Finally pulled my head..top cylinder ate a ring

[scout-j-m]

Let me get the background info out of the way from the start. My motor is a 1994 70HP Force. I have owned it for the past 5 or 6 years and honestly, it never really ran good. I have dealt with a surging problem at speed which I initially was locked in on having to be an ignition problem. Who knows, that may be partly true. I periodically checked the compression when working on it and always got readings of 90, 110, and 110 psi from top to bottom. I will admit I always left the plugs in except for the one I was testing. This year I decided to commit to fixing this thing right so I troubleshot the entire ignition system statically and got acceptable readings on the resistance and voltage of every ignition component as well as spark on all cylinders that jumped a 1/2" gap on a tester. A couple weeks ago I decided to to eliminate fuel delivery so I cleaned the carb, replaced fuel lines and filters, checked the fuel pump, checked reeds, and hooked up to a portable tank with fresh fuel/oil mix. So I did all of that, and started her up in order to fiddle around with the idle mixture adjustment. She seemed to run worse than before in terms of idling so I did a proper compression test with all plugs removed.After a compression check I only got 20-30 psi on the top cylinder while the bottom two were now 120. I decided to wait to pull the head until my new gasket came in.

So on to yesterday, the gasket had still not come in (***************** is SLOW) so decided to go ahead and pull it while I had some free time. There isn't much to say that the pictures don't say for themselves. The top piston is messed up at probably 7 or 8 places around the edge where the ring broke, a couple ports on both the intake and exhaust have actual chips/gouges in them from where the ring caught, the cylinder has multiple linear scores as well as a couple quarter sized indention/scores which must be from the chipped of metal from the ports. The picture also show that the head is damaged at that cylinder. The gasket was intact and in good shape. The other two cylinders looked good as well aside from some carbon build up. Their rings looked good and there were no cylinder scores that I could feel with a fingernail or see.

I would like to attempt to rebuild this myself but honestly, I am worried those quarter sized blemishes may be too deep. I have zero experience though so what do I know. My initial question is what should my first move here be?

 

[Jiggz]

From the look of the top cylinder piston crown, it looks like it hasn't been firing for the longest time. Back in 2012, I had the same problem with my number 3 cylinder, i.e ring catching on the exhaust port. I've never opened let alone work on replacing a piston on 2 cylinder motor. But with the help of forum members not only I was able to do such a feat but also learned a lot in doing so. As for your next move this is my recommendation. First things first, you need to set a goal first and a goal that you will actually do your very best to accomplish it. And that goal is TO FIX IT ABSOLUTELY RIGHT! Again your best friend for doing this are two, your digital camera and this forum. Now for your next move after genuinely swearing to fix it right are:

1. Remove the top piston to further assess the damage - removal is through the intakes. Dismount carb and reeds and you should have access to the con rods. You will need a 12 point 1/4" socket and extension. Unscrew rod caps. But before doing so make sure you have a white rag place either under the crankshaft (inside the crankcase) or have the motor tilted slightly and place the rag just under the carbs to catch the roller bearings if they fall off the caps. With caps off, push the piston out and re-assess the damage. If bearings fall off inside the crankcase, use a long rod with heavy grease to pick them up. DO NOT USE MAGNETS. Or in my case, I have one of those long handled, angled needle nose from HF which costs less than $10 each.

2. Chippings on the ports (in or exh) are not that critical. This can can just be filed and smoothened, in fact, it is highly recommend to chamfer (vertical edges only) the rest of the ports to prevent future rings from catching again. The chipping on the head can be filed and smoothened. Luckily it doesn't seem to break or compromise the sealing surfaces of the head. However, if you want you can also take it to a machine shop to have it rebuilt.

3. The linear grove is critical and should be assessed prudently. Rule of thumb is to ran your fingernails through them and should not catch. I can think of two ways to fix this. One is through machining which will require you to dismount the entire block and take it to a machine shop. The other is to try to correct as much of the linear grooving with a cylinder stoning or honing kit. The stoning/honing kit is about $40. Try to get one with multiple stones of different roughness. Finish the honing with the finest stone and even better using the brush honing.

I think this is a good start for you next move. The next major moves will be to measure the bore and from there order a new piston/ring kit. The Wiseco kits are highly recommended. They come with the rings already installed and all you need to do is to get the new wrist bearings. You will need to save the old conn rod. DID I SAY TAKE A LOT OF PICS? YUP YOU WILL NEED TO TAKE PICS EACH STEP OF THE WAY TO ENSURE YOU HAVE A REFERENCE WHEN PUTTING IT BACK.

