Datsun Roadster Forum-311s.org

OK, so we know we're dealing with R16 pistons, and it has a history of blowing the thicker head gasket which must have been selected to reduce compression in the first place (I assume after they had compression/detonation or clearance related issues)... been there, done that. Also noted it said "mild dome", suggesting that they may not be as high a compression ratio as the original "R16 piston in U20" modification... let's hope so. I'd like to see cranking compression numbers... mine is 205-210lbs, approx 11 to 1 ratio (*B cam duration). I think a healthy stock U20 is around 185-190. Of course, the numbers will change given the thicker gasket, and will increase when he uses a standard gasket. We have no idea how much the head has been cut yet.
Can be many reasons why it's blowing the gasket, not just compression. "98" fuel may not be measured in the same way as in the states and we need to confirm we're comparing apples to apples, but better fuel will help a lot. (JT just noted same).

Options: 1) remove and machine the existing pistons to reduce compression. I have no idea how much can be safely removed. 2) replace with stock U20 pistons. 3) replace with higher quality U20 pistons. 4) figure out why it's blowing the gasket and replace with new thicker one if they exist anymore (?) and cross fingers (not suggested). 5) have a copper gasket made (good luck!). 6) replace head gasket with stock one and run octane booster and/or try a 123 dizzy to better control ignition timing and pull out more advance under load (maybe). We also don't know if we'll have clearance issues with a stock gasket... likely per JT.

That's how I see it... others?

unleaded fuel SP98 in France contains 98% of octane and 2% of heptane
how do you measure this in US

right but the calculation for octane in the US is an average method (R+M)/2 which is measured differently from EU RON from what I understand:

EU ratings are 95/98, which are equivalent to US ratings of 91/93. So, EU 95 octane = US 91 octane and EU 98 octane = US 93 octane.



On the pistons, to clarify: “Mild dome” is advertising speak for “cast R16 piston copy”. Those are R16 replacement pistons with a modified skirt.

Those pistons are NOT at all the same as our Mini-dome forged pistons. The Mini-domes are specifically designed to clear a significantly cut R16 head (barely) or even a U20 head with no or minimal work.

In a U20 application, the R16 dome piston will hit the head unless a thick gasket is used and/or the head is carved for clearance (making high compression)

Regardless, since we are talking about 91-93 US equivalent octane, flat-tops are the better plan in this situation.

Also- so everyone knows, we currently have 6 types of U20 head gaskets IN STOCK. 3 versions are slightly over .100 thickness, a solid copper version at .093, the Nissan .05" OEM gasket , and low cost aftermarket gaskets. I'll put up a separate post for those.

Hope all this helps, my recommendation would definitely be flat-top pistons and the Nissan gasket unless the head has been cut a lot.

I agree building the compression ratio correct for the street is smart.
Hopefully Labrube can do a light hone or dingleball hone and salvage the existing pistons by having the top milled.
A good machinist should be able to machine the domes down to appropriate size/compression ratio if not hollow underneath.

My avgas comment was more for borderline cases temporarily, not running all the time. Or storage.
You can hear the difference between premium and avgas on my engine.
Part of the the issue is adding alcohol changes the properties of the gas and it runs a little leaner with carbs.
More chance of detonation/ping. and rejetting my be appropriate. Fuel injection has a feedback llop that corrects for that.

I guess splash was not a good word, should have said some gallons
And a tank of avgas is great for that Datsun or mower or edger or generator if unused for a while.
We can have bad corrosion and varnish sour gas problems here in California with our holier than thou formulations.
Avgas or unleaded race gas avoids that. I can go in more detail in anyone cares.

There is a new version of avgas that is coming to market 100ul unleaded.
Also good for storage as it wont varnish and corrode like the gas with alcohol.

I have also used non alcohol race gas for storage or getting a car going with some old bad gas in the tank.
but it costs me more than the avgas. I also add Techron to help clean and get out the varnish.
A good high octane race gas or 100LL can be mixed with a tank of older stale gas to salvage it sometimes.
Usually its cheaper and better to pump out the remainder of the tank and just put in what you need.
But sometimes that is not practical so when we can't get to the tank or pumping out is a problem
we can just add appropriate high octane gas to get us through.

