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MG Midget and Sprite Technical - Another piston broken and hoping to race next week
|We rebuilt our mg midget engine over the winter and since putting it back in have experienced two pistons breaking between the rings on the 3rd chamber and then again on the 4th. We purchased the powermax pistons from a local mini specialist and were told they made by Nural now and the box has a German stick on it and they are the correct size The first time it went straight away so we replaced it and now another one has gone. Oil and temp were fine so I drove it a couple of times of distance of about 50 miles. I took the car out the weekend and when giving it a bit more revs I noticed smoke coming out of the catch tank. On a compression test the 4th was down to 110 and when taking the piston out it had broken between the rings. |
I have entered silverstone next weekend but I'm reluctant to put another piston back in. The company we brought the piston said they sell them all the time and have no problems so we are none the wiser.
Has anyone got any ideas.
Appreciate the help.
|Seems unusual to say the least. |
What are your bores like? is there any chance the rings have nipped up in the bore and pulled the pistons apart?
|Robert (Bob) Midget Turbo|
|bores are okay and we checked ring gaps|
|Let me ask this question then|
Looking at the damaged pistons and I can imagine there was a lot of damage done after the seperation. Is it possible to determine if the top has been torn off or is it a simple crack?
|Robert (Bob) Midget Turbo|
|some decent pics would help... then hope Pete B comes along...|
|please see picture attached
|What revs are you using?|
|no more than 4,000 on the 40 miles i did since replacing the piston.|
|I'm sure a lot of people on here know the answers to the questions below, but I don't.....|
Could you give a bit more info on the engine spec eg what size pistons, what CR you running, fuel type, any sign of pinking when running, what bore to piston clearance, what carb set-up you have etc, etc?
|forgot to say... |
I have a 1380 with similar damage! Only I bought it like that(!) and only found the problem when doing a quick check for sticking rings as it has been stood for a while.
Now a full rebuild with kent 276 and possibly some other weight loss work....
|Looking at the surface of the failure site on the piston may hint at the cause, but it's far too low a cycle count for conventional fatigue failure..but the picture needs to be much much better.|
you would expect the rings/piston to pick up on the thrust face...not above the gudgeon pin, assuming that it true (cos I'm guessing about A series here) it points to the bore rather than the piston?
Are the rings still in one piece?
pinking would take out the top land and crown and not at 40 miles (unless you have converted it to a diesel!)
have the pistons failed in the same place each time (the same "o'clock" location)?
certainly this number of failed pistons is a catastrophic issue, you have my sympathy. it suggests a problem with the block....if the pistons were iffy 1 & 2 would have chucked their toys to?
I would try heating the block up to operating temp and checking the bore variation, I assume you got a rebore done druing the rebuild, maybe that was too much for the wall thickness and it's hogging when up to temp....would an xray help...if the failures are all at the same spot that tells you where to look.
All of the above are 'things to contemplate' I'm glad to say that I've no specific experience of this!
|I wonder if this is an issue more to do with inferior rings now that they are made in china by small slave childrens hands|
I went thur this last year... but i broke my pistons because i didnt gap the rings correctly and they micro welded momentarly ...just long enough to bust the landings.... but ive seen this story from multiple people not just british cars
What did your ring gap ends look like.....where there signs of micro welding ???
|Thanks all for your comments and help. I will check the gap rings tonight and see what they look like. We did worry the pistons maybe from china but look like they are made in Germany, maybe the rings are from china.|
The block could be the issue I suppose as the engine did get quite hot at Oulton last year.
My dad and his friend are suggesting they carefully put it back together then run it standing for at least an hour on tick over.
I'll let you know how things turn out and also try and get a better picture of the piston.
When you rebuilt your engine was there any wear in the bore - like was there a ridge at the top of the bore and if so was it completley removed
To me the picture of your piston looks like ----
1- The top ring has come up against a ridge left in the bore and broken the landing
2- The piston has been damaged by the second ring catching on the top of the block during assembly but in this case the ring probably would have been damaged as well and you would have had to have noticed it wasn't going in properly ??
3- Detonation - either - poor fuel quality
too much compression
over advanced timing
Any chance of a pic of the top part of the bore.
|Personally I would ensure the ring gaps are sufficient on assembly, the CR has not been raised too high and that ignition advance is suitable. I cannot see the angle of the break unfortunately otherwise I could have a better guess maybe. A set of better piccies would help Usually the lump comes out with a Vee shape pointing upwards this is too much pressure on the piston from above. |
|I've not had chance to go to my dad's but he said had the piston turned down and put onto the Rod. We realise the car has a very high compression but we haven't had any issues with pinking as we use an octane booster. We have had issues though with the engine running on after switch off, could this be the cause of the piston breaking??|
|What CR do you actually have?|
Assuming the pistons are not faulty and it is a pretty good guess that they are fine then there must be another problem.
