[DGR-Z246-98]

Hello All,

Just coming to the end of a part rebuild on my 10 year old Merc 5.7L, during which a groove was found on one of the cylinder walls. To cut a very long (and expensive) story short, the outcome is a 30 over rebore.

Am curious what that takes my official capacity to now? anyone have any experience in such matters??

All the best,

Steve.

[WetWilly]

Hi Steve,

Ok, this is taking me back quite a bit but if I remember right, your 5.7L (350Ci) with a 30 over would bring it to 5.8L (355Ci).

WetWilly

[DGR-Z246-98]

Many thanks for the info WetWilly, any detriment to having this done? will there be any noticeable difference do you think or is it too small to have any effect??

Hoping to get her back late next week - finally!

Steve.

[WetWilly]

Hi Steve,

No problem, I like to clear the cobwebs in my mind from time to time... As for your questions,

#1) No.

Again, if memory serves me right, your 5.7L is a Chevy small block 350Ci and a 30 overbore is pretty much standard for rebuilding any engine. I don't believe the 350 is a thin wall block, that being the case, it should be able to handle a 40 and at the most, a 60 overbore if it's never been rebuilt before. As long as all the other components are in good shape, rebuilding an engine is a cost effective alternative to a new engine.

#2) No.

As for any gains... from my experience with land vehicles, if all your doing is a standard rebuild, you will not notice any difference with the rebuilt engine due to the overbore alone once it's broken in properly verses a healthy stock one. Of course, if your were running around with a very tired motor, you should see a marked improvement in performance and fuel usage with the rebuilt motor.

Remember to break it in right then let us know how she performs!
WetWilly

[il_Mio]

Hi,

I often notice people refering to breaking in correctly.
What is the best way of breaking in a new / rebuild engine ?

Cheers,

Johan

[indykoch]

I don't really know what the "best" break-in procedure is, but here's what I've always heard...

For the first 20 hours, keep your RPM's varying. Don't keep a constant speed for long periods.
Some say *no* full throttle... others say break it in like you'll be running it afterwards. I always seem to think a few, short bursts at full throttle couldn't hurt.
Change engine oil after 20 hour break-in. You'll see a lot of fine shavings in the oil or on the drain plug (if magnetized like most are).

I think that's pretty much it. Main thing is to be easy on it, run at different RPM's throughout the range, and never stay at full throttle for long periods.

[DGR-Z246-98]

Thanks for the reply WetWilly, I'm glad you mentioned the breaking in, because after all the fuss it has taken to get to this point I had completely overlooked this!

The original work was simply to replace the oilpan due to a small leak from corrosion, but as this was an engine out job I decided to replace the risers at the same time as she's always been run, and berthed, in saltwater so I knew they'd be starting to deteriorate by now.

Unfortunately this was probably a step to far for the engineers here, and although everything appeared fine on idle we ended up with a fairly bad overheat on the trip back to the marina which resulted in the part rebuild.

The root cause has never been determined (or maybe admitted too) but I'm guessing that a bunch of riser corrosion worked it's way down into the system when they originally removed the old ones and that they hadn't flushed it properly.

This time we'll have their engineer on a sea trial before taking handover, but naturally I will be keen to test her as close to full load as possible before I sign off on the work with any confidence.

Indykoch - thanks for your reply, I'll make sure I vary the throttle but what would you think the maximum time I'd be safe running full would be in the test or should I just avoid it all together? I'll be sure to ask them about the oil change too!

Thanks for all your help guys,

Steve.

[Z 202]

I would be encouraging some WOT running during break-in. Remember, you are dealing with a rebuilt here. You need to get the rings to seat well early in their life. That won't happen if you don't get that block good and hot. Take it out, get it up to temp and then drive it like you stole it for a bit.

I don't know where the fear in going WOT during break-in comes from. If it has been put together correctly is shouldn't be coming apart at the rpm limit.

You also want to avoid using a synthetic oil during break-in. They are slippery enough to be missed by the rings and begin a glazing process on the cylinder walls. Once all the rings seat this no longer an issue.

[il_Mio]

Thanks Indykoch & Z202

Johan

[DGR-Z246-98]

Thanks very much everyone, I'll let you know how I get on...

Only problem now is that I've been reading the posts on 'best add-ons' and my shopping list is getting out of hand!!

Steve.