I had a little bit of blow by (and the oil smelled like fuel) so I decided to tear down the motor to see what was going on. I found everything to be in good shape except the top ring on the #4 cylinder had about half the gap of the others (out of the bore.) The ring also had a slight ridge on the bottom of it and the others did not. Here are the specs on the motor.

400 .030 over
SRP pistons
about 1300 miles and 3 passes since I put it together.

The motor didn't have blow by until after I went to the track. There is a good chance is was a little lean in the upper rpm range. All the plugs look good. The bores show a little wear from piston rock but you can't catch your fingernail on it.

Any ideas?

 

bubble hone the cyls and get some new rings and slap it back together

 

That sounds good, but I got a bore gauge and I will verify everything is still round and the Piston to wall will still be ok. Any suggestions on which rings? It is mostly street driven... 11.4:1 compression.

 

I measured the bores today and the piston to wall seems way out. The bores are measuring 4.165 to 4.166. The pistons are 4.150. It looks like I'm about 3x the recommended piston to wall clearance. The guy who originally machined the block must have gone .040 over.

So here is the question... Should I look for a new block, or just get some new pistons? I have srp forged with floating pins right now.

 

Did when you went to the track hit this engine with nitrous??. Power adder's require more ring gap to prevent he top ring from butting together.A lean condition could do that too.The top ring gets hot.

Did you measure the bores at three locations in the bores while checking their sizes??.

Did the machine shop check the deck of the block for square/flatness then have the pistons IN HAND boring and finish honing with torque plates??. Note:in hand for the correct size of the bore fitted to the pistons in use.You can't bore a block in advance not having the pistons.And forged pistons require more clearance.Are you sure you using the correct spec to check the bore sizes??.

Good 400 block cases are getting harder to find.Yours being at .040 over already I do not recommend bore that to .060 unless you sonic check it for wall thickness.And if it is good to go at .060,then following what I outlined above.That is critical to these 400's.

Are you using the stock 400 heads and if not did you drill the steam holes??. When a engine gets hot it's not only the cooling system that gets hot,it cooks the oil losing it's ability to lube the engine.

With the short run time on this,as you said,there certainly is something radically wrong with the build.As a after thought,the measurement you made was only on number 4 bore or on all the bores??.

 

If the bore is smooth with no ridge at the top then I agree with above- measure every bore at multiple points to make sure it's actually a true 40-over, not just tapered from wear or a single-cylinder phenomenon. If that all checks out, proceed with 40-over pistons and rings (which also gives you the opportunity to change your compression ratio, if you want to) after doing a quickie hone on the cylinders.

Good thing you have a bore gague! Not everybody does. This is exactly the kind of situation for which it exists. KNOW, don't guess.

Lots of shops will bore and hone a block without the pistons in-hand, unfortunately. They know if they hit an exact 40-over the pistons are designed to produce the proper piston-to-bore clearance in that hole. That is to say, the piston is designed to be slightly smaller than the overbore it fits into, not that the piston is exact to spec and the bore has to be taken slightly oversize to get the required clearance. It's kind of a "sloppy" way of doing things, but it usually works so shops just nail the bore spec exact and ASSUME the pistons will come the right size to fit that bore with proper clearance. Then they move on to the next job.

Not sure if I explained that very clearly, but that's how it works.

 

Yes, I did check the bore at multiple locations (horizontal and perpendicular axis at top middle and bottom.) They heads are performer RPMs and they have steam holes drilled in them. I actually only measured number 2, but I will get back in a measure them all.

I definitely think there was a fueling problem at high rpm. Unfortunately my laptop was acting up and I couldn't datalog the runs. From what I could tell though AFRs were up around 16:1 over 5 grand.

I highly doubt the machine shop had pistons in hand when they bored and honed the block, and from my calculations it was not decked at all.

What do you guys think about getting another block? Rather than dish out another $500 on pistons for this block. Would it be better to find a virgin block and have it bored, or one thats already .030 over? I'm seeing 400 blocks on craigslist around here for 3-500.

Thanks again for your help.

 

H311 no! If the bores measure out good on all 8 you buy new slugs (nowhere near $500! What pistons are you buying??) and put it back together. If you previously had 30-over slugs in a 40-over block you were running with one leg tied behind your back. A set of 30-over rings will BARELY seal against a 40-over block AT BEST. And with all the piston rock from the excessive clearance.... no way.

