Twincam - Bore size and pistons

[europatek]

Ask a question in life about how to do something and more often than not one is left wandering what decision to make with all the varying ideas. Aware of this and perhaps at my peril I ask for opinions and ideas on the following. The twink in my Cortina, a stroker 1700cc with 83.5mm bore is going great but has developed a piston rattle. One piston - no.3. Over about 2400rpm you can hear it develop - under load. The pistons are modified Datsun from an L16 engine and have about 80000km on them. Now the engine is out of the car for some minor body repair work to the floor so I thought it may be a good opportunity to do something about the piston.
This is where you guys come in. Here's my ideas;
- do nothing and hope the piston survives.
- remove said piston and ceramic coat the skirt. Reassemble.
- purchase a complete new set of Datsun pistons at 84mm, modify to suit and bore the block. But will it bore to 84mm?
I'm trying to this economically whilst providing a sound, long lasting solution.
So, what do you guys think?

[rgh0]

what condition are the bores in ? Can they be rehoned and new 83.5 mm pistons fitted?
If it really needs a bore to 84 mm I would do an ultrasonic thickness check of the bores to ensure you have enough wall thickness to make it worthwhile doing.

cheers
Rohan

[Esprit2]

(Snip)... Here's my ideas;
- do nothing and hope the piston survives.

Three automotive rules to live by:
1) Never trust anything that fixes itself.
2) Left to their own devices, everything proceeds from bad to worse.
3) Hope is not a plan.

(Snip)...
- remove said piston and ceramic coat the skirt. Reassemble.
- purchase a complete new set of Datsun pistons at 84mm, modify to suit and bore the block. But will it bore to 84mm?
I'm trying to this economically whilst providing a sound, long lasting solution.
So, what do you guys think?

Do you know the knock is piston slap rather than rod knock (bearings) or a wrist pin/ bushing going bad? Until you know what the problem is, planning a solution is an academic pass time. Day dreaming.

The engine is out, so that's an opportunity. Open it up and evaluate the condition of the bores, pistons, rod bearings and wrist pin/ bushing. Then go from there.

If the the wrist pin is offset, and if the piston was originally installed backwards, piston slap could be a matter of an incorrect thrust angle.

If the piston slap is a matter of excess clearance, it's possible that ceramic coating (Nikasil, Alusil, etc) the bore and AF coating the piston skirt might improve the fit. I can appreciate the need to fix on a budget, but high-tech coatings aren't cheap. For the cost of doing a proper job of coating, you could probably buy a set of pistons.

But all that is guessing. Open up the engine and evaluate the situation. Then make an informed decision about how to deal with it.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[europatek]

Thanks for the feedback gents.
Rohan - I agree regards the ultrasonic test. As for as fitting another set of 83.5 pistons if the bores are OK do you really think this could/would solve the problem? I would assume the rattle has started because the bore is now worn so fitting another set of same size wouldn't work. Please pass on your thoughts. I have located NOS Hepolite Datsun pistons at 84mm at a very reasonable $260. Would just need to flycut, balance and bore block to suit. Might be the best way forward. Have you taken any blocks to 84? I did have a crossflow block at 85 and ran it for many years without problem.
Tim - I agree the best way forward is to crack it open and have a look. My comments about doing nothing really come from the old adage - "if it ain't broke don't fix it". I've learn't this lesson the hard way a couple of times. Technically it isn't broken - but it may if the skirt comes adrift. And it is running so well at the moment. Definitely worth checking the little end bushes.
FWIW - I've added a couple of photos of the state of the workshop at the moment.

[Esprit2]

(Snip)... Tim - I agree the best way forward is to crack it open and have a look. My comments about doing nothing really come from the old adage - "if it ain't broke don't fix it". I've learn't this lesson the hard way a couple of times. Technically it isn't broken - but it may if the skirt comes adrift.(Snip)...

I guess I disagree on the definition of broken. Perhaps nothing has shot out through the side of the block yet, which would clearly constitute being broken. However, if you're hearing something rattle in the engine, piston slap or otherwise, then something is clearly not right, and in my mind that's "broken"... and soon to be "Broken" in a bigger way if not addressed. Not fixing it isn't much of an option.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[rgh0]

Whats causing the noise could be a range of issues with wear or damage to piston or bores. The is a chance the bores could be Ok you will not know until you take it apart and measure it.

With offset boring 84 mm is possible in a reasonable percentage of blocks, may be 50%. 85 mm is possible in around 10% of blocks.

