General engineering on rebuilt engines

[gudgeon]

Advice very greatly appreciated, sorry for the long post.

I'm getting to the point where I will be starting a rebuild of an engine. The books I've read go to extreme lengths, sending every single part to the machine shop for testing and replacing almost every moving part with new (including pistons).

I know that many of you have done perfectly good rebuilds without going to ridiculous lengths and I was hoping I could get some guidance on how to go about this sensibly.

I will be sending the head to a machine shop because at the very least the valve seats need re-cutting. (pressure test and deck at the same time, possibly guides depending on findings)

I am replacing the crank and pistons (stroking the engine), which parts would you send to a machine shop for checking? Pistons, conrods, crank? I was planning on visual inspection on most parts with dial guage, vernier caliper, and some crack revealer, is this sufficient? How would you inspect the piston pins?

What lengths would you go to on the block, I'm aware I will have to re-hone most probably (at least because of the increased stroke), what about the crank main bearings etc...

How much effort do you go to to balance the crank and pistons?

[gudgeon]

Ok I'll narrow it down to one simple question:

"How much effort do you go to to balance the crank and pistons?"

None?/static weight?/dynamic?

[GeoffBob]

You don't say which engine, so I will have to guess a bit here.

I assume your replacement crank isn't new? If so, then check all the crank journals for roundness. On older well used engines they may have become oval. If so, a regrind of the crank will be in order, requiring the obvious oversize bearings when fitting. This, and the balancing of the crank can be done by your local engineering works. Your flywheel will also affect the balance of your engine.

Oversized pistons are advised if you plan to buy new. When I buy new I always go oversize on a cast iron block. Having to sleeve an aluminium block detracts from this option, but if I can go over-size I do, that way I get to erase a lot of the engines "history". Depending on the type of piston I fit, I am very particular about the rebore size, since different material pistons have different coefficients of expansion. However, if you are on a tight budget, you'll likely get away with same size pistons and new rings, provided the cylinders aren't already worn out of tolerance. This is a perfectly good option if the cylinders still have some life left in them, and also if you know the history of the engine, but in many respects you are just postponing the inevitable.

Also, check your cams if its an older engine. Chemical cleaning of the block is in order before you go looking for cracks. The rest I think you seem to have covered.

Since your engine is open it's good to be as thorough as possible. Oh, and fit a new oil pump. It's always sad to see all that hard work go to waste because your new, tigher tolerance, parts aren't getting oil supply they should from an old pump.

Best of Luck

[Gunni]

As GeoffBob says,

Alot of this comes down to how much history you want to remove from the engine and the components you want to use.

For a quick plain rebuild you need to make sure that
everything is within tolerance and then replace wear items.
Bearings and rings for instance.

[madaboutthe30]

What books have you been reading gudgeon? As Im interested in how to rebuild an engine aswell

[gudgeon]

Thanks for your help.

I've got some American book on Engine rebuilds, lots off the internet, small snippets out of less relevent books.


The engine is M20, the crank and pistons will be 2nd hand. I'm mostly worried about re-balancing I guess.

EDIT: Maybe best bet to send the crank straight to the machine shop and then they can balance it too.

Someone recommend me a machine shop in the Reading Area.

[GeoffBob]

I see,

With regard to balancing then:

Provided you extract a crankshaft, pistons and flywheel from the same donor engine, a rebalance will not be required. Off-hand, I can't remember if the bolt pattern at the flywheel/crank junction is circularly symmetric or not. If not, then there's only one way you can put the flywheel back on the crank. If it is, mark the position of the the flywheel on the crank with a centre punch to make sure you put it back in the same position it came off. A new set of oversized pistons should weigh the same as the old (should you decide to fit new), and will not throw your crank out of balance, at least not to any degree that you need to worry about (I doubt you are building a race engine out of second-hand parts?).

However, should you attempt to fit together a number of odd-ball aftermarket parts such as a custom billet crank, obscure flywheel, forged connecting-rods and forged pistons, then both a static and dymanic balance should be undertaken. This can be a difficult process to get right, requiring a fair bit of calculation and weighing of parts on your part, but eventually you should be able to input your collected data into a balancing machine in order to achieve a crank that will be balanced when attached to your collection of obscure parts. I mention this only out interest to you as I suspect that this sort of operation is what your books are aimed at. It's an essential process to go through on a top-end racing engine an can account for both a fair increase in HP and increase in the engines rev red-line. But for the operation that you have described, its really not necessary.

[gudgeon]

Great post GeoffBob thanks.

I wouldn't call the parts oddball but the crank, pistons, and flywheel will be from different engines.

[GeoffBob]

I understand. Oddball was the best word I could come up with to describe an ensemble of parts that were never designed to be used as such Yours are far from oddball.

However, I recommend you have the flywheel balanced anew since its off a different engine. If it was originally balanced separate from the engine, it should still be alright, but it's worth checking in case it wasn't.

Then have the crank main and big-end journals ground (if out of round) and then have the crank dynamically balanced. A well equiped engineering works will be able to do this for you. Hand them your pistons and con-rods and they should be able to set up the balancing machine to balance your crank to work with your chosen pistons and rods. Unless you are really really really sure that the OEM intended your chosen crank, pistons, rods and flywheel to work together in harmony (or others can tell you from experience that they work well together), then I suggest you go this route.

BTW, I like the look of that book of yours. The bloke on the cover has a workshop cleaner than my bedroom. Now that's the way to build engines! I have a mate who puts the hoover round his shed before he works on his engine, I wonder if that's a bit extreme?

[gudgeon]

I frequently use a hoover in my garage, but the amount of filth I get each time hardly indicates obsessive cleanliness.

Thanks a lot for your advice.