done on it and all of the arrows on the pistons say front and point to the front of the motor
Are they cracked?
With cheaper type forged pistons like TRW, there is allways more clearance made when boring the block. This is beacause the cheaper forged pistons expand more than cast pistons. That is why some people think they have piston slap when cold, but really they just more clearance till it gets hot. That is why you are meant to take more care warming up motors with these pistons. If not, they can suffer a broken piston skirt due to them "rattleing" in the bore while cold. I use TRW, and refer to them as cheaper type because compared to other race forgies, they are cheap to buy.
 
#16 ·
ebxr8240 said:
Sounds like too much clearance....OR pistons put in wrong way around and pin off centre is wrong, causing side thrust-piston slap....
I've tried to work out what you are referring to? If it was the carbon comment, he was talking about the marginal amount on the top of the piston. At this stage, no one has heard the beast running or slapping.
 
#17 ·
ok thanks for the comment but yeah the engine doesnt even look like it has been run much at all and the person that sold it too my friend has assumed that it has cracked pistons in it so at the moment im wanting advice on what to do as i have a limited budget seen it is springnats this weekend coming in my local town and i have three weeks to get this motor in and running do you guys think i should just bolt it all back up and put it in and hope that it was just teething issues on first couple of fired ups or should i take the pistons out and get them crack tested
 
#19 ·
I have seen these pistons crack before.Both the TRW L2379 and sealed power 2298 are notorious for cracking around the skirts near the pin.Overly generous piston to wall clearance is usually the culprit or as Brenx said revving the engine before the pistons have had time to heat and expand properly.They are a relatively cheap but heavy piston ,but if set up with the right tolerance will put up with a lot of abuse.
To see if they are cracked you would have to pull the pistons and rods out(Yes,all eight) and look real close at the skirt section as the cracks can be somewhat difficult to see,moreso if they are still inside the engine.Before I pulled them out though i would certainly be measuring the piston to bore clearance and see if it is within tolerance.some machine shops give these pistons way too much clearance.
 
#20 ·
the previous owner said that the reason the original owner said they were cracked was because forgies only have a lifetime of 20,000kms so supposively they have done that many k's but must have been cleened when put into motor about 1000k's ago
 
#24 ·
Forged pistons are harder on the bores than cast pistons so bore wear will be higher, more wear = more clearance and excessive clearances can crack the piston, in theory there is some merrit in the statement that forged pistons don't last as long but there are allot of other factor including how hard the motor is reved that will determine it!!!
V8 supercar pistons would be buggered after 2000 k's!
 
#26 ·
4Vman said:
Forged pistons are harder on the bores than cast pistons so bore wear will be higher, more wear = more clearance and excessive clearances can crack the piston, in theory there is some merrit in the statement that forged pistons don't last as long but there are allot of other factor including how hard the motor is reved that will determine it!!!
V8 supercar pistons would be buggered after 2000 k's!
Sorry, but there is no way that forged pistons are "harder" on an engine than cast pistons, especially the bores. Pistons do not contact the bores, so the material difference in terms of hardness can not be an issue with regard to bore longevity. Additionally, since forged pistons expand less than cast pistons (including hypereutectics), they are less prone to "rattling" in the bores on cold start-ups. If the oiling system is doing its job and slinging oil into the bores, then the skirts of the piston will properly float on a light coating of oil between them and the bore.

The idea that forged pistons do not last as long is utter BS. Perhaps more true is that engines with forged pistons tend to be hot rodded a lot more and that, not the piston material or production method attributes to lower life including faster wearing bores. Also, since you can usually rev and engine with forged pistons higher than cast pistons (assuming the rest of the engine is built to take it), more revs per minute account for greater wear over a given period of time.

Rings are what wear bores. Cast pistons expand a good bit more than forged pistons. This expansion process leaves the rings either loose or tight depending on how you look at it. Ideally, an engine builder builds for typical operating conditions and that is going to be the temperature that the piston routinely sees during normal driving conditions. When pressed into "harder" service, cast pistons oftentimes expand beyond reasonably safe limits and cause additional friction, heat and resultant wear on the bore. For this reason, hot rodders with a clue run forged pistons. Attention to proper bore clearance is of the utmost importance when fitting pistons. However, more than bore clearance attributes to fitment including rod side clearances and definitely ring end gap clearances. Another factor that can often be seen in novice engine builders is ring ends that are not filed squarely.

Bore oiling in a Cleveland is a bit whimpy, comparatively speaking. It is not uncommon for NASCAR and other kinds of all-out racers to place bore oilers in their engines.

If excessive wear is responsible for cracking pistons, then the hypereutectic pistons with their fairly notable clearance requirements (due to large expansion) are subject to such situations. However, excessive bore wear is likely to produce ring sealing issues long before pistons crack.

Every good engine builder that I know absolutely won't touch a hypereutectic piston for any purpose...and they all swear by forged pistons. The only people I've ever heard of who push the "hypers" are trying to make a quick buck and sell cheap cast crap to someone who is on a tight budget and doesn't understand what junk they truly are with regard to high performance engines. If you want to rebuild a stocker, but stocker cast pistons, not hypereutectics. If you want to build a hot rod or a race car, use forged pistons. If you're spinning your engine above 5000-5500 RPM regularly, then forged is definitely the way to go.

I wouldn't use hypereutectic pistons in my worst enemy's engine. I think that they are cheap cast junk with the thermal qualities of a high silicon cast piston. Anyone want to guess what the added silicon does to piston strength? It reduces it!

:davis:
 
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