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MG MGB GT V8 Factory Originals Technical - Cylinder Bore expansion

Most of us know that pistons expand as the engine heats up and information is available for various types of pistons. However, does anyone know how much the actual bore increases as the engine warms up to normal temperature. Obviously the larger the bore the greater increase so I am interested in the amount for a Rover 3.7 inch bore (3900cc ali block with liners.)
Denis Perrott

I suggest you Search coefficient of expansion of aluminium or thermal expansion of aluminium. Then aply it to the circumference of the piston. Do the same for steel with regard to the sleeve bore.

Thanks for your comments. I wish it was that simple. In the case of the Rover block the liners are heat shrunk in so there are unknown stresses in the liner at cold. Therefore the interaction between the liner and block would be difficult to calculated as the temperature rises. I am hoping to determine the change in bore size to within .001" and hoped there was someone out there with info for a Rover Block in particular. As a last resort I will stick the block in a heated/boiling bath (If er in doors does not object)and physically measure the bore increase.
Regards Denis
Denis Perrott

Beware of overly close tolerances on the Rover Block. I had a very nice 3.5 lunch itself due to the builder making a "tight" engine. the tinnest parts of the piston is the lower skirt and this in a "tight" motor will hit the small lip area where the cylinder liner exits the bottom of the block. My next engine will be built on the upper side of the tolerance band.
M Mallaby

If you do I'm sure we all would be very interested to know.

Dennis, I am quite sure that the actual measurement method you suggest will give the most useful results. My guess is that you will see less increase in the bore size than there is in the piston but there's no telling how much, and it is even concievable that the size could remain constant, but not very likely.

Jim Blackwood

"My guess is that you will see less increase in the bore size than there is in the piston but there's no telling how much, and it is even concievable that the size could remain constant, but not very likely."

That's my guess too. Surely the pistons (and liners for that matter) operate at a much higher temperature than the aluminum block (and water jacket, etc.), and also at a much higher temp than your water bath, no?

But, if the bore size does change appreciably... I'm curious about whether the water bath test would yield the same results if you tested with and without heads installed. In other words: does the block distort and expand unevenly, and do the heads restrict or change this effect? Isn't there also higher heat at the top of the cylinders/block than at the bottom? Shouldn't this differential cause distortion too?

Guys. Thank you all for your various useful comments.
As Curtis correctly states the temperature at the top of the bore and piston is higher than at the base and this is why pistons are tapered with greater clearance at the crown compared to the skirt. I'm sure that the heads will affect the expansion but how much is difficult to ascertain given that they also expand albeit at a different rate.
Consider you are cruising at a light load and then you suddenly put the pedal to the floor. The first element to heat up is the crown of the piston (As it has no direct water cooling) followed by the upper cylinder walls and combustion chamber walls and exhaust ports. Clearly when the load on the engine is increased suddenly all parts do not heat up and expand at the same rate. This is probably the situation most likely to cause a piston siezure.
The reason for my initial question is that I have built an engine using forged pistons using skirt clearances recommended by the piston makers and they slap like crazy even when hot. The piston skirt expands .005" in boiling water and cold clearance in the bore was .004" (recommended by maker for this type of piston) so the bore must be expanding more than .001" otherwise they would have siezed by now. I intend to bore/hone another block to to a SLIGHTY smaller size and I was hoping someone would be able to advise from their past experiences. It maybe that when my engine is working very hard, the piston slap is minimal but of course in such conditions other noises drown out the slap noise.
Anyone got any comments on amount of slap noise from forged pistons?
Regards Denis

Just to make sure,were the pistons measured at the manufacturers recommended position on the piston to attain the .005" clearance? For example,at the pin,at the pin boss,a given distance from the bottom of the skirt,etc. IMHO,.005" clearance on a street car with a bore that small is too much.

In all probabliity the bore expands at a rate very close to that of the piston, and here's why I think that. Machined press fits are zero clearance or interference fits between round parts. Any running fit has to have clearance. The least possible clearance for a 1" diameter shaft under optimal conditions is going to be no less than .0005" except in special cases and as the diameters increase so does this number so about the least you would want for a 3.5" fit under best conditions would be about .0015", but this is if it is perfectly clean. Add any dirt and it may seize.

Machined fits often take into account materials with different coeficients of expansion but the expansion may not be as much as you expect for a temperature change of 100* F. which is in the range of what we are talking about here. When shrink fitting parts, often a temperature differential of 100*F is insufficient for interference fits of .001" or more and differentials of several hundred degrees are more commonly used.

What this tells me is that your cylinder bore, which has it's temperature held in check by the coolant is limited in how big it will get for several reasons. The metal around it is at a stable temperature and so will restrain it. It is true that a boundary layer will get hotter and so expand more but some of this expansion will of necessity be in an inward direction. A small percentage no doubt, but still there. Finally this boundary layer will be made up of stable temps at the outside, a thin hot layer at the inside, and a gradiant in between. This defines the cast iron liner and aluminum water jacket. The aluminum, being a better conductor will be cooler. But the liner, having a thin oil coating on the combustion side will not be nearly as hot as the piston crown or surface of the combustion chamber and may rise in temperature only another hundred degrees or so. Much more than this and the oil film would scorch, making frequent oil changes an absolute necessity and creating excess smoke. Then the cylinder head, having a coolant layer directly over the combustion chamber is mostly going to be stable as well except at the roof of the combustion chamber so I would expect it to have little effect on cylinder expansion.

