Cracked pistons

[gtblast]

I am rebuilding a stock 1970 engine for my GT. While cleaning up the pistons, I found that two of the pistons have cracks that are just to the side and above the wrist pin area. These cracks run up and through the bottom land of the oil ring.

I have other good piston and rod combinations from another 70 engine that fit fine. My question is, do I need to worry about the weight of each piston/rod assembly? This is for a stock engine that I am not racing or anything like that. Can I just substitute the two piston and rod combinations in from the other engine and go on with the assembly, or do I need to weigh each one and make sure they are all within some range?

[First opel 1981]

Originally Posted by gtblast I am rebuilding a stock 1970 engine for my GT. While cleaning up the pistons, I found that two of the pistons have cracks that are just to the side and above the wrist pin area. These cracks run up and through the bottom land of the oil ring. I have other good piston and rod combinations from another 70 engine that fit fine. My question is, do I need to worry about the weight of each piston/rod assembly? This is for a stock engine that I am not racing or anything like that. Can I just substitute the two piston and rod combinations in from the other engine and go on with the assembly, or do I need to weigh each one and make sure they are all within some range?

Here's my opinion for what its worth.
You will be happier with balanced pistons but an engine will run with a mixed combination.
Everything in between is a matter of what makes you feel the best.

If I was going to suggest what to do, i would tell you to weigh the pistons and find which 4 are the closest in weight and just go from there.

[tekenaar]

. . . I would only use the entire set (already balanced) out of your second engine after checking their condition, use new rings, check existing con rod bearings' clearances and condition, surface hone cylinder walls to seat new rings and reassemble short block . . . no worries about crank, already in balance, so it's good to go for your use, too.

While apart, check the condition of head and cam/valve train and repair/refresh as necessary. Reassemble the long block and you should have a very reliable engine for many, many miles and minimal cost and effort!

[gtblast]

Originally Posted by First opel 1981 Here's my opinion for what its worth. You will be happier with balanced pistons but an engine will run with a mixed combination. Everything in between is a matter of what makes you feel the best. If I was going to suggest what to do, i would tell you to weigh the pistons and find which 4 are the closest in weight and just go from there.

If I can find a decent scale, I will weigh them and try to find the 4 closest in weight.

[gtblast]

Originally Posted by tekenaar . . . I would only use the entire set (already balanced) out of your second engine after checking their condition, new rings, check existing con rod bearingS' clearances and condition, surface hone cylinder walls to seat new rings and reassemble short block . . . no worries about crank, already in balance, so it's good to go for your use, too. While apart, check condition of cam/valve train and repair/refresh as necessary. Reassemble long block and you should have very reliable engine for many, many miles for minimal cost and effort!

I only have three piston/rod assemblies from the other engine, the forth piston/rod assembly was destroyed, that is why it is a parts donor engine. The cylinders are already honed, and the crank is back in the block with new mains. I have new rod bearings and rings ready to go. I am refreshing the head at the same time. As stated above, if I can find a decent scale to use, I will weigh the piston/rod combinations and try and get them as close as possible.

[First opel 1981]

Originally Posted by gtblast If I can find a decent scale, I will weigh them and try to find the 4 closest in weight.

Harbor Freight advertises a good scale for less than $20. Or there are new scales available on Ebay for less than $20 including shipping. I think I paid $17 for mine.

As an alternative, if you know anyone who works in a print shop, they all have very accurate scales.

[gtblast]

Originally Posted by First opel 1981 Harbor Freight advertises a good scale for less than $20. Or there are new scales available on Ebay for less than $20 including shipping. I think I paid $17 for mine. As an alternative, if you know anyone who works in a print shop, they all have very accurate scales.

Thanks for the suggestions. I had not thought of Harbor Freight. We have one not too far from here, so I may just run down and see what they have. I will look online to see what they offer.

[gtblast]

Originally Posted by gtblast I am rebuilding a stock 1970 engine for my GT. While cleaning up the pistons, I found that two of the pistons have cracks that are just to the side and above the wrist pin area. These cracks run up and through the bottom land of the oil ring. I have other good piston and rod combinations from another 70 engine that fit fine. My question is, do I need to worry about the weight of each piston/rod assembly? This is for a stock engine that I am not racing or anything like that. Can I just substitute the two piston and rod combinations in from the other engine and go on with the assembly, or do I need to weigh each one and make sure they are all within some range?

I picked up a nice little digital scale off Ebay and set about to weigh my piston and rod combinations. I figured if the piston/rod combinations weigh within a gram or so, I should be fine for a stock engine.

The two piston/rod combinations out of the original engine weigh within 1 gram of each other. The two piston/rod combinations out of the spare engine weigh within 1 gram of each other. BUT, the difference between the two sets is about 27 grams! That is a big difference.

Just looking at the pistons and rods, it is not obvious where the difference is. I did make an attempt to measure just the piston end of the assembly and it would appear that the weight difference is in the piston or at least at that end. In my attempt to measure just the big end of the rod, they all appeared to be pretty close.

So, now what? Do I ned to take all for assemblies to a machine shop and have them disassembled and balanced, each piston and rod? I really hate to do that because it is more money, and I have already spent more than I wanted on this rebuild.

