Engine inspection/rebuild.

[dougal9887]

Hi all, restoration pretty much finished on my 1973 Plus 2S 130/5, off the road since 1980 . I've decided to use the tall block conversion I bought some years ago and would be grateful for any advice. It has been balanced and has QED + 30thou pistons which by my calculations give 1692cc. I've removed the sump and pistons / rods and measured the bores at 83.3mm. Ovality checked with a cylinder dial guage shows +. 02mm in cyls 1-3 and +. 04mm in no.4 about half way down the bore in thrust direction; how does that sound? The pistons look as new. Big end and main bearings look fine; I'll check clearances with plastigauge. The water pump turns freely so unless the bore ovality is a concern, I'll re-assemble for use as is. Deck height measures .014" which is somewhat less than recommended. I've done a trial assembly of the head using the old gasket and measured the compressed volume through a plug hole at 42.4cc's. This gives a compression ratio of 11 :1. I calculate a compressed volume of 44.5cc's is required to achieve 10.5:1. So I need to gain 2.1cc's. Cometic appear to provide thicker gaskets, 1.7mm uncompressed would be required and although this might provide a simple solution there may be downsides such as the timing cover/spacer/head seal. Alternatively the pistons could be machined and a standard gasket used. If going down this route I would gain 1.2cc removing the raised crown and .9cc by removing a further .22mm, a total depth removed of .6mm. Both volumes allow for the cut outs. The deck height would be about .57mm. Is there anything I should look out for? Whats the best way to mark the pistons for identification once they're machined?
Thanks.

[rgh0]

.02mm ovality is OK and .04 mm is getting to the top end of what is acceptable I believe. However the ovality can also change a little when the head and main bearing caps are bolted down so No4 may reduce a bit. Blocks should be bored with a torque plate replacing the head and main caps in place to maximise bore roundness in the assembled condition.

It sounds like you have a decked 1600 tall block with standard 1600 rods and twin cam pistons is that correct?

You say you measured the combustion volume through a spark plug hole - how did you do that?

I normally invert the head and measure the chamber volume with valves and spark plug in place. Then I insert a piston a known distance down the bore to measure the net volume of cut outs and intruder above and below the top of the piston deck. This may be a positive value for a flat top piston with cut outs or negative volume if you have a large intruder above the deck. I then calculate the cylindrical space between the top of the piston deck and bottom of the cylinder head face at TDC and add the three elements to get the overall compression volume.

Machining the pistons or using a thicker head gasket are both OK solutions to achieve your target. You can get different thickness front cover top gaskets or get / make a different thickness spacer plate to accommodate the change in the head gaskets thickness if needed.

I would either stamp to etch any markings needed on the top of the piston in the same way new pistons are typically marked.

cheers
Rohan

[dougal9887]

Rohan, thanks for the reply.

It sounds like you have a decked 1600 tall block with standard 1600 rods and twin cam pistons is that correct?

Yes.

You say you measured the combustion volume through a spark plug hole - how did you do that?

I set no.1 at TDC using a dial gauge then sealed the pistons and old gasket with vaseline before replacing the head. I filled the chamber using a burette with the engine tilted, spark hole highest to let air out of the top of the chamber. When shaking and tilting and filling produced no more bubbles I continued to fill and lower the block level till the parafin was level with the top of the hole. I calculated the volume of the spark plug less the .1ml it contains and deducted this from the filled volume.

Machining the pistons or using a thicker head gasket are both OK solutions to achieve your target. You can get different thickness front cover top gaskets or get / make a different thickness spacer plate to accommodate the change in the head gaskets thickness if needed.

Is there a preferred solution? Im just thinking that getting the pistons machined is a one job solution whereas the alternative involves sourcing a suitable gasket and dealing with the front cover spacer issue.

I would either stamp to etch any markings needed on the top of the piston in the same way new pistons are typically marked.

I was thinking about the best way to mark them for identification after removal from the rods and whist being machined before re-marking on the crown?

cheers
Rohan[/quote

Thanks.

[rgh0]

Yes I would machine the piston tops I think, as it makes future maintenance easier with standard head gaskets. The amount you say you need to take off should not affect the piston strength significantly but I would check the crown thickness just to make sure there is plenty of metal there after machining

What cam are you using/ You should check valve to piston clearances also before machining the pistons to see if that is all OK also.

To mark the pistons I would etch inside the piston on the underneath of the crown in a suitable location. If you don't have a vibrating tip etching tool then a scratched number with a blunt nail in true lotus tradition should suffice.

cheers
Rohan

[bill308]

1-to-4 punch marks on the bottom of the crown would also work and avoids penmanship issues.

Bill

[dougal9887]

Thanks again for the replies guys.
My feeling was for machined down pistons so that settles it. Re. crown thickness, I imagine QED will be able to confirm if the amount I will require machined off will be acceptable.
I have standard sprint cams at present but considering QED 420, deliberations on which will be subject of next post! The pistons are the QED large cut out type so should be ok. I'll not have the pistons machined however untill head work is completed and volumes re-checked together with valve clearance.

[dougal9887]

When I started to assemble the above, first check was for piston ring gaps. They were well outside spec and the top of the bores appeared to have a strange shape so I think perhaps there has been a diy ridge removal. After a visit to the machine shop a +.040" rebore and new pistons were decided on. Since so much was now being done and the crank was at the limit of acceptable wear (already +.020") it got a 30/30 regrind. So all maxed out now but a nice round 1700 cc.
After balancing and assembly a check of the combustion volumes indicated a .030" gasket for a 10.5:1 compression ratio. Cometic appeared the only option for this thickness in an 84mm bore size. This eventually arrived and the head was assembled using ARP studs.
The head has had a mild porting job, valve guides and seats, QED 420 cams and Q55 springs. Offset dowels were used for timing.
The engine was finally plumbed in last weekend.

As the coolant was added a steady drip emerged between engine and gearbox. This, I hope, is a leaking rear core plug which I didn't replace. It looked ok. There's definitely no leak at the cylinder head so I can't think of any other cause?
I decided to carry on with a first fire up of the engine since now was a better time to find any other issues requiring attention with the engine having to come out again.
First problem was that the engine wouldn't turn over with the plugs in. Jump leads to the solenoid and block gave a temporary fix. It took a couple of shots and full choke to start and it ran for 20 minutes at 2500 rpm to bed in the camshafts. Temp hovered 85 to 90 degrees with the fan cutting in and out. Oil pressure just below 50. Pretty satisfied, I tried to slow to a tick over which it clearly didn't like so I switched off and pulled the plugs out for a check. 1 was a bit black, 4 was pretty normal, 2 and 3 were shiny clean and clearly never fired! Checked the leads and found 2 and 3 reversed Sorted that and it started immediately and ran for a short period at 800-900 rpm but a bit lumpy. My time was up so didn't attempt any tuning but pretty satisfied that the only problem was the coolant leak.
So engine will come back out, and hopefully go back in, this coming weekend.