I wouldn't think you would see more than .002"-.003" of expansion in the cylinder under working conditions. Starting with .004" clearance and expanding .005", you would have to have .003" of cylinder expansion to maintain at least .002" of cylinder to piston clearance, and this would be right about what you would want for a running fit.

The piston OTOH is a completely different animal. Cooled by oil spray on the bottom and piston skirts and directly exposed to combustion, it operates at higher temps, but consider this. Some metals have a negative coeficient of expansion. Antimony is one, it gets larger as it cools, and is used as an alloying agent with other white metals such as lead and aluminum. Certain alloys are produced which have a zero expansion as well. So it is likely that the alloy of your pistons was selected to give a rate of expansion that is very close to the rate of expansion of your block, probably allowing more clearance when cold to make cold starting possible under sub zero conditions and closing down the clearance a bit once warmed up.

I once had a water cooled motorcycle that was designed for .0015" of piston clearance but this was a 2 stroke with relatively small pistons and should be considered an extreme case. Typical clearances run about .001" for each inch of piston diameter, so for a 3.5" piston .0035 would be just about right. In the case of the above motorcycle I had the cylinders bored and they came back with .003" of clearance. The pistons practically fell in the holes and there was no slap that I could tell, but I only put a few thousand miles on it after that and am sure that it didn't last as long as it would have otherwise.

The bottom line is, if you followed the MFG's specs of .004" and have piston slap conventional wisdom would say go to .0035" or possibly .003" and it should solve your problem, but be sure to gap the piston rings correctly. This is probably the biggest cause of seized pistons. The top ring especially is subjected to much heat and must be adequately gapped. Forged pistons are expected to grow more than cast pistons and are usually allowed more clearance for that reason, but alloys are continually getting better so pistons are becoming more stable and closer fits are becoming more common. Finally, the skirt design will have the biggest effect on piston slap. Modern pistons aren't round or cylindrical anywhere except at the ring lands and the skirt can act as a spring to absorb the impact of contact with the cylinder wall. A piston with a different skirt design may work better.

One last footnote, are you quite certain that what you are hearing is piston slap? In my experiance there really aren't that many people around who really know for sure what it sounds like and so it is used as a catch-all for unidentified sounds, some of which are quite harmless. It sounds to me like your clearances are correct. Where I would expect piston slap to arise is where a backyard rebuilder decides that 5-7 thou of taper is within acceptable limits, flex hones the engine and slaps in a new set of rings. But not on a fresh engine with proper clearances.

Jim Blackwood

Thanks Dale & Jim for your comments.

First to answer Dale's question, the piston dia was measured at 0.5" up from the bottom of the skirt and at this point it did expand by .005" from room temp to 100 degrees C. The cold clearance in the bore at this point was .004" which is what the piston maker recommended. Actual piston size 3,717" and bore 3.721"
I now suspect the piston maker allowed for racing conditions and made the upper diameter of the piston smaller than would normally be suitable for a road car. Probably my fault as I did mention the occasional "run what you brung" at Santa Pod drag strip.

Picking up on Jim's very comprehensive advice, last point first, it would "make my day" if the noise was not piston slap as the pistons cost megga bucks compared to the other parts of the engine. I did consider other causes of the noise but as the noise decreases as engine temp rises this eliminates elements affected by oil thickness such as crank bearings. The noise also increases with load and is very noticable when the engine is cold as I reverse up my steep driveway. I am running adjustable pushrods (probably hollow tubes) and I did think that these may be amplifying sound from the cam lifters. These are Hydraulic Type.
I have determined at idle with a sound stick that the left bank (looking from the drivers seat) is louder than the right bank. This I think points to piston slap as in a v8 the left bank pistons are hanging away from the thrust side and the right bank pistons are laying on the thrust side. This may a rubbish theory but would welcome any comments. If you can think of any other engine parts that usually get quieter as the oil thins, please comment.

My plan, once I am sure the noise is piston slap, is to have another block bored and hones to a slightly smaller diameter and your suggestion of .003" cold clearance goes along with my thinking also.

Regards Denis


I don't think you can go by the skirt measurement. You should contact your piston manufacturer for recommendations. How much smaller is the head of the piston at the ring lands? It is likely that the design preloads the skirt slightly, remember it is designed to act like a spring and it is likely that it springs outward when it warms up as well. Check how much larger the head of the piston gets when heated. I'd bet it's less than .005".

Jim Blackwood

"If you can think of any other engine parts that usually get quieter as the oil thins, please comment."

It may be way off base from what you are experiencing, but my 3.5L rattles considerably with tappet noise which goes away as things warm up. This happens only when the car is started from cold.
Edd Weninger

Edd, Thanks for the comment on the tappet noise. The noise in my engine is a heavier sound and is load related so it should not be the tappets.