I believe there are only 2 options.
The pistons have nipped up in the bore due to excessive heat produced by pre ignition. Sounds doubtful from your description.
The pistons have nipped up in the bore due to inadequate mechanical clearence (rings or bore?)
I don't believe there are any other options?
|Robert (Bob) Midget Turbo|
|I'm sure PB will be a long soon with a detailed opinion but until then....|
With a very high CR, running octane booster and experiencing run on! Yes this will be the cause....
I think you should be investing in forged pistons from a reputable source..
and to prevent the run on, you will need to fit some form of anti run on valve in the intake to ensure any unused mixture in the manifold is weakened to the point where it won't burn.
|Not much you can say without looking at the 'bits' involved.|
I usually associate running on with high idle speed followed by too little idle ignition advance.
|Over the weekend we have now put the piston carefully back in. It was run for at least an hour and half without going anywhere. I have then taken it for out for two short trips and have gently increased pace on the last trip. Everything seems fine and I'm still hoping to be at Silverstone on Saturday.|
General consensus is that the compression of 230 is to high and we would of been better fitting forged pistons as suggested earlier on this thread. The timing has been adjusted and I just hope this is enough from preventing another one to break over the weekend!
We'll probably have to bite the bullet and purchase some forged pistons after the weekend (you live and learn).
I've attached a better picture of the piston and there is a link for Silverstone below if anyone fancys going as we have about 80 cars entered including several MG's.
|Are you sure you don't have ridges at the tops of your bores?|
I was interested to see you had the piston turned, what was done and by who. I ask as typically pistons haven't been round or parallel for a long time to account for the differing expansion at the top and bottom of the piston and from side to side and front to back.
|No definitely no ridges, checked a couple of times.|
|The picture still looks like too much pressure from above, the 230 is not too high for a race engine,more like ignition issues, I still cannot see the angle of the break although it sort of looks like the two breaks per land might well form a tip up Vee. Implying excess pressure from above the lands. What was trhe piston make? And why havent you rolling roaded it?|
|I think the crack also shows it started top to bottom looking at what is left of the ring land. It would be nice to see the piston.|
Ridges in the bores would break the top ring only. The piston land area does look smaller than the skirt which is how it should be.
You must get fuelling and timing etc set properly or you are wasting your time and the cost invloved with racing.
|Whenbi broke my 2 front pistons... it took the entire chunk out of the piston at the landing.|
Yours busted off at the inside edge...ill see if i got any pics
|okay... im wrong, it does look like yours|
|I've got a simulair compresion figure last time ( all above the 200) and is does not seem to be a problem in my road 1380.|
I do have the ruddy expensive 73,5 omega pistons though
|Interesting one.... Are the rods in the correct order and in the correct way around...?|
Worth a thought...
|M T Boldry|
|Thanks Prop, does look like ours and thanks Peter for the suggestions, we have had all set up on the rolling road but not since the end of last year.|
Going to fit a decompression plate and see how it goes.
|It isnt a matter of cranking pressure Onno. from memory combustion pressure can reach or exceed 1500 psi on a race engine. This pressure is nothing compared to the incredibly high pressures associated with detonation. Imagine this on the ring land as the piston is still on its way up being forced up against the pressure by the other three pistons. That is why the ring land snaps with an upward Vee indicating excess pressure from above. Like when you miss what you are aiming at with a hammer....think how easily fins snap off air cooled engines! The ring lands also seem to show 'peel' downwards showing the force was from above.|
If the piston had been subjected to some unknown side loads enough to snap the lands I am sure the piston waould have some appaling skirt damage as the skirt is at least 20 thou per side wider than the ring lands. Friction tends to smear the piston rather than snap lands which has to have either an upeward or downward very high force.
|Sure looks like detonation|
Different camshaft grinds affect cylinder pressures at cranking speed -- you can't rely on cranking pressure to be an accurate guide of compression ratio -- it has to be cc'd up It's no use fitting forged pistons to try and disguise another problem , measure it up so that you know exactly what you have and then if you are going to get forgies you can get the correct set for the compression ratio required
As an example my old hillclimber had a very radical cam grind which resulted in a cranking pressure of only 180psi with 12.5-1comp.