 

[HotTommy]

scout,
I was as novice as you with much the same problem 15 months ago when I came to this forum for help. It took a while and there were several frustrating false starts along the way, but I worked my way through the engine rebuild. My point is that the experts here can and will help you do this if you choose to try. The only advice I can add is this. When you finally get to reinstall the head, be sure the gasket is aligned with the cylinders before you tighten things down. I found out the hard way that hanging the gasket on the bolts does not assure that it is properly aligned. .. Good luck.

P.S. One more thing - stuff a towel or cloth in the engine bay beneath where you are working. Its real easy to loose small parts down into the engine if you don't have something there to catch them.

 

[gm280]

scout, here is my .02 cents worth. Your head looks good and there is only a little smoothing work on the damages cylinder head section to make it work great again. However, the piston with the ring problem is toast...period! Don't even try to reuse it. It will take a new piston for certain. The cylinder is very questionable because if it can't be rebored to remove all the gouges, it will have to be sleeved to work again. But if I were doing this rebuild. I would check the other two pistons and cylinders and probably have all three bored oversize with new pistons and rings if that will fix the bad cylinder. And the only way to find out is to take it to a competent machine shop for them to make that assessment and determination. If they say they can bore it out and clean it up, then you have to make the decision to buy new parts and rebuilt, or not. But understand you have to make that decision. If you really like that engine, go for it. But if you couldn't care less, then part it out and look for another one. But unless you buy new, another engine can have it's share of issues as well... JMHO!

 

[scout-j-m]

Thanks Jiggz. That falls right along with what I had in mind.

As far as my goal for the motor, I want to rebuild it and do it as right as possible or as my budget allows. The only doubt I had is whether to do a full rebuild or not. I have seen Frank A post that these motors can handle only one single cylinder being bored over. So aside from doing a complete rebuild like you mentioned I was curious if it truly is possible to bore, hone, and install new piston/rings in the failed cylinder while only honing the other 2 cylinders are installing new rings on the existing pistons (and chamfering the ports on all 3 in the process). I would obviously prefer to do it complete like you mentioned though. I would hat to do all of that and somehow end up with uneven compression or whatever else it could possibly cause.

As for sizing the current pistons, can someone help me locate the identifying marks on the two pistons which are ok? Obviously the bad one will need to be measured but I do not have the tools to do that nor know of anyone to borrow them from. I kind of just assumed I could get that squared away when I took the head into the machine shop.

Regarding the big time scoring just above the ports in pictures 2, 3, and 5, I assumed it was just guaranteed they were bad enough to require a bore. The long thin scores everywhere catch my fingernail noticeably but these bigger patches feel 5 times as bad and look worse in person than on camera. Maybe I can get a better picture with the piston removed. This kind of fell along the lines of the machining in my mind as well as I could help but wonder if a cylinder can be scored so bad it requires a bore of over +0.010"...like what if that one was already at +0.020" and would need a bore of +0.040" to get rid of the scoring and which is too thin of cylinder walls according to what I have read.

I'm happy with your prognosis on the head as well lol. It was another area of big concern. I also figured I could see how much the machine shop would charge to fix that. I know head milling for warped car and truck heads caused by overheating is pricey but no clue on this type of thing.

 

[Jiggz]

No Title

I believe for your motor the std piston is 3.375" bore with a 0.010" increment in oversized pistons up to 0.040" or 3.415" bore. The stamping on the piston crown indicates if the piston is oversized or not. Standard size is usually indicated by number "1" and "2" indicates an oversized piston of 0.010" or a bore size of 3.385" Here's a pic of the piston I replaced showing the "1" stamped. Need to be aware tho the stamping is only if they are the original pistons. For Wiseco pistons the stamping is actually on the piston deflector.

 

[scout-j-m]

scout, here is my .02 cents worth. Your head looks good and there is only a little smoothing work on the damages cylinder head section to make it work great again. However, the piston with the ring problem is toast...period! Don't even try to reuse it. It will take a new piston for certain. The cylinder is very questionable because if it can't be rebored to remove all the gouges, it will have to be sleeved to work again. But if I were doing this rebuild. I would check the other two pistons and cylinders and probably have all three bored oversize with new pistons and rings if that will fix the bad cylinder. And the only way to find out is to take it to a competent machine shop for them to make that assessment and determination. If they say they can bore it out and clean it up, then you have to make the decision to buy new parts and rebuilt, or not. But understand you have to make that decision. If you really like that engine, go for it. But if you couldn't care less, then part it out and look for another one. But unless you buy new, another engine can have it's share of issues as well... JMHO!

