raindance654

HI.
I'm looking to rebuild a 305 from an '88 Firebird and I'm trying to do so on a fairly tight budget.
I also want to get the longest life from this build so I'm gonna bite the bullet and spring for a rebore and new pistons.
I'm trying to avoid the need to re-balance the reciprocating assembly and the consequent extra costs.
To this end, I'm looking for replacement pistons which weigh in the same ballpark as the stock factory items. Since replacement pistons seem to vary pretty wildly in advertised weight, I'm trying to get some idea of what the stock ones would weigh before I commit myself to removing and destroying the existing pistons.
Any offers on the weight of factory 305 pistons or where I could find this information...........???
Rainy.

AlkyIROC

Unless your replacement are exactly the same as OEM, a few grams will make enough difference that it should be rebalanced to get a perfect 100% balance.

Keeping it as a daily driver and rpms stay well below 5000 rpm, a stock replacement set will be close enough that a rebalance isn't really required. All OEM engines are not 100% balanced.

raindance654

Thanks for the reply.
This is what I figured, AlkyIROC, is that since it's gonna be a daily driver that replacement stock pistons would be fine with the factory balance job, taking into acount the less than spot-on job from the factory.

The problem is that replacement style OE pistons seem to have reduced pin height to allow rebuilders to machine generous amounts off the deck height and/or dished piston crowns meaning a plummet in CR and a corresponding drop even further in power output.
Flat top correct pin height pistons seem to vary more than 100g in advertised weight which is way more than can be tolerated.
This brings my thinking back finding a set of suitable pin height pistons with decent CR with a weight in the neighborhood of the originals.....hence the need to determine their weight without removing them from the rods atthis point to weigh them.

Any offers here.......???

8Mike9

For just a just a stockish rebuild, I'd probably get a jobber kit spec'ed to my machine work, I wouldn't even bother asking about the piston weights when ordering, as they'll be pretty close to what was in the car before.

You can then put them on a scale when you get them and match them closer if you like, but probably not needed.

One thing I'd probably do though, is if you have the rods resized, I'd have the shop match
them up, or at least see if they need any adjustments.

raindance654

Ordinarily, I'd agree, Mike............but it's the pin height and piston crown changes that are making the problems.
I'm having problems finding replacement flat top pistons with OE pin height and comparable CR...........
........ones that have OE pin height and flat top crowns can be as light as 580g or as heavy as 685........and with variantions as large as this, I can't just throw in a random set and hope for the best.

If I know what the originals weigh, I can pick a set of pistons that are of similar weight and stand a chance of 'getting away' with doing this.
I don't want to press of a piston and weigh it as this would remove the option I have at the moment of not reboring (one I'd rather not take).
Pls guys ...........any number at all for this......??

8Mike9

Let me clarify something that may put your mind at ease.

The crank is factory balanced, turning it .030..won't chance it's balance.

The rods are matched at the factory via weight, that's about all..there is a chance if resized, you could toss a few out of the comfort zone..it's why I mentioned it.

The pistons are all spec'ed to weight.

So for grins...let's say you took a brand new car with 11 miles on it, pulled the engine, dropped the crank to change the piston from a dish, to a flat top, to get some better CR.

Let's say the pistons you put in were 100 grams heavier, or lighter than what you took out.

How would that change the balance of what was there before?

It can't and won't

AlkyIROC

100 grams spinning at 5000 rpm would feel like pounds on the end of the rods.

For balancing, the crank is balanced (internal, external depending on the application) depending on the final bob weight.

Main bearings are not part of the equation

Piston weight is matched with wrist pin weight for a total weight. If weight needs to be adjusted, it's removed from the pistons to match the lowest weight. Not all wrist pins weigh the same amount so mixing and matching may be done to find a similar weight. Once the pistons and wrist pins are matched up, those pins need to stay with the pistons.

Rods are weighed at both ends. Total rod weight means nothing other than to the final bob weight. Each rod is reduced to weigh the same at each end as the lightest rods. Small end weight all reduced to the same weight as the lightest. Big end weight all reduced to the same weight as the lightest. Just because the rods weigh the same doesn't mean they have the same weight at each end.

