Books of engine component dimensioning and tolerancing

[womble]

Hi All,

I was wondering if there was any information available on dimensioning and tolerancing practices for engine components (gasoline engines)? I need some custom parts made for an engine. I could just leave the tolerancing up to the machine shop but I like to keep detailed records of everything so I want to produce some drawings of the parts. I've done engineering drafting before, but not anything that involves these sort of fine tolerances.

If you know of any books available, papers, guides, etc. please let me know. I can order stuff from both the SAE and ImechE if that helps.

Thanks for any information.

 

[EngJW]

I have lots of engine books and papers but I don't think this subject is covered in any detail in any of them. Normal machine design tolerances work for most parts. For example, crankshaft journal diameter tolerance, finish, shape, and runout to other journals. Same as any other shaft that is supported by bushings.

Parts that bolt together- tolerances on hole diameter and position, same as any other assembly.

Of course there are some very specialized features like hone patterns and the alignment between decks, bores, and crankshaft axis.

One book that you might want to get is the ANSI Y14.5 book on geometric tolerancing so that you can use the right terminology.

If you have a specific feature in mind, please post again.

 

[womble]

It's the more specialised features really, like the ones you've mentioned or as an example, say a connnecting rod; How much tolerance for twist, flatness, distance between centres, etc. is allowed? Are there any industry or company "standards" for tolerancing certain items?
I hope people don't mind the questions but as you've said there doesn't seem to be much info on this.

Thanks

 

[womble]

My main worry is over doing the tolerancing on the part and it becomes too expensive to produce or impractical.
I suppose I want to know whats acceptable within the industry. If any one has any examples please post a URL.

Thanks again

 

[thundair]

My understanding is that tolerancing is where you break away from a design concept and start engineering. The tolerance studies are pretty basic about bolt patterns and and fits and finnish but when it comes to component interfaces such as piston to bore and crank to insert you maybe able to get help from the manufactures recommended fit tolerance. Some tolerances are based on empirical and others on calculations of thermal expansion or oil film Viscosity. So as you can see there will be no set values that you can pigeon hole.Like eating an elephant one bite at a time.

Regards

 

[Pressed]

See the first two links in my reply:

http://www.eng-tips.com/viewthread.cfm?qid=138270

The title includes the word concentricity but the question and solution are basically the same. If you are dealing with only a few parts I wouldn't worry what the manufacture does for thier operation. Some make parts to a wide tolerance and then measure them into small groups and mate them with similar sized parts; saves them lots of money. So the clearance is your focus and the links cover that. If you have sample parts you can measure the clearance directly. Good luck.

_______________________________________
Feeling frisky.........

http://www.tailofthedragon.com

 

[Zapster]

If you have a machine shop that does work for you ask them. They are the best resource for information regarding when do tight tolerances become expensive for them to maintain. There is no one general answer because, the machine shop will have standard fixtures and jigs for many parts that enable them to easily maintain tight tolerances with little additional expense for some parts. In contrast, if special jigs are required or difficult reference measurements are needed, things start to get more expensive.

With that information in hand, determine what tolerance is required for your parts. State the required tolerance on the drawing. If the required tolerance starts to make the cost of machining expensive, take a closer look at this area of your design and verify that you really need a part with associated expense of the tight tolerance.

 

[EngJW]

Some of the things you asked about connecting rods can be found in various overhaul manuals. On the center distance, at the two engine companies I worked for the distance was plus or minus .002 inches.

Get an idea of the basic process that will be used for a part- lathe, mill, grind, etc. That will determine your tolerances. Bearing fits for example- if you are working in the range of .001-.003 inches, you are going to have to hold the bore and shaft pretty tight. Something like a piston pin would have to be ground where they can hold .0005 or less, and the bushing in the rod would have to be honed after it is pressed in.

Also check out some of the "how to" books about building race engines. A copy of Machinerys Handbook could help a lot.

 

[patprimmer]

I would start with the OEM workshop manual, then consider the intended use, then talk to a few contractors who could do the work, then decide where the suitability for purpose and cost benefit breaks are.

Only you can decide that. OEM specs would be a good starting point if the engine is modern or current design. For antique or obsolete, modern tighter specs might cost no more than original OEM, but might work better or not work at all, depending on many factors.

If you have a specific problem, please give details so we don't repeatedly try to second guess.

There are quite a few members here who modify or adapt engines, or who repair engines when original parts cannot be obtained.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[womble]

Thanks for your help guys.
The specific problem I have is with an engine that has minimal aftermarket support.
We reached the limits of some of the stock parts and have been modifing components from other engines to work with ours. I no longer wish to do this and would like to develop parts specifically for this engine.
Components required first;
Pistons
Conrods
Valves and valve guides
Head studs

Future parts include;
Steel main caps
Mains girdle

With the pistons and conrods I could just do a drawing with the general dimensions, send it to the manufacturers, and let them sort out the tolerances.
Headstuds are not a problem.
I've checked through all my info on the engine and found information on tolerances for the valves and guides.
Main caps; I guess measuring the caps and blocks of several engines would give me the tolerances that are used.

I'll pick up a book on geometrical tolerancing and take it from there.

Thanks

 

[EngJW]

Pistons- the skirts are cam ground so you have to specify the contour, both down the skirt and around it. You have to determine at what point on the skirt you want to specify the diameter that will be used for checking bore clearance.

Rods- they have to be machined with the cap bolted on. If the cap is made separately or is cut off from the rod, the mating surfaces have to be smooth and flat, and the bolt holes should be drilled or reamed through both parts together. That way, everything will line up better for final sizing.

 

[thundair]

Back to my pestering thermal sizing

If you have a tolerance for a valve guide is it for a frosted bronze or a cast iron guide? The same goes for con rods are they steel, titanium or aluminum..Do your home work and eveything will be fine..

You did not say if you were trying to restore to original or make into a modern engine. If so the methods and tolerances 45 years ago are differnt now.. (better materials)..

Regards

 

[womble]

The engine type is around twenty years old at the most...
The pistons and rods I've leave to the manufacturers after reading a bit more about them.
As for the valve guides... Well they are cast iron as stock but of course we want them in bronze.
However, I've found a supplier for valves and guides so I'll speak to them for advice on exact material choice etc.

Thanks again.