FYI- oil in the combustion area is DEATH when nitrous is involved. Even a modest amount past the rings can drop the effective octane of the mixture dramatically. Detonation is much more likely.

Get the right pistons and rings in those bores and you won't believe it's the same engine. Well, OK, it won't run much different than before but it'll last this time.

If you really want to do a fresh block I would save your pennies for an aftermarket one. Especially so if you want to run a 400. Reworking ancient factory 400 blocks is getting silly when compared to the cost of a GOOD NEW BLOCK from the aftermarket. And then you'll have a foundation that can handle anything you're likely to throw at it. You'll use it again and again and again trying different combinations over the years.

 

OMG Damon!!!. Did you read what I posted or just ignore it??. Look it isn't just for race applications those steps have to be followed,it is all 400's from what was learned over the 40yrs the 400's have been around.Listen and learn. This posting I made wasn't from just another guy.Rather from someone who's been there done that.

First your suggestion is not follow the machining steps.WRONG!!!

Then to slam pistons into bores that the bores maybe can't be fitted to the pistons.WRONG!!!

Explain how in god's name is he ever going to get ahead of his problem??.

Come on Damon...........

 

A good shop should be able to machine a block without having to match it to a specific set of pistons. The factory never did that. Thats why they have tolerancing. The bores absolutely should have a torque plate when machined, but matching it to a specific set if pistons is a bad idea in my opinion. Instead it should be machined to the correct size for the size of pistons that one intends to use. If you decide to put the cart in front of the horse, then you could end up with a one-off block matched to an out of spec set of pistons.

Measure the bores and pistons to make sure they all fall within the recommended tolerances so the clearance is what it should be. The pistons SHOULD already be sized to fit the specified bore hole size. If theyre not, then whoever made them is smoking crack, and their parts should not be used. As an engineer, I never send a shop mating parts for the components Im having fabricated. I put the dimensions and tolerances I want on my drawing, and then have the parts made. If the shop does it right, then I get what I ask for and it all works. This is the same thing GM does when they make engines so the robots and CNC machines don't need to grow a brain and measure a bunch of stuff.

 

No,No,No!!!. The aftermarket pistons,even the good sets,are all over the map.Well really the really good sets are sold in a set of 10 so if you have to replace one you have one that measures the same.
I don't have a clue what your engineering has anything to do with this situation other than it is spoken like a true engineer.

We aren't talking about GM specs..........are we??. Production specs are never acceptable for use in high performance applications.Race,street,or anything else.

I'll stake 40 yrs in high performance engines and working for GM Chevy engine plant in the provided advise I have posted.Every set of pistons has to be in hand BEFORE a bore can be done.That is even more important with aftermarket forged sets like I think he already owns.Here is a chart on just a mere set of I-Cons:



There are only two exceptions:

http://www.venolia.com/

http://www.wiseco.com/

So hows about taking the advise from a no B.S.old pro??.

 

I'm looking at forged sets of 4.165 pistons. They are upwards of $450 for the set. I also called the guys with the 400 blocks on craigslist and they all have been, or need to be repaired. So I'm leaning towards a new set of .040 pistons. The car ran a 12.1 at 117 the way it was set up before, so I dont have any qualms grabbing a set of .040 pistons, checking all the pistons and bores, and throwing it back together.

 

Have whoever you use to do the machine work also measure the bores to see if they can find a source for a piston that size and then follow the steps of looking at the deck for flatness and least re-honing the block using the torque plates.It's a compromise,but a good one.

 

I'm going to justify what I am suggesting.The Siamese bores on this 400's move around a ton.The torque plates simulate the stress on the deck/bores like as if you have the heads torqued to the block.Now you already have a sealing issue with what you say are the wrong parts.

BTW-I'm having a hard time understanding a 12 second engine,that is .010 off with the pistons,and wonders behold,no bore damage!!. No piston damage!!!. Just one ring damage. Those pistons had to be rocking like crazy. Especially revving up high enough to run 12's.Still thinking there is something wrong in the measurements your making.Might want to have the machine shop measure it too anyways.

We have bought a bunch of 9.90 roller cars that we raced for a yr and sold.That spans over best guess 30yrs.And in a number of them we did use a 400 engine.To help stabilize the bores we filled the blocks about 1/2,then followed the steps I outlined.We have also build I would guess over the yrs about 1/2 dozen street/strip 400's for customers.I'm telling you it is a huge mistake not to torque plate them 400's.