Orgers in Bayswater can do the thickness check and offset boring and will advise you what they think. I would want to aim for a minimum of 2.5 wall thickness after boring. Start going less than that and you risk cracking the bore sooner rather than later. 2.0 mm is the absolute minimum i would risk and only if the thin spot was only over a small area and low down in the bore

cheers
Rohan

[europatek]

I pulled the engine down yesterday. I had forgetton that the bore is already 84mm. 83 +0.40" Datsun pistons. This bore was done in 1992 and with about 80000kms on it still looks ok. No ridge and no obvious signs of ware. There is a change of colour/ware at the thrust point. I will take a trip to Orger in Bayswater and have a complete assessment made to determine the best way forward. As I determined it was no.3 with the noise I'm wandering if that piston skirt could be ceramic coated, the block honed and some new rings installed. We'll see.........

[rgh0]

At 84 mm the block will probably be near its limit even with offset boring. Orgers will be able to help you with the options after they measure up the bore to determine if new 84 mm Datsun pistons will fit with just a hone or wether it needs something else. e.g. Sleeving back to 82.5mm is possible, ceramic coating or custom made pistons also possible. Boring to 84.5mm or 85mm may be possible depending on block wall thickness

regards
Rohan

[europatek]

Received the bore/piston report today. .0045-.005" running clearance. Too much. So am having the block sonic tested to see if their is any chance of a bore. Datsun pistons are available in +.060" so 84.5. Lets hope I can bore it. Crank also needed a grind they say. So it continues.............

[europatek]

Had some more feedback from Orger's today. The block sonic tested ok - except for no.4. One area is down to 1.89mm. Thin - but has survived 20yrs and 80,000kms. Next thinnest point was 3.4. Most areas, particularly in the thrust area are near 5mm. So it looks like a sleeve for number 4 and a bore to 84.5 - so 1740cc.
They reckon the crank needs a grind. They are talking rod resizing, tunnel checking/boring etc... which I've never done before and struggle to see how the dimensions would be any different to how it was made. Anyone with experience here?
Previously all I've done is had a crank ground and bought shells to suit. Check with plastigauge and go. Never been a problem.
Stripped the head down. Guides, valve stems and seats all good. I new it was worn in the followers so am fitting new follower sleeves and followers.
More to come as it progresses....

[rgh0]

Can they offset bore to leave the 1.89 mm thin area almost untouched.? you want about 2mm minimum in the non piston thrust areas but if its lasted a long time already the metal is probably ok as long as you don't touch it in the thin area. Boring and sleeving back to 84.5 will leave very little metal behind the sleeve in this thin area and you risk splitting the sleeve - been there done that, with one of the many blocks Orgers have done for me. Personally not keen on sleeving if it leaves a thin spot now behind it.

Personally I would start again on another block if I had a wall thickness below 2mm but new blocks are expensive and good old blocks hard to find. I have the advantage of having a dozen blocks on my shelf that I have thickness tested for engines if I need them. That option not available now for most!

If you do a lot of work on the block such as sleeving you do need to line bore it in a race engine. In a road engine you can take the risk on the crank tunnel not being affected to much but you may find the crank tunnel moves with the boring and sleeving.

Always worthwhile rechecking the rod end sizes and correcting if needed in a high mileage engine. Again you can take the risk but worth doing I think.

cheers
Rohan

[europatek]

UPDATE - decided to sleeve No4. The thinest area after sleeving and boring will be around 2.6mm in a localised area. Decided on a bore of 84.5 with new pistons to suit - 1740cc Crank is being reground and rods resized. Head only needed two new follower sleeves plus a new set of followers. Will add some photos for those interested to follow the rebuild.

[europatek]

UPDATE - Collected all the engine parts from the respective machinists. Started to set the tappets tonight. Pistons to balance and waiting on a couple of items before assembly can begin in earnest. Settled on a sleeve in no.4 and a bore to 84.5.

[europatek]

UPDATE - Completed cylinder head assembly and tappets. Balanced pistons and assembled to rods. Dummy assembly to check piston/valve clearance.
I've been having some of my machining done by a chap who works from home. He has quite a workshop setup with lathes, mills etc..... and alot of experience with twincams, engines etc.....
From what I can gather he is the chap who pioneered the Datsun conversion. I inspected one of his twincams that is 2.0ltr. Uses a Datsun L20B crank - 86mm stroke and an 85 bore.
He modified the Datsun pistons for me. Flycuts, circlip grooves etc.... Handy chap to know. PM me if you are in Melbourne and want his contact details.

[europatek]

UPDATE - Rebuild completed. Put the bottom end together yesterday and finished the build today. So final specs are:-
- 84.5 bore
- 711M crank
- 125E rods
-10.5:1 comp
- 1740cc
- Bigvalve head with mild porting
- Wade 205 cams - 30-70/70-30
-New pistons, rings, bearings, timing chain, cam followers, two new follower sleeves, seal, gaskets etc.....
The only real change from before is the slightly bigger bore.
Photos show work of the this weekend. Looking forward to turning the key.