[First opel 1981]

Removing the pistons from the rods and reinstalling them could damage the pistons. You could minimize the damage by using heat and cold but I don't know how to do that so I won't go there.
You could take them to a machine shop and ask them to remove 27 grams off of the heavy pistons/rods but that is a lot of meat.
Since there are two of each, you should be able (I would think) to put them in in a balanced situation but, personally, I wouldn't go that route.
I think your best, cost effective, choice is to beg someone on the forum for a set of pistons. Make sure you specify what version you are needing, high or low compression pistons.

[tekenaar]

Originally Posted by gtblast . . . The two piston/rod combinations out of the original engine weigh within 1 gram of each other. The two piston/rod combinations out of the spare engine weigh within 1 gram of each other. BUT, the difference between the two sets is about 27 grams! That is a big difference. Just looking at the pistons and rods, it is not obvious where the difference is. I did make an attempt to measure just the piston end of the assembly and it would appear that the weight difference is in the piston or at least at that end. In my attempt to measure just the big end of the rod, they all appeared to be pretty close. So, now what? Do I need to take all for assemblies to a machine shop and have them disassembled and balanced, each piston and rod? I really hate to do that because it is more money, and I have already spent more than I wanted on this rebuild.

OK, here's my thought on the weight difference . . . one set with cast rods (heavier) and one set with forged rods . . . the weight difference sounds about right, if that's the case!

Hate to say it, but I wouldn't mix forged and cast rods, even in sets of two . . .

[gtblast]

Originally Posted by tekenaar OK, here's my thought on the weight difference . . . one set with cast rods (heavier) and one set with forged rods . . . the weight difference sounds about right, if that's the case! Hate to say it, but I wouldn't mix forged and cast rods, even in sets of two . . .

Both sets are out of 1970 engines, so they should be forged. I am not an expert, and I have looked at the various threads and pictures, but all the rods appear the same.

[opelnut10]

Originally Posted by gtblast Both sets are out of 1970 engines, so they should be forged. I am not an expert, and I have looked at the various threads and pictures, but all the rods appear the same.

To make them work, you need to take to assemblies to a machine shop and have the pistons and rods seperated, weight them out and match up the lightest rods with the heaviest pistons, then weight them out to find the lightest assembly and make them all weight the same as the lightest.

[gtblast]

Originally Posted by opelnut10 To make them work, you need to take to assemblys to a machine shop and have the pistons and rods seperated, weight them out and match up the lightest rods with the heaviest pistons, then weight them out to find the lightest assembly and make them all weight the same as the lightest.

Yes, I am thinking this is the only answer, short of obtaining another set of pistons/rods from another engine as a set. I hate to cough up more money, but I can't turn back now.

[wrench459]

Originally Posted by gtblast Yes, I am thinking this is the only answer, short of obtaining another set of pistons/rods from another engine as a set. I hate to cough up more money, but I can't turn back now.

I would pick the later part of your statement (another set of rods & pistons).

[DennisGardiner]

It might be easier to just get a full new set of pistons.
Just my 2 cents worth.

[GTJIM]

If two pistons of a set are visibly cracked then it is a fair bet that the other pair are well on the way to being 'used up' - they have done the same service!

Being 'cheap' can only lead to greater expense when another piston lets go.
Best to get a NEW set. Is your motor STD bore size or has it been reconditioned at some time?

If STD then be aware that not all 'STD' Opel pistons are the same size but are selectively fitted in several slight increments of size. So you have to be sure of bore size before just bunging in any old set of pistons - even 'STD' new ones may not be the correct size.

RE-using 'old' pistons is not cost effective ... it is Irish Roulette!
(loading all the chambers .... and hoping for a misfire .. )

[gtblast]

Originally Posted by GTJIM If two pistons of a set are visibly cracked then it is a fair bet that the other pair are well on the way to being 'used up' - they have done the same service! Being 'cheap' can only lead to greater expense when another piston lets go. Best to get a NEW set. Is your motor STD bore size or has it been reconditioned at some time? If STD then be aware that not all 'STD' Opel pistons are the same size but are selectively fitted in several slight increments of size. So you have to be sure of bore size before just bunging in any old set of pistons - even 'STD' new ones may not be the correct size. RE-using 'old' pistons is not cost effective ... it is Irish Roulette! (loading all the chambers .... and hoping for a misfire .. )

Well, my low budget rebuild is getting out of hand, especially with this latest issue with the pistons/rods. I am not going to cough up $400 for a new set of pistons, plus the machine shop costs. I will advertise here on the Forum for a good used set of rods/pistons. I am aware of the incremental piston differences. If I come up dry, I will scrap the rebuild project. The car is not worth pouring that kind of money in to it.

[opelwasp]

Originally Posted by gtblast The car is not worth pouring that kind of money in to it.

Really, how much did you think you would have to spend? Just a basic engine rebuild will cost at least $1000. A freshen up of an old engine will cost around $400 and that is just buying you a little time till it completely lets go. There really is no such thing as a budget rebuild. You either rebuild it right or you half @$$ it, which will cost more in the end. Just save up your money and do it right.

