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240z engine rebuild options


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Hi guys

Well I decided to have a look at the original engine from my 1970 240z a couple of weeks ago and I 've got it partly stripped. Problems is engine dropped a valve before I bought it which has damaged the E31 head and smashed one piston.

My first thought was to get the knackered bore sleeved and to buy one new piston ( which are apparently still available) and a full ring set. And get the block tanked to get all the bits of broken piston out.

However the engine has done at least 120,000 miles ( could be 220,000 miles!!!!), and only replacing one piston seems a bit of a false economy if the rest of the engine is starting to wear. And I was concerned that fitting one replacement piston should not be done anyway as perhaps replacement pistons should only be fitted as a set of 6?

I was thinking whether to overbore it for 280z pistons but wouldn't the compression then be too high ( particularly with an E31 head.)

Any assistance gratefully received.

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I had the original L24 in my car do the same thing a couple of years ago. And I was thinking the same things you are. A few things more things to consider, was the crankshaft dammaged? You will also have to replace the con-rod. How bad was the head damaged? When It all came down to it, it was cheaper to replace the entire engine with a L28.

But If you must rebuild this engine. If you do replace just the one piston you will have to have it balanced along with the other pistons. If the engine does have 220,000 then most likely the sleeves will need to be machined to get them nice and smooth. I'm not sure if you can use the L28 pistons. You might have to machine too much material from the sleeve. I don't know. Also If memory serves me right the L24 had flat top pistons so if you are going to retain the original head then you will need the flattop pistons from a later model 280. I'm not sure if it would be cheaper to use over sized pistons and bore the sleeves. Or replace all the sleeves and use L28 pistons.

Good luck.

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Sounds like you have got some hard decisions to make. I have no idea what you do or do not know about these engines.

Firstly is cost a priority or are you going to keep that engine. cheapest option could be another engine. If you want to keep the block you need to get the damaged cylinder bore surveyed. You need to know what the recovery number is ( the amount of machining required to restore the surface for continuous use).

Best case light hone and find a fresh piston. Worst case you are already looking at scrap.

With any bottom end re-work always do more than less, always change out the bearing set, even change out the rods on the early 240 engine with a major rebore you are up for six of the oversize variety pistons. then the oil pump should probably be changed out for new(200000miles). all those corroded alloy parts should go for new. new water pump maybe. That is not all but it gives the idea.

Other scenario contact a major zed parts supplier, price and availability of a rear sump L28, (hot run and with compression test data) mortgage your next palace and you are away.




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thanks for the replies guys

I have another 240z engine ( from my 1972 car) so I don't NEED to rebuild the original yet, but Its one of those things that bugs ya!!!! and I'd like to know whether it can be rebuilt or whether it is too far gone. This car will probably be kept original if I can redo the original engine. Cylinder head has been chewed up and needs some welding and a new guide/exhaust valve

Sorely tempted to go for L28 etc but cost might be prohibitive. Plus I don't think there are many places here in the UK that do L28's. Perhaps I'll post to find somewhere over here who knows their stuff. Hmmmmm!!!! got me thinking. I can always swap the original engine back in if I rebuild it in the future.

SJCurtis I'll get you my HS numbers shortly

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Hi abas,

I'm based in central London - so a little closer to you than most of the people that frequent this site! Maybe that will be useful.

If you are determined to save the original L24 block that came with your early car, then Steve's ( sjcurtis ) advice to have the damage assessed by a professional machine shop is going to be the only way to go. I don't think you can do anything with it until you know where you stand. Maybe its beyond saving already.

Alternatively, I have just about all the parts you would need to build up either an L24 or L28 engine. I have blocks, cranks, rods, pistons and cylinder heads. I have these items in component form as well as assembled heads and short blocks. Once you decide what route you want to take, then I can possibly help you out with any mechanical parts that you might need.

Going for a fairly peppy L28 build up might not be as difficult or expensive as you think. All the machining work you might need, as well as the cost of the rebuild parts like bearings, gaskets and piston rings etc are going to cost just about the same whether you are building an L28 or an L24.

I've even got some good used and new performance parts that you might want to spice it up with.

I think the hardest part for you is just deciding which way you want to go. However, most mechanical installations can be reversed - so you could take the car back to a more original spec in the future if you wanted. Body, paint and trim are slightly different in that respect.

Good luck!

Alan T.

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Good to hear there is someone less than 3000 miles away!!! I'm near Bournemouth on the South Coast.

I'm definitely interested in following this up. Do you have an E31 head for sale? Was going to get mine repaired but not sure now.

I would lilke to keep my original engine if possible, do you know what the max overbore/piston size is for a 240z engine? I'm thinking of increasing the cc by overboring (which it needs anyway) but by keeping the crank the same.

You have some great pictures in your gallery!!!!



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Hi Andrew,

Ah, Bournemouth. The English Riviera. I've had some good times down there. Its a bit closer than 3000 miles that's for sure!

I've always found that the Bournemouth and Poole area had a real lot of serious car freaks. Lots of really good Rods and Rod builders. Is Dorset the British California?!...........

I can understand your sentiments for wanting to keep the original engine in the car if possible. Its such an early car ( in comparison to the average UK car ) that it has to be worth thinking along these lines. However, you can still pep it up a little and have something that's very satisfying to drive but still tractable and relatively docile. I always think that its worth taking advantage of this country's so-far lenient laws with regard to modification of old cars. This will free up your options on the spec. of the engine.

I'll PM you my direct e-mail address and we can take it further. I'm sure I have some parts that you might be able to find a use for.

All the best,

Alan T.

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