Don't guess it measure it--
Also you say you have adjusted the timing - to what??
|I cant remember what mine checked out at ... it was well over 200 ... im thinkng it was like 250...270 psi.... ill be checking it this weekend...my guess is advancing the sw5 07 camshaft by an extra 3.5 degrees to 102.5 degrees must have really maxed out the dynamic CR.... I found my old crank shaft rhe other day so i think i might have enough parts and specs that i think i can hobble to gather enough numbers that i can calculate the dynamic CR.|
Hopfully i can do that the next time im off work when its bad weather out. Certianly will be interesting to see.
|As Willy says you can not assume anything from simply measuring the cylinder pressures on cranking.|
You must know the real value of C/R simply by calculation. Then you need to know the actual cam profile. The cam will determine what C/R will be acceptable. A high overlap cam of 280/290 area will take more C/R than something as mild as 260.
That said if it has been rolling roaded then that surely would have indicated if detonation was a problem. All a little strange?
|Bob Turbo Midget England|
|I cant remember what mine checked out at ... it was well over 200 ... im thinkng it was like 250...270 psi.... my guess is advancing the sw5 07 camshaft by an extra 3.5 degrees to 102.5 degrees must have really maxed out the dynamic CR.... I found my old crank shaft rhe other day so i think i might have enough parts and specs that i think i can hobble to gather enough numbers that i can calculate the dynamic CR.|
Hopfully i can do that the next time im off work when its bad weather out. Certianly will be interesting to see.
|This site is getting sqiurrlyier by the day... i hope its not serious or a harbonger of something worse to come.... cause you know its not a case of if theres a major cyber melt down but when.|
|Has the spec changed in anyway since you had it on the rollers? Why did you strip it? Did you measure the ignition timing then put it back the same? get it onto the rollers to make sure.|
|I don't believe any of this! (didn't believe Prop's analysis of his failure either)|
But the pics are not good enough, Wash the piston and give us a pic straight on to the ring section, showing at least one of the broken lands straight on (as at the RH side of the last pic). The last pic from about 1-2 inches higher would do, if it were clean and well lit. It looks to me like the lands are bent upwards, especially above the oil ring. Are either or both of the comp rings tight in what remains of the groove? The piston top did not bend downward, so tightness in the grooves means the lands bent up.
Nothing about this damage suggests any influence of excess compression, preignition or heat. There is no indication of excessive temps. I have some pistons from a 1275 that I ran out of water just at the start of a rade, which I raced anyway (long story); while the 3&4 pistons are melted at the siamese section from the top down through the pin bore, and the pin looks like it was hit with a cutting torch, the rings and lands are unbroken.
It all tells me installation error. What is your ring compressor like? Are the bore tops chamfered and how much? Pics of all! Not to P*ss anybody off, but How hard did you have to beat on the piston top to get it in the bore? The answer likely accounts for Peter's "The picture still looks like too much pressure from above"
And while we are here, what does this mean: "he said had the piston turned down and put onto the Rod"
|I think the angle of the break will tell you more about the sequence of events. I think it would take a lot of effort to snap the lands on assembly.|
If the damage occured installing the pistons the two break points will have to form a downward pointing vee would it not? I would have thought the complete lack of compression if the lands had been snapped on installation would have been very obvious to the owners when they started the engine. It seems the owners are saying this occured during operation rather than it was there from start up? Your suggestion is too much pressure from below to lift the lands upward, I feel from the not so good pic the pressure was on the top of the land not on the bottom imparted from the top of the block as you suggest. As you say pic is not very clear.
"I think it would take a lot of effort to snap the lands on assembly." Well. I was lying under a Ford pickup to guide the rods onto the crank while the owner put the pistons in. After some cursing and delay, he started banging on the piston tops with a dead blow hammer. I screamed and told him to look at the ring compressor, finally got out and refitted it myself, and installed the rest of the pistons. We had a terrible fight, with him telling me about how many engines he had rebuilt, and me telling him how many I fix after guys like him rebuilt them. I told him it was a good bet the pistons would fail, but he was not interested in buying new ones again. He would only look at the one I intervened in, and it looked OK. The truck started and ran fine, but lost compression on several cylinders including that one, and started using a lot of oil a month later. The bad holes were all ones he put in, but none of the ones I put in from the start. Having more money than brains, he sold the truck; I think it was mostly a matter of not wanting to pull the pistons to find the predicted breakage.
"Upwards" or "downward" forces are subject to interpretation as to where you are applying the forces. I'm considering the force on top of the piston as "downward".That is the only force there is if the bore has no ridges. Combustion pressures act equally on the ring lands and the piston top, so other than the force transferred from ring land to gudgeon pins, or actual seized rings, there is never a down force on the lands.