[First opel 1981]

Originally Posted by opelwasp Really, how much did you think you would have to spend? Just a basic engine rebuild will cost at least $1000. A freshen up of an old engine will cost around $400 and that is just buying you a little time till it completely lets go. There really is no such thing as a budget rebuild. You either rebuild it right or you half @$$ it, which will cost more in the end. Just save up your money and do it right.

Not everyone has enough cash, disposable or otherwise, to pour into a 35+ year old car.

It's more helpful if you can help this member meet their goal of a running engine within their budget rather than berating them because they aren't doing it the way you would (new parts and machining work).

I think what this member is asking is reasonable. He needs someone with an extra set of usable pistons to offer them if so inclined. If I had a set, I would gladly send them his way. I have a set I would give him but one of the bearings scored up the rod.

Perhaps he can keep this car running long enough to improve his ability for a "complete rebuild" but in the meantime, lets step up and see if we can't help him with what he needs.

[opelwasp]

berate
Pronunciation:
\bi-ˈrāt, bē-\
Function:
transitive verb
Date:
1548

: to scold or condemn vehemently and at length
synonyms see scold

Please show me where I berated him. All I suggested was budget rebuilding is nigh impossible, and those who have have paid the price, just ask greensmurf20. His "budget" rebuild caused him all kinds of heart ache and cost him much more than he anticipated.

Also, usable used pistons is a gamble. So you slap a set of "good" used pistons in the car and 6 months later the engine craps out on you. Now you have to get towed home and it costs a mint because you are 300 miles from home. Plus now you have to fix all of the motor and get the metal chunks out.

I had this happen 6 months after a $2700(got royally screwed) rebuild by a shop. So I do have experience here.

[First opel 1981]

I'm not going to play the game.
I just want to help the guy.

[opelwasp]

Originally Posted by First opel 1981 I'm not going to play the game. I just want to help the guy.

Whatever.

I am trying to help him too. It is best to learn from others mistakes from cutting corners. Everyone doesn't have to repeat the same ones. There are some parts that are safe to reuse, but in my opinion pistons are not one of them. They take too much of the brunt of the engine to chance it. If he was able to reuse the pistons from the same engine I would say go for it. But that clearly is not the case. I do not think it is a good idea to introduce unknown "good" pistons to the mix.

Plus, who wants to be the member who gives this guy some "good" used pistons and 6 months later they grenade on him? He is gonna be pissed and feel cheated. While the other member is going to be looked at like he gave him bad parts. It just isn't worth the aggravation or bad feelings all the way around.

[First opel 1981]

Originally Posted by opelwasp Whatever. I am trying to help him too. It is best to learn from others mistakes from cutting corners. Everyone doesn't have to repeat the same ones. There are some parts that are safe to reuse, but in my opinion pistons are not one of them. They take too much of the brunt of the engine to chance it. If he was able to reuse the pistons from the same engine I would say go for it. But that clearly is not the case. I do not think it is a good idea to introduce unknown "good" pistons to the mix. Plus, who wants to be the member who gives this guy some "good" used pistons and 6 months later they grenade on him? He is gonna be pissed and feel cheated. While the other member is going to be looked at like he gave him bad parts. It just isn't worth the aggravation or bad feelings all the way around.

Well, for what its worth, I have NEVER had a piston give out on me. I have swapped pistons from engine to engine. I have done this on over 20 engines and NEVER had a piston fail. Most of these engines were Opel 1.9 engines. My own engine in good ol #2 GT went 50K+ miles on swapped pistons including towing an ATV trailer for at least 5K of those miles.

Why would you suggest that a piston swap would be a fatal mistake when, obviously, your $2700 rebuild blew a piston? By your own admission, new pistons aren't a cure-all, and by my own experience, used pistons aren't a likely failure.

Sure, they could blow...either way. Many things coupld happen whether using new parts or usable used parts. Why speculate?

I offered this guy a solution if he can't, and by his admission he isn't willing to, spend an extra $400 (plus the cost of rings, plus the cost of boring/honing, etc) on his engine. Perhaps someone else will offer him a set of pistons with the understanding that they have no liability in the matter.

I won't argue any more. It's like the special olympics....remember?

[opelwasp]

Originally Posted by First opel 1981 Why would you suggest that a piston swap would be a fatal mistake when, obviously, your $2700 rebuild blew a piston? By your own admission, new pistons aren't a cure-all, and by my own experience, used pistons aren't a likely failure.

Actually it isn't obvious I blew a piston, because it didn't. It actually sucked a valve which turned the piston to Chiclets sized pieces and annihilated the head. I admitted nothing of the sort.

If he does put in used "good" pistons he will still have to pay to have the cylinder bores honed/rebored. He also needs to check and see if the cylinder bores are even usable. They may be excessively worn and putting "good" used pistons will be a waste of time.

Reusing pistons you know the history of is fine if you feel confident in doing that. But like I said, using unknowns in a motor is a bad idea.

I also never said I wasn't retarded.

[First opel 1981]

Originally Posted by opelwasp I also never said I wasn't retarded.

I will say as much. In fact, I'm an uber genius.