Hope that is more clear and helps someone.

One other option witht high compression pistons can be to put in a "hotter" cam with more duration and overlap thus reducing the effective compression in the low and midrange.

The cam described would be beyond a B or C cam, so that compromise would just make it even less pleasant on the street because of markedly less power in the 2k-4k range. Track engines (excessive CR, excessive cam) are pretty much never a great solution for street cars-just annoying.

"I agree building the compression ratio correct for the street is smart." This is very well put. Same goes for the camshaft.

If he wants the most labor intensive method, that is to cut off the domes. (take all the pistons off the rods w/o any piston damage, remove all the rings, clean the pistons and ring lands very clean and carefully inspect for cracks/zyglow, if not cracked-cut off the domes, balance 3 heaviest pistons to the lightest one, reassemble everything with new rings and circlips). That's MANY hours of work at current shop rates and you wind up with a cheap cast piston with the domes chopped off.

I'd say, buy some good u20 pistons and forget all that nonsense. Labubre,you could also check with Richard in the Netherlands, it is possible he has some .060 U20 pistons. He and Erica are great folks.

The low dollar way is .060 H20 pistons, those would probably be fine under 5.5krpms and would correct the CR if the head is cut a lot. (Chain slack could still be a problem). Depends on what he wants out of the engine. With the overbore and a cut head, the CR would be in the low 9's most likely, which would be great with 89-93US octane. I don't generally recommend h20 pistons, they are heavy and cast, but they are the cheapest solution if that is the goal.

Irrelephant

anybody knows how much the 0.06 R16 dome pistons would need to be surface to come back on good compression rate?

I think if you were to go with forged piston, they usually require just a little more clearance and you used a flattop piston it be alright. So to get that tiny bit of additional clearance you could have the block diamond honed and not necessarily go to the next size bore. Talk to Richard in the Netherlands about a machinist and he might have pistons as well. You can also reach out to Jim Tyler about wall to pistons clearance requirements.
Dave

I think he is asking if anyone knows approximately how much the dome of an R16 piston would need to be machined down in order to create a decent street compression ratio in a U20 using a stock head gasket.

He knows the options regarding pistons. Getting parts to France can be a barrier, so it makes sense to see what can be done with what he already has... I agree Richard may be helpful!

It seems that i avoid a US buyer from some big trouble as this car was offered last year on BAT
https://bringatrailer.com/listing/1969- ... adster-48/

High compression rate with +0.06" overbored, R16 dome pistons, + smog distributor on a de-smogged engine...none of this was mentionned
What else will i discover next???

Anyway, "lesson learnt"
Thanks for making me feel less lonely this is bad experience
I will do whatever is necessary to put this car back on the roads!

forgot to mention the non operational clutch MC and slave + brake MC & rear cylinder...

Gregs672000 wrote: ↑Mon May 08, 2023 8:12 pm I think he is asking if anyone knows approximately how much the dome of an R16 piston would need to be machined down in order to create a decent street compression ratio in a U20 using a stock head gasket.

He knows the options regarding pistons. Getting parts to France can be a barrier, so it makes sense to see what can be done with what he already has... I agree Richard may be helpful!

The whole dome needs to go to make normal U20 compression. If the head has been cut you would have to take more. I wouldn't recommend cutting significantly more than the entire dome off, (since a some point the remainder won't be very durable).....If he wants to fix it correctly, by far the best play is to get correct pistons.

Plan B, take the head and have the combustion chambers opened up to reduce the compression. I would if money was limited use that approach, it drops the compression ratio without having to take the bottom end of the engine apart and use all of the existing parts and you could cc all of the cambers to the exact same size.
Dave

That could work too, but you'd have to remove a lot of material (basically the same volume as the 1600 dome-which is considerable) and any additional needed to correct for a cut head. Many hours of fairly skilled labor- That would cost as much in labor as replacement pistons.......Again, by far the best solution is correct the real problem, which is to get rid of the dumb piston choice..( no matter what piston you replace them with) Even h20 pistons would be a better choice at this point. Hopefully Richard can help somehow.