Thanks for the advice. That seems like reasonable expectations.

My only follow up is in regards to if the one cylinder can't be cleaned up good enough. What is this boards general opinion on buying a used block and just doing a hone and ring job? At first glance through used blocks on ebay and pairing that with needing one new piston plus all new rings it doesn't seem like a good move in terms of risk versus reward. At that point parting it out would be a serious consideration because I am not too crazy about the boat this motor is on. My end game is really to try to fix the engine, use the boat/motor for a few more years, and upgrade boat and motor. I bought this back in my really broke college days...now I am just kinda broke so maybe I can afford it

 

[scout-j-m]

Hopefully I can find a good local machine shop. While having never dealt with one, I do know that my wife's cousin is dating the son of a very reputable local one. Actually my vehicle mechanic also uses them for his machining work.

 

[scout-j-m]

I believe for your motor the std piston is 3.375" bore with a 0.010" increment in oversized pistons up to 0.040" or 3.415" bore. The stamping on the piston crown indicates if the piston is oversized or not. Standard size is usually indicated by number "1" and "2" indicates an oversized piston of 0.010" or a bore size of 3.385" Here's a pic of the piston I replaced showing the "1" stamped. Need to be aware tho the stamping is only if they are the original pistons. For Wiseco pistons the stamping is actually on the piston deflector.

This is the only shot I have with me of one of the good pistons. I believe the lettering on the middle left says "B4" whatever that means. It also looks like that may be a "1" at the 10 o'clock position but to my untrained eye it also looks like it just may be some carbon lol. Are there any markings on the bottom side to look for when I remove them?

 

[Frank Acampora]

These engines are very tolerant about only one cylinder being over bored. Let the machine shop guys tell you if it can be bored. Do not think about sleeving the cylinder because a sleeve costs about 100 bucks and installation not only takes special expertise but special tools--and the ports need to be cut. If the cylinder can not be bored, and from the looks of the photo, I think it can be salvaged, then buy a used block. As far as boring the other two cylinders, that is up to you. There is usually a set-up charge for the first hole and then a set fee for the others.

HOWEVER: Since these 3.375 standard bore pistons use a semi-keystone top ring and since they do tend to break them regularly, EVEN if you do not bore the cylinders it would be a prudent move to replace the other two pistons with better after market ones.

Don't let the machine shop tell you they need to measure the pistons. For example Wiseco pistons are sized from the factory to fit standard overbores (.010, .020, .030) with correct clearance. Add to that the fact that the pistons are cam ground to an oval shape to account for uneven expansion and if the machine guys have never done an outboard they will be confused. So bore to standard size and order the correct piston. Think hard before overboring to .040. This does not leave much wall thickness and may lead to failure later on. Earlier Chrysler and Force manuals recommended no more than .030.

Question whether or not the machine shop is experienced in blind hole boring. That is what must be done on these engines because the boring bar and hone can not pass completely through the cylinder.

If you are not satisfied with the current machine shop, seek out a racing shop. They usually do all sorts of special stuff.

Note that to bore a block and there is no question but that yours does need it, the block must be bare. So, you might as well go ahead and start disassembling it and removing it from the leg.

 

[jerryjerry05]

Your probably looking at boring .020 over.
Try not to go too much past that.
Wiseco pistons and the parts they suggest.
Wiseco rings only fit wiseco pistons(found out that the hard way)
The other pistons are usable(depends on the amount you want to spend) just need new rings.
You also need to figure out(if possible) why it took a dump?
Change all the gaskets, exhaust too.

 

[scout-j-m]

These engines are very tolerant about only one cylinder being over bored. Let the machine shop guys tell you if it can be bored. Do not think about sleeving the cylinder because a sleeve costs about 100 bucks and installation not only takes special expertise but special tools--and the ports need to be cut. If the cylinder can not be bored, and from the looks of the photo, I think it can be salvaged, then buy a used block. As far as boring the other two cylinders, that is up to you. There is usually a set-up charge for the first hole and then a set fee for the others.

HOWEVER: Since these 3.375 standard bore pistons use a semi-keystone top ring and since they do tend to break them regularly, EVEN if you do not bore the cylinders it would be a prudent move to replace the other two pistons with better after market ones.

Don't let the machine shop tell you they need to measure the pistons. For example Wiseco pistons are sized from the factory to fit standard overbores (.010, .020, .030) with correct clearance. Add to that the fact that the pistons are cam ground to an oval shape to account for uneven expansion and if the machine guys have never done an outboard they will be confused. So bore to standard size and order the correct piston. Think hard before overboring to .040. This does not leave much wall thickness and may lead to failure later on. Earlier Chrysler and Force manuals recommended no more than .030.

