Recommendations, boring, ballancing

[droyce]

Well if I understanding you right the stock was over 7mm and the new piston only extends down 5mm, Id bet your okay. Looks great man, cant wait to see this thing run.

[Mekanix]

Thanks.

The only reason I was concerned is because the stock piston skirt is smooth and the new one is ribbed like any new piston.

When I put the needed shims on the bottom and rotated the crank I could hear a little bit of the piston rubbing on the bottom of the bore.

I actually think its like running a file over the end of the sleeve by the sound of it.

So then I did all the measurements to see if maybe I cut the sleeves too short. But I actually left them longer than stock.

I'm just thinking that the bottom of the skirt and the top rings are the only think holding it straight and if the skirt went too far down that it would scrape and fail.

[Mekanix]

If you haven't seen my other thread about cam's, here is what was discovered.
The cams for an 1100 or 750 will fit with the end lobes cut off.

But the middle lobes on an inline 4 are 180' apart.

Mine are 90' apart.

It just wont work and wont be easy to modify.


So far the suggestions are to

A: send it to web and have it ground
B: cut and re-weld an 1100 or 750 cam to 90'
C: find the best cam already made for a twin.


I think I'll look into gs400's A, B and C model engines , then the gs450 and 500.

These are all two valve engines.
The general cam specs say they will fit but I don't know what the numbers really are yet.

[droyce]

Thanks.

The only reason I was concerned is because the stock piston skirt is smooth and the new one is ribbed like any new piston.

When I put the needed shims on the bottom and rotated the crank I could hear a little bit of the piston rubbing on the bottom of the bore.

I actually think its like running a file over the end of the sleeve by the sound of it.

So then I did all the measurements to see if maybe I cut the sleeves too short. But I actually left them longer than stock.

I'm just thinking that the bottom of the skirt and the top rings are the only think holding it straight and if the skirt went too far down that it would scrape and fail.

How tall is your new skirt? The ribs rubbing would worry me, but if theres enough skirt still in the sleeve it should keep it square.

[droyce]

Heres my stock pistons, consider the extra length to keep it square when the rod is at its sharpest angle. Maybe talk to a piston expert, im just armchair engineering here.

[GregT]

The pistons extending past the liner bottom shouldn't be a problem - I've seen them coming out further than that on other motors. Just make sure the edges of the liner on front and rear are smooth.

Cams - as far as i know all the 400 8V twin cams are the same. You can't run the 2V/cylinder cams as the profiles are quite different. The 8V use an assymetrical profile to compensate for the rocker geometry - leading and trailing rockers. The 2V/cylinder profiles are symmetrical as they are direct acting onto the buckets.

[Mekanix]

How tall is your new skirt? The ribs rubbing would worry me, but if theres enough skirt still in the sleeve it should keep it square.

There is 5mm in the difference between the stock and new piston.

Cometic can make me a 2.38mm coper head gasket, the rest will be a base shim of 1-2mm.

Cams are going to be a later issue by the looks of it.

[Mekanix]

I ran into a completely new problem today.

While mocking up the head I noticed the cam chain tensionner was too tight and there was no slack left in it.


Looks like I nee a longer chain

There is enough slack in the chain without the tensionner.
I was thinking of having the pushrod ground down a bit and then thought I might as well buy a new chain.

Mine already was a brand new DID 219 fts with 120 links. Now I need to buy a 122 or cut a 128 link chain to make up for the difference in height change.


Never thought of it till now that there would be such a difference. But I suppose I'm moving the head up by 3mm. that means I need 6mm of chain or 1 link.


I have my old and new chain, I could splice the two together with 2 master links. I would only need to use 1 link from my old chain and to buy 2 master links and the tool for crimping them.

[GregT]

A manual camchain tensioner should solve your problem.

Don't mix old and new camchain.

[Mekanix]

A manual camchain tensioner should solve your problem.

Don't mix old and new camchain.

Ok, I wont. Turns out the chains are different size pins anyway.

I'm working on a manual tensioner just for fun, but the chain is straight against the guides with not much slack at all, 2-3 cm maybe

I've done some more math and it looks like the bigger the arc the chain has to travel around, the less the chain defflects. The tighter the chain is, the more it deflects so i think its a good idea to set it up to work with the original tensioner and then use the manual one.


I ended up ordering a new chain with 124 links. That should be 2 links too much. Got it at a better price than another headache

[Mekanix]

Minor setback on the cam chain.

Onto the next challenge: deciding on comp ratio and shimming.

Remember you need .040in minimum piston to head clearance - you may have to machine the piston crowns to achieve this particularly where the bore overhangs the original head area - these areas become squish bands.

Then the fun starts with trial assemblies, dialing in cams and checking valve to piston clearance.....Worry about finished CR only once all the clearances in the chamber are OK.

Well the fun is beginning

What comp ration should I go for ?

I know that With a piston to head clearance of 2.0mm I get a compression ration of 11.2 :1

With 2.4mm I get 10.5 :1 which is what the stock one was.

The stock piston had about 1mm clearance and the valves cleared with enough room but the valve timing had to be perfect. 1 tooth off meant a hit.
Adding 1-1.4mm more clearance means I won't have to worry about valve clearances as much but I still might extend the valve recesses if I think they are too close.
need a cam chain to test with to see it for sure


I'm aiming to have the piston top out level with the block and use the head gasket for the amount of squish.

The base gasket will be 1.45mm and the head 1.8-2.5mm

1.8mm=11.6:1
1.9mm=11.4:1
2.0mm=11.2:1
2.1mm=11:1
2.2mm=10.8:1
2.3mm=10.7:1
2.4mm=10.5:1

15cc comp chamber
2 cc per piston valve recesses.
total 17cc
bore 78mm
stroke 56.6mm

http://www.msgulfcoastvwclub.org/TechPages/Tech10.html

http://www.johnmaherracing.co.uk/enginecalc.htm

[ArttuH]

Well, over 2mm is way too large piston to head clearance, IMO. It will ruin the squish band and hurt performance. I would aim to 1.0-1.5mm range.

On the other hand over 12:1 is quite high compression ratio for an air cooled street engine. It may work or it may cause detonation. I don't have much experience on high compression N/A engines so I can't say for sure.

So ideal solution would be to adjust piston to head clearance to 1.0-1.5mm and then machine piston tops or combustion chambers to get suitable compression ratio, maybe something like 11.5:1.

[7981GS]

I have pushed these GS's to their limit on the street and a couple of my bikes are at 10.7:1 comp ratio which I think is right at the ragged edge for these air-cooled engines.
They don't knock or ping unless stuck behind a Hardly-Ableson parade when it is over 90 degrees out.
All future creations will be limited to 10.5:1.

Eric

[platinum2]

they ran over 12:1 in racing with out problems,it will just need tuned properly, new bikes are 14:1,timing and jetting ,need to be altered ,check into old school racing there should be a lot of useful info to work with
but high compression means more strain on cranks,and rods ect... so good luck with crappy designed metals,a thick head gasket,or some head chamber relief work would bump it down to a smoother 11:75:1 maybe i dont think 12:1 will be bad if the crank is good

[7981GS]

So you have no problem finding 100 octane gas there?
How much do you have to pay for it?

Eric