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205 block engine build


LewisH
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  • 2 months later...

Block has been re-bored +1mm, new mahle pistons to suit, reused the con rods. Crank ground .5 mains & .25 big ends, put it all together yesterday but something isn't right.

The pistons are still sitting 1.5mm below top of block when at top of stroke. I've tried contacting the company that did the work but discovered they have since closed the doors and gone under! Having spent nearly £2k on this so far, feeling a tad frustrated to say the least.

My understanding is that the low compression engines had pistons sitting 1.5mm below block and standard were .5mm below. Can anyone confirm this?

I was hoping to achieve 10.1 compression ratio, but with pistons sitting this low, I understand that this won't be possible. Question is, what should I do now? Skim the block face 1mm? Change rods to YB type? Buy another set of pistons? Not really sure which one is going to be the best option, considering cost and feasibility. 

Any advice much appreciated. 

Pics below of pistons at top and also side by side comparison of new vs old pistons.

20221104_091331.jpg

20221104_091944.jpg

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Perhaps the extra 0.5 mm radius of the piston has increased the volume (pi r squared h) of the stroke so that now the standard compression ratio is when the piston top is a little lower ?. How about trying to measure the CR ?, seal the valves in one of the cylinders with thin layer of silicone sealant and have the spark plug in, and with the head level, flll the chamber with water and measure exactly the volume of water used. Add to this volume, the thickness of a gasket and the gap above your piston (pi r squared the distances) and you have the volume of the combustion chamber. Then measure the stroke to calculate the induction volume.  You will then have the two volumes to work out the CR. Costs nothing to try, and then with luck you will know from where you are starting in your quest for high compression. I'd have thought that you could make an accurate liquid measurement if you can use a medical syringe to empty the liquid as they should have accurate volume markings and also perhaps a liquid with low surface tension, like say paraffin, so that you can fill level without that bulge on the edges that water produces, or maybe water with detergent or dishwasher clear glass liquid.

Edited by Sparepart
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If you want to increase the compression ratio more, you should have the head skimmed. You've already got the high compression pistons, the only other thing that will increase the compression is to reduce the combustion chamber volume.

The crank journals are now undersize, this does nothing for compression. If the big ends were offset ground, you'd gain a bit more capacity but you've got more from the larger bore.

The deck surface of the block looks cleaned up already, if the head hasn't been skimmed, I'd just get that done and put it together.

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  • 2 months later...

I ended up getting some correct HC pistons, which are now fitted and flush with block. I am now pretty close to getting the engine back together, bottom end is all done, sump fitted. 

I am going to fit new core plugs and have purchased a set from Burtons, but they seem to large to me. I understand that it has to be an interference fit, but they seem way too big. The holes in my block are 33mm and the core plugs are 42mm. Is this right? 

The block is a 205, but it doesn't have the plug on the back of the engine near the flywheel. Not sure if this is a different type with smaller core holes?

If the one's from Burtons are too big, does anyone know where I can get the correct plugs and what size they should be to match my 33mm hole?

Thanks

Lew

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Please have a look at the following link, I am not sure if it will help or further confuse, according to this parts catalogue there are an assortment of sizes depending on position and year of manufacture. All the sizes are bigger than your measurement, like 38 and 48 mm.

https://www.fordopedia.org/parts-catalog/pinto-ohc/B1.40

 

Edited by Sparepart
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Sorry, just measured again and they are 38mm, not sure how I got 33, maybe digital calipers should have been on my Christmas list!

Anyway, 42mm into a 38mm hole is going to be difficult. Do I just need to persist with a big hammer? Or are these plugs too big? I can only seem to find one size for pinto but I am a bit worried about cracking the block. 

From the little info online that i can find, plugs should be less than 1mm larger than the hole, but no idea if this is correct or a general rule of thumb? 

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