Jim, I have rechecked the piston in the boiling water test with my spare new piston this time starting at 16 degrees C and rising to 100 degrees C. The crown expanded .005" and the skirt .0055". However the diameter of the crown (COLD) at the piston ring area is only 3.674 and 3.717 at the skirt. The skirt dia extends up to u/s of the oil ring but it seems like a very loose crown diameter in a bore of 3.721. I think the makers designed for all out racing conditions when I mentioned Santa pod and allowed for large crown expansion.

When I built the engine I was expecting to hear a little piston slap when cold because the pistons are only 2.78" tall. This is because I am running a 90mm stroke TVR Crank and the shorter con rods do not help. However, the pistons are the same proportions as the TVR items except they are forged and need greater cold clearance. One thing is for certain, I will need to sort the problem before the winter when cold starts will be at below Zero degree C for a month or so where I live, giving a further .001" calculated clearance at cold. Of course the oil will be a little thicker !!!.

Regards Denis



Before spending money on a fix, spend it on a run up to Derbyshire and let Peter Burgess have a listen to the noise - he should be able to diagnose and advise on the solution and may also have some info on expansion rates etc.

At least drop him an e-mail!

Alternatively Real Steel in Uxbridge or even RPI might be able to help.
Chris at Octarine Services

Dennis, I believe you are correct in your thinking, but if you can, take Chris' advice and also talk with your piston maker before doing anything. The short piston length certainly aggravates the situation but it still should be possible to get it to run quietly. Just how close you can run the clearances is something the piston maker should be able to help with and it might have to be set up tighter than you think to do it. Nothing more bothersome than fixing it and finding it wasn't enough except finding it was too much.

Jim Blackwood

Your battling two issues here. Forged pistons are noisy by nature due to they're expansion rate. The crown is always smaller due to the way a piston is cooled. The crown will expand more due to the heat from the fire. While its true some oil is sprayed on the bottom of the piston to cool it and lube the wrist pin. Most of the cooling is done through heat transfer between the piston skirt (the coldest area) and the cylinder wall. I would be very cautious about tightening up clearance with forged pistons. If its to tight there wont be enough room for a proper oil film. This can lead to piston scuffing and rapid piston/cylinder bore wear.
Since you set the clearances up to piston builder's spec's, the main source of your noise is probably the short piston skirt. I see this quite often in newer engines. They are building pistons with almost no skirt. The skirt barely reaches below the wrist pin area. Its no wonder they're noisy, even when the vehicle is brand new. This is because there is not enough skirt length to prevent piston rock. They make them this way to reduce friction and improve fuel mileage.
One way the new car makers are reducing the noise, is to use the powder metal forged process to make pistons. They have the strengh of a forged piston, but the heat expansion rate of a cast piston. They can then run the tighter cast piston clearances. It helps reduce the noise, but some of the engines still have piston noise (mainly when cold).
GM is telling us that the noise, dosn't effect long term durability of the engine, and I have seen some of these engines with well over 100,000 miles. They still run like new and do not use any oil.
Not sure for your application, if they can make a piston with a longer skirt. Possibly extending the skirt longer in certain areas just to clear the crank throw or con rod, would be enough to help. Any length you can add to the skirt will help reduce piston rock, without the risk of running to tight of clearance. Draw back is you would have to buy new pistons. However if the noise is really lound and bothersome, it might be worth it.

bill jacobson

Guys thanks for your comments

I actually ordered the pistons through Real Steel who got them made by Probe in the States. They were to my design except for the matter of running clearance which I left to the experts !! I did provide the bore size to them of 3.721 and piston skirts were supplied at 3.717. I shall be going to see Real Steel soon to let them have a listen to the slap and get their views.

The pistons are as tall as they can be and currently clear the counter weights by only 60 thou. I'm told this is the normal for the TVR 5.0 lump with a 90mm stroke. My pistons are slightly more stable than the TVR items as they have a small skirt extension each side. The wrist pins are close to the bottom so as you have suggested, they will probably be noisy.

Having read all your comments and advise it seems that some slap is inherent with these Forged piston proportions and it may be that I will have to put up with it unless I change back to cast pistons.

Alternativly, once I establish the actual bore expansion expected I may choose to use a block with a cold bore 1.5 to 2.0 thou smaller.

Regards Denis

Denis, do your pistons have an offset pin and do you know if they are installed correctly in the engine, i.e., with the offset to the thrust side of the piston?

Also, you can play with the oil weight by going a bit heavier; maybe a different brand.

Wayne Pearson

Yes the pistons have offset pins and I had this designed in to match the TVR pistons and to follow suit with later Rover thinking for the 4.6 & 4.0 pistons. When I heard the slap, the first thing I suspected was that I had put them in the wrong way round. So, off came the sump to check and I was probably dissapointed to find that the pistons were all installed correctly with the offset towards the major thrust side. (Left side of the engine looking from the front.) When the sump was off, I also checked the skirt clearance at cold which was .0045".

I am using a 15/40 oil (Castrol Magnetec) and maybe I could try a 20/50.
Thanks for your comments.
Regards Denis

This thread was discussed between 13/10/2006 and 23/10/2006

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