So if you already have had a sealing issue with alleged wrong parts,didn't follow the procedure necessary for 400's and you know the 400's have a sealing issue,now is the time not to go back to what you have by compounding it.

If you hot leakdown that engine five different times and don't do what I am suggesting,your going to get 5 different results and find low cylinders that have moved to different locations.

If you want further variation of what I'm telling you,look at the aftermarket blocks designs that addressed the issues the 400 OEM blocks have had.

 

I agree that is might be a good idea to have someone else check the dimensions just to be sure. It might actually be good to have several others check them, since even with a bore gauge, much of the result is due to setup, procedure, and feel. I know I can take two measurements at different times and get different results, even with thermal consideration.

I would further suggest that you have an somewhere other than an automotive machine shop check your measurements. If you are going to split hairs, it would be best to work with someone who does it routinely. Most automotive machine shops can't hold (or accurately/properly measure) a tenth tolerance across even a 22" span, let alone a half-tenth over a 6" long bore. In 98% of the work going through an automotive shop, that is sufficient since it is a pretty forgiving assembly. In your case, more accurate methods may be required.

Find someone/somewhere that deals with metrology at this level. Most shops that perform ID/OD grinding, EDM shops, or diemakers work will have this capability. Ask around.

 

My mistake, I thought you where referring to taking measurements from the actual pistons, rather than giving the shop the specificaitons called for by the piston manufacturer. Thats what I was trying to convey in my post, is that the bore size should be given to the machine shop based on whats called for from the manufacturer for the specific piston and application.

Agree?

 

Well really both the manufactures spec and measuring the pistons to be used having those pistons in the hands of the machine shop.Consider this.Most of the guys on here are buying the lowest dollar pistons and those sizes need to be verified and in alot of cases the bore sizes adjusted to the pistons that where purchased.It is a export of the market and the parts that are being sold.

 

I agree 100% that they need to be verified to make sure you get what your asking for, and they're in tolerance. But, at the same time I would not let the actual size of the pistons drive the final bore size. Goes without saying but I've seen crazier stuff done. If the pistons are out of spec, its best to send them back.

In my mind, the way to go is to select your pistons for your applicaiton. Purchase and measure the pistons to make sure you get whats called for. Then machine the block to the specified bore size for the piston and application and measure that too when they're done to make sure its all copasetic. Sounds like in the case of the OP that was not done.

 

I'm fairly confident in my measurements. I did it over 20 times on that one bore in different spots and was getting consistent results. I also used a caliper at the top of the bore and it was right at 4.165.

The pistons were really loud especially when cold, but they would quiet down a bit at operating temp. There is definitely evidence of piston rock on the bores and the skirts but it doesn't look terrible. I'll snap some pictures of the bores/pistons/measurement process so you can let me know what you think.

 

The thing that gets me and the reasons why I am thinking something is wrong with the O/P's measurements,is at .010 oversized bore to the pistons on the first start-up at a idle you would think is would run like a raddled paint can no less run 12's.

 

All the pictures are taken from the front of the block (top of the photo is the rear of the block.)


http://www.flickr.com/photos/91135913@N04/8276341322/http://www.flickr.com/photos/91135913@N04/8276341322/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8276341584/http://www.flickr.com/photos/91135913@N04/8276341584/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8276341736/http://www.flickr.com/photos/91135913@N04/8276341736/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275276369/http://www.flickr.com/photos/91135913@N04/8275276369/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8276342062/http://www.flickr.com/photos/91135913@N04/8276342062/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275276703/http://www.flickr.com/photos/91135913@N04/8275276703/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275276921/http://www.flickr.com/photos/91135913@N04/8275276921/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275277057/http://www.flickr.com/photos/91135913@N04/8275277057/ by http://www.flickr.com/people/91135913@N04/, on Flickr

 

Here are the measurements of the bores. #2 is the first one I did, so I made about tried about 3 times in each spot and wrote them all down...

http://www.flickr.com/photos/91135913@N04/8275326037/http://www.flickr.com/photos/91135913@N04/8275326037/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275325921/http://www.flickr.com/photos/91135913@N04/8275325921/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275325811/http://www.flickr.com/photos/91135913@N04/8275325811/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8276390346/http://www.flickr.com/photos/91135913@N04/8276390346/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8276390200/http://www.flickr.com/photos/91135913@N04/8276390200/ by http://www.flickr.com/people/91135913@N04/, on Flickr
http://www.flickr.com/photos/91135913@N04/8275325457/http://www.flickr.com/photos/91135913@N04/8275325457/ by http://www.flickr.com/people/91135913@N04/, on Flickr

I have 2 pics of each piston as well, this seemed like enough pictures for now.