Question whether or not the machine shop is experienced in blind hole boring. That is what must be done on these engines because the boring bar and hone can not pass completely through the cylinder.

If you are not satisfied with the current machine shop, seek out a racing shop. They usually do all sorts of special stuff.

Note that to bore a block and there is no question but that yours does need it, the block must be bare. So, you might as well go ahead and start disassembling it and removing it from the leg.

Thanks Frank. That is good info on the communication between the machine shop regarding the correct sizing. Definitely something to remember. Do you buy directly from Wiseco or dealer or parts store?

Also, getting a used block somewhere was my back up plan if the damage is too deep. There are always a handful on ebay so hopefully some are sold by reputable/trustworthy sellers who can confirm their current sizing and quality of the cylinders. I will start tearing it down this week.

 

[scout-j-m]

Your probably looking at boring .020 over.
Try not to go too much past that.
Wiseco pistons and the parts they suggest.
Wiseco rings only fit wiseco pistons(found out that the hard way)
The other pistons are usable(depends on the amount you want to spend) just need new rings.
You also need to figure out(if possible) why it took a dump?
Change all the gaskets, exhaust too.

Jerry, I'm not sure what caused it however that cylinder has always had lower compression the entire time I have owned the motor. My guess is that the ring broke a long time ago and contributed to that and then in it finally snagged bad on the ports and that's what finished it off. The only other thing I can think of is that for the first couple of years of owning it, I never adjusted the carb....when I finally did it the idle mixture screw was only 3/4 out. From what I have read on here, 3/4 out is the absolute farthest screwed in you want to go. Plus I have a single carb motor and have also read that the top cylinder in motors that share a single carb run even more lean than the others. Does that sound like a plausible cause?

 

[gm280]

I don't know if I believe that you ran your engine for years at an overly lean condition, even at idle. Otherwise things would have gone south way before this. I do believe you had ring issues before this event though. But that is just my warped thinking...

 

[Jiggz]

Personally, instead of trying to locate for a block of the same engine, I would rather just look for a power head instead. Of course, the decision will be based on cost of the machining and rebuilding the old one and the amount of time and effort spent in doing so compare to the price of a used power head. The one thing you need to make sure with a used powerhead is to check the compression and other system integrity checks. The powerhead doesn't have to be running either because of electrical or some other problems since the old one has perfect running parts. At Evilbay there are bare powerheads selling for less than $300! You can always inquire about the compression checks on powerheads being sold. Or try to make a trip to a boat salvage shop for even better examination of the item. Of course, this is all my personal opinion since I am a cheapskate mech, which is the reason I am an avid member of this forum! Lol

 

[scout-j-m]

I just wanted to say that I won't be able to get into the teardown until this weekend unfortunately. So I have been spending what little free time I have had just looking at parts on ebay as a reference to how much is out there for my model/year. I did find this service manual though for basically $20. Is this the factory service manual everyone always recommends picking up? All I have is the Clymer one, which is decent but not very model specific.

 

[Nordin]

Yes it is.

 

[scout-j-m]

I got it. Shipping today.

 

[scout-j-m]

I got the shop manual in the mail this morning and read through the rebuild chapter over lunch. I didn't get into any teardown this weekend as I decided to wait until the manual came in.

I was going to spend this evening removing the flywheel, ignition system components and linkages and fuel lines. However, I do not have a big enough socket for the flywheel nut and none of mine are close enough to make an accurate guess either. Can anyone tell me what size I need? ***Edit: ********** lists the flywheel nut as being a 3/4 - 16 size. A quick search showed that the head dimensions for that but require a 1 1/4" socket. That sound right? Seems too big. I was thinking 1 1/16".***

Also, I picked up my car from my mechanic Friday afternoon and got the name of the machine shop he takes his blocks and heads to. When I asked him about their prices he said he can get a v8 motor bored, honed, and cleaned for around $170. He thinks the cost per cylinder to bore is $12 each and the cleaning is $40. Doing the math that means it's about $20 or less per cylinder which would be great. I was budgeting for $100 for the machining work so it sounds like I maybe overestimated. The shop is also just a few miles down the road from me which an added bonus.

 

[jerryjerry05]

Yup 1 and 1/16
Before you start.
PB Blaster or a good corrosion buster on ALLLLL the screws, bolts, nuts and anything that turns.

Once you get to the exhaust(if you haven't already) a propane torch and a hammer and PB Blaster might be needed.
The exhaust chest is held on with 1/4-20 and they BREAK EASILY!!!