Weight of pistons, wrist pins, rings, rod bearings, rod bolts, nuts etc are all added up to get a bob weight.

When I went from a 540 to a 588 engine I did a few changes. Piston was bigger for the bigger bore but was lighter than the ones for the 540. I also went with a thicker wall wrist pin. I switched from H-beam rods to billet I-beam rods.

When everything was done, the bob weight was less with the 588 than is was with the 540. The engine balancer I use got my crank balanced down to a few grams. He doesn't like seeing anything above 10 grams out of balance for a high reving engine.

For your street engine, you can roll the dice and slap it together without getting the rotating assembly balanced. The antenna may vibrate at an idle if it's out of balance enough. Chances are it will run just fine but I wouldn't gamble on it being a lifetime mileage engine if the balance is too far out.

You can assemble an engine, you can build an engine, you can blueprint an engine. It all depends on your budget as to how it's put together. You can also go buy a basic stock replacement crate engine for a fraction of the cost and with a warranty and drop it in. If you're lucky, you might even get one that's been dyno tested.

8Mike9

Well sure, I think what you might be missing...heck, even I may be missing something, is he is doing a budget rebuild.

My assumption...have parts machined, and he's assembling.

He's sticking to the 305, chances are he's not looking at 400HP...he may be wanting a bit more pep from his 305...thats all.

Do you think GM did any more than I let him know?

Ofcourse not...GM certainly did not have a room where guys were checking mass on both ends of the rods..packaging a set..and sending it with the "kit".

He's looking at a rebuild, not a performance rebuild, just a DD that will last 200K, like GM does them.

Don't scare the guy into spending a few hundred unneeded dollars and having everything balanced not necessary for what he is doing...unless I missed something, he want's a sock rebuild.

Why have him spend all the additional dollars for a 215 HP engine, like he's building a 500 HP unit?

I guarantee you that any stock replacement GM engine is "just slapped together"..as are the majority of non performance rebuilds in every flavor of engine.

AlkyIROC

If he really wants to save money and avoid future issues, a rebuild isn't the answer. Like I already said, go buy a stock replacement crate engine and drop it in. It will cost at least half of what a rebuild that you expect to last will cost.

By the time you buy all the parts, gaskets, seals, machine shop work then the time to assemble it all properly and hope you don't make any mistakes, it will cost more than buying a prebuilt engine.

Yes you can go with a dirt cheap rebuild to try and save money but if it fails, was it worth the cost? An engine that's just slapped together isn't going to last the 100,000+ miles that's expected of a proper rebuild. To me, if you're really asking these kinds of questions to do a rebuild, you probably shouldn't be doing it. Too many people get way over their head and end up spending far more than they expected trying to do a budget rebuild.

8Mike9

I agree completely...but there is "budget done right" which is far different than slapping an engine together.

Thing is, for a proper "budget rebuild"...not a high performance "how I'm gonna make 400 HP on 200 bucks" thing, he'll have to have the machine shop go through/check out the bottom end and machine as necessary.

He can save additional dollars by taking the heads apart, and just take in the valve and heads, have the heads rebuilt, have them snap check and label the valves, and assemble with new springs if needed.

When all is said and done, his out of pocket cost will be 1/2 of a crate engine...then he has to put in his time and rely on his ability to recheck work that was dome when assembling...and his ability to properly assemble the engine.

If the machining is good, if the parts are in spec, and the assembly is done properly and checked along the way, it should be like new.

Now if the OP is thinking just bore and hone the cylinders, toss in new pistons and rings...I agree, that's slapped together and most likely won't get him a 100K...might quit smoking though

Edit:

I might add a jobber master rebuild kit which will have just about everything he needs, sans an oil pump...and maybe a timing chain set...can be had for 400-500 for a SBC

Machine work on the block and heads, should run (in my area anyway) more than 400 to 500...add a 100 for a new melling pump an timing chain set..is still less than 1/2 the cost of the lowest Hencho in Mexico crate engine available