 

Kerplunk- you keep right on doing what you're doing.

Gary's having some sort of "moment" here. He's absolutely right, but he's also barking up the wrong tree, with all due respect.

Your block was, given your very careful measurements, punched 40-over and stuffed with 30-over pistons. And worse than that, it was probably stuffed with 30-over rings, which will do a crappy job of sealing in a 40-over bore.

Yes, it might rattle some, especially when cold, but how loud is your exhaust?

If you would like verification, take your new 40-over pistons, when you get them, to a machine shop (or several machine shops) and have them measure them to verify your ow measurments. You'll find they are a few thou smaller than your 40-over bore. The pistons will measure ~4.160 (assuming forged pistons which have larger clearance than cast or hypereutectic pistons) which will put your clearance just about perfect for a 4.165" bore. Which is exactly what they are supposed to be.

Is it perfect and exact? No. If you want perfect and exact, you have the bores matched specifially with the pistons that are going in them. Otherwise you're right back to where you are right now.

And since you're going to get a new set of 8 you can always play the old game of putting the skinniest piston in the skinniest bore and the fattest piston in the fattest bore to optimize the clearances you have to work with. I have!

 

Not to get all up in your business, but I would take the block to a competition automotive machinest who does this day in and day out. Will cost very little to just double check your measurements with his measuring device that he uses daily and probably has a standard to check his measuring device against to make sure its correct.

Holes should be just right - not too tight and not too loose. Either one will leave you wanting...

BTW unknown to me, I ran a block loose once back in the early 80s. It was around .010 off. Bought the car with the engine in it running like a scalded dog, made it run like a pissed off cat, showed it NO mercy, and it ran daily for about 14 months. Think 20 year old with no bills and heavy foot daily. Rings finally gave up the ghost.

 

Really can't tell if it is a reflection,but the dark spots in the bores looks like where the pistons where rocking.

 

That is where the pistons were rocking. I just ordered some .040 pistons and rings. My next step will be to take the block pistons and rings to a machine shop and have them measure and torque plate hone it. What do you all think? How much do you think the bores will distort with the plate bolted down?

 

Not much. But if you're going racing you want the best ring seal possible, and that's how you get it.

 

I'm far from any "professional" engine builder, but I've assembled a few engines...including the 400 I have in my car now.

I do know forged pistons are fitted a little looser in the bore due to expansion as they heat..I was told by more than one experienced machine shop guy that the forged pistons expand more than Hypers do. I also have SRP pistons installed with a -21cc dish..I kept the compression down to 9.1:1 using 72cc Dart heads.

Looking at your pics, you do have quite a bit more scoring on the cylinder walls than I found in mine...you're definately running looser pistons or something for the pistons to rock that much. I agree with Damon...it really does sound like the shop used .030" over pistons in a .040" over bore.

A new set of .040" over forged pistons do hit the wallet hard since it's just not a common overbore that's sold every day...I believe I paid a bit over 500 for my set 4 years ago. I recommend Total Seal gapless rings...I love those suckers! They do have a normal second ring, which I opened up a bit for my boosted application. Good luck with it!!

 

Just talked to Bub over at Burtonsville Performance machine shop. Sounds like a nice guy here is what he quoted me:

Parts:
Main bearings
Rod bearings
Cam bearings
Freeze plugs

Labor:
finish hone
hot tank
File fit rings
square the deck
Install cam bearings and freeze plugs
balance rotating assembly
check all clearances (obviously)
assemble short block

$1500

What do you all think? I have heard they do good work.

 

Should be and this is a important detail that you be sure is done,"finish hone with torque plates". That sounds about what the average price would be.

Be prepared for someone to come on here and post they had "what" done for so much less.If it is good work,then it is a decent price.

 

Just thought I would post an update. Machine shop did really nice work.




Don't use the cheap chrome timing chain covers! This shot is with it bolted up with 2 of the bottom bolts.


This is me pushing on it at the top:


The factory cover fit flush against the block.






Got a quicktime bellhousing and had to measure the runout.



By the time I got the .014 pins adjusted the total runout measurement was .009 (cant see it because of the flash.) The instructions say to divide by 2, and that number should be less than .005, so I'm just barely within spec.