My interpretation is down force on top of the piston via hammer, and up force on the land from the ring sitting on top of the block.
The crack will always start at the inner corner of the groove, but the fracture may well not be complete for some time,since the stack of rings and (cracked) lands transfers all the load to the top of the piston, keeping the whole thing from falling apart on assembly. In this scenario, I would expect the crack to propagate inward and upward from the start at the upper inner corner of the groove, then tend outwards toward the top of the land; this is what I see, especially above the oil drain hole to the right of pic rwo.
|"We had a terrible fight, with him telling me about how many engines he had rebuilt, and me telling him how many I fix after guys like him rebuilt them."|
Brilliant, just brilliant !
I saw you asked about "he said had the piston turned down and put onto the Rod", I asked about that earlier in the thread as it seemed odd but I haven't seen a reply to that question yet. Enquiring minds need to know.
|Dave B -|
There does seem to be a failure of the data acquisition link. A modified engine with no CR numbers, no end gap numbers, bad pics, most of our Qs left unanswered; but,we try.
|Ill see if i can find those pistons tomarrow and take a better detailed photo... but im 98% sure mine was the result of the wrong ring gap... and general in-experiance... i think i still have the rings that show the mirco welding at the end of the rings where they gap.|
If the ring land snaps from jamming on the top of the bore as it is battered into the hole the force which breaks the ring would be from below, ie the block, the hammer or whatever provides the downward force, the block resists this force and the ring snaps the ringland. This would have run like a bag of !!!! from the moment the engine started.
The give away will be a close up of the angles of cracks...if an imaginary line is drawn from each crack....if they touch below the piston the force would have been from below, most likely scenario battering the pistons in like you said. If the lines join above the piston the force was from above most likely scenario excess pressure from uncontrolled combustion. The failure is tensile one side and compressive the other under a bending moment, just need to study remains of piston to find direction of bend.
As you say lack of pics and descriptions makes it difficult to help with plausible reasons for the failures.
|Ehm, why does the piston in no1 pic have oil return slots and the one in pic no2 holes?|
|Alex G Matla|
|Had the same Q. I think #1 still has a slotted oil ring on it. Fuzzy pics not good for detailed examination!|
|Nicely noticed Hercule Matia.|
Looks like cheapo in pic one and better quality pic 2.
A N other thought....shame we don't get answers to our questions....say the dizzy was marked prior to dissassembly then the cam was put back in one tooth advanced.....dizzy put back in on markings, the timing would be 18 degrees advanced and the 18 degrees advance on the cam would be bad too :)
Engine assembly requires practice not always get away with follow book.
Just a thought!
While the first picture is a bit fuzzy I can see it has the oil control ring still in place and the ring is slotted, the second picture has no oil control ring in place so you can see the return holes in the piston so I don't think you can say that the first picture doesn't have the same holes in the piston.
It looks like the bottom of the photo shows a typical return slot (unless that is another lump of land missing twixt oil control ring and compression ring) for oil but as you say very fuzzy !
I assumed that was the ri ng gap on the oil ring on the first pic.
|Have you considered fuel.Many years ago when I started racing I had problems when I started to eat away head gaskets and destroy pistons (squeeze cast Omegas). |
Then I changed fuel and all was well. What I think had happened is that I had used Leaded 4 star that had been sitting in the petrol stations tanks over the winter (I was getting problems like you at the start of the season). Anyway switched to super unleaded (+cvl) from the petrol station at the top of the parade in leamington and never looked back!!! (and have always used either powermax or squeeze cast omegas)
I have always run 11.7:1 CR and never had any problems and have religiously bought new fuel for each meeting (maybe max 2-3 weeks old in the fuel cans)
|Oh also for info, |
1) What Cam
2) What ignition advance at 5000rpm
3) What jets in the carb?
But I agree with Peter Burges you need to rolling road.
Give me a call if you need any help, im only down the road in Banbury
|Yes, I see now... oil control ring still on in 1st pic|
|Alex G Matla|
| Many Thanks to all that posted help and I'm sorry for the lack of replies and information as I was away with work most of last week then went to Silverstone on Friday|
I'm happy to say that I made it through practice and had an enjoyable race with a Morgan, Porsche and a MGB finishing 2nd in class to a Lenham.
We were told by a couple of people that the compression was too high which breaks the rings and fitting a decompression plate has got us running okay for now.
Thanks for the offer, I may take her for a spin over your way when she is running reliable as want a 2nd opinion on our brakes and have got your number.
This thread was discussed between 03/05/2011 